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Blood Vessels

Structure of Blood Vessels


The Heart

Cardiac Tissue


The Chambered Heart

Structure and Function of the Human Heart


Gross Construction



The Intrinsic Conduction System

Evolution of Vertebrate Circulation

Circulation through Gills

Circulation through Lungs

Human Ontogeny Recapitulating Phylogeny

The Fetal Circulation Pattern

Changes at Birth

Regulation of the Circulatory System

Regulation of Pressure

Pressure of Ventricular Contraction

Blood Volume and Viscosity

Blood Vessel Volume

Regulation of Blood Flow

Arterial Pathways of the Body

Arteries of the Heart

Arteries of the Head and Neck

Arteries of the Upper Limb

Arteries of the Body Wall

Arteries of the Visceral Organs

Arteries of the Pelvis and Lower Limb

Venous Pathways of the Body

Venous Drainage of the Heart

Venous Drainage of the Head and Neck

Venous Drainage of the Upper Limb

Venous Drainage of the Body Wall

Venous Drainage of the Visceral Organs

Venous Drainage of the Pelvis and Lower Limb

Circulation and Thermoregulation

Countercurrent Exchanges: Maintaining a Temperature Differential

Cooling the Body: Regulating Cutaneous Blood Flow

Cooling the Brain

Lymphatic System

Lymph Production

Lymph Vessels and Circulation

Lymphatic Tissues

Lymph Nodes




Abdominal Lymphatic Tissues


Summary of Important Concepts



Table 1 Characteristics of the Circulatory System

Table 2 Major Arteries of the Body

Table 3 Major Veins of the Body

The circulatory system is more elaborate in vertebrates than in other animals (Table 1). This is a direct consequence of body size. The simple mechanical demand of moving greater quantities of blood over greater distances has required the evolution of a more complex and stronger heart with multiple chambers. Larger masses of tissues reduce the effectiveness of diffusion and have required the development of closed circulation within an extensive system of blood vessels and capillaries. The efficiency of the blood in its ability to carry and deliver oxygen has been finely adjusted by the controls on blood pressure and the molecular mechanisms of oxygen transport.

The vertebrate system was able to make suitable adjustments for the greater demands of the high metabolic levels of mammals, including their need for an even higher rate of oxygen delivery and the new requirement of finely tuned thermoregulation. Many of the details of oxygen transport and that of other gases and nutrients are more suited to a physiology text. The following chapter will explore the contributions that anatomical design can make to circulatory function.



Vertebrate Characteristics

Hemoglobin contained in blood cells

Chambered heart

Closed circulation

Aorta ventral to the spinal column, dorsal to the viscera

Tetrapod Characteristics

Reduction of aortic arches

Pulmonary circulation

Mammalian Characteristics

Complete separation of pulmonary and systemic paths through the heart

Aortic arch passes to the left

Elaboration of counter-current exchange to regulate body temperature

Human Characteristics

Increased efficiency of emissary connections to cool the brain


The blood vessels in the body form a closed circulation, in which blood does not leave the vessels. Although some white blood cells do actively roam outside of the capillaries and plasma routinely escapes from them, the erythrocytes and the bulk of the plasma continues to follow an endless circuit away from and back to the heart. Many invertebrates possess open circulation, in which blood is pumped initially through an aorta but then flows freely among the body cells before finding its way back into the heart. Closed circulation is a verebrate characteristic.

Arteries and veins are defined according to the direction of blood flow relative to the heart. Arteries carry blood away from the heart. Arterial blood is usually richer in oxygen than venous blood, but this does not hold true for the pulmonary vessels or for the vessels in a fetus. Because arterial blood is under pressure, the walls of arteries have to be reasonably sturdy. As the vessels course farther from the heart, they branch into smaller and smaller vessels. The smallest branches of arteries are called arterioles. Arterioles have thinner walls and distribute blood among small divisions of tissues.

Veins carry blood from body tissues back to the heart. This blood is under very little pressure and the walls are relatively thinner. The smallest branches of veins are called venules. Blood carried by venules and veins converges into larger vessels in its route to the heart.

Arterioles and venules are joined by microscopically small vessels called capillaries that pass beside or close to nearly all the cells of the body. Passages through capillaries are commonly smaller than the diameter of red blood cells, and only the elasticity of their cell walls permits these blood cells to pass through. The small diameter provides a resistance to blood flow that reduces pressure nearly to zero. The slow movement of blood in capillaries facilitates the exchange of gases and nutrients with surrounding tissues.

Structure of Blood Vessels

Arteries and veins have a similar structure. The walls have three layers. The inner most layer is the tunica intima, or endothelium. This is a simple squamous epithelium whose function is larger vessels is simply to provide minimal resistance to the passage of blood. The tunica intima also includes a layer of elastic connective tissue that gives it greater strength.

The middle layer of vessels, or tunica media, contains smooth muscle and another layer of elastic fibers. This muscle is responsible for manipulating the diameter of the passage within the vessel. Narrowing the passage increases the resistance to blood flow and reduces its volume. If major vessels throughout the body are constricted in a systemic response, blood pressure will be elevated. The muscle and elastic tissues of an artery cause its wall to maintain a round shape and an open passage within. After the artery is squeezed, it immediately resumes its shape.

The outer layer of the vessel wall is a supporting layer of connective tissue called the tunica externa (or adventitia). It helps to anchor the blood vessel within the body.

Veins differ structurally from arteries primarily in the thickness of the muscular and elastic layers. Because venous blood is under considerably less pressure, venous walls are correspondingly weaker. An empty vein usually collapses and remains in that state until it is filled with fluid. Many veins also contain simple valves to maintain direction of the blood flow. The valves consist of a pair of flaps that are passively opened by the movement of blood and equally passively closed by any backflow in the opposite direction.

Smaller blood vessels depart from this basic structure by reducing or eliminating the outer layers. The smaller arterioles and venules have lost the outer layer and reduce the muscle of the tunica media to a single layer of fibers. Capillaries dispense with all but the epithelial cells of the endothelium. The thin walls and the flat shape of the squamous cells encourage the distribution of oxygen and other small molecules through them. The endothelial cells of the endothelium are also more loosely joined to facilitate the escape of fluids and white blood cells to escape into adjacent tissues.


An anastomosis is a direct connection between two blood vessels that are larger than capillaries and that permits blood to flow from one to the other. Anastomoses may occur between arteries, between veins, or between an artery and a vein. Each of these pairings serves a different function.

Arterial anastomoses provide alternate pathways, or collateral circulation, for blood to reach a given tissue. Under ideal circumstances, we expect the arterial blood advancing into the anastomosis from opposite directions to slow to a stop at a point where the blood pressure is zero. The location of the zero point depends on the relative pressures in the two arteries that have been joined. Realistically, however, those pressures change constantly from one moment to the next and the zero point also changes constantly.

Consider, for example, the brachial artery crossing the elbow and dividing into the ulnar and radial arteries. Any contraction of the skeletal muscles adjacent to one of these arteries will inhibit blood flow and decrease pressure downstream of the contraction. Flexion of the joint will also compress the brachial artery and interfere with blood flow because the artery passes on the anterior aspect (inside the flexure) of the elbow. Collateral circulation enables blood to travel through smaller vessels that branch off the brachial artery above the elbow and join the radial and ulnar arteries in the forearm below the compression. The anastomoses therefore permit immediate compensation for the effects of muscular and skeletal activity on the circulation.

Arterial anastomoses are commonly found where such interference is likely, at the major joints in the limbs; at the ends of long vessels where blood pressure is likely to be low, as on the thoracic wall; and in vital organs, such as the brain and heart, where a temporary interruption of blood flow could damage tissues.

Venous anastomoses serve a similar purpose in providing alternative pathways by which blood may return to the heart. Because venous blood has very low pressure, its flow is easily blocked by the activity of body tissues or by external pressure. There is a much greater development of collateral circulation among veins than arteries. Consequently, injuries or surgical procedures that damage or block a smaller vein are much less serious than disruptions of arteries.

Arteriovenous anastomoses occur at the level of arterioles. The circulatory system needs to be able to supply more blood to a given tissue than it needs in a resting state. It can reduce overall blood pressure by shunting that surplus flow directly into the venules. When the tissues are very active and require more oxygen, the shunts can be constricted by muscular sphincters to direct an increased flow into the capillary beds. Actively manipulating these passages therefore permits the body to adjust the flow of blood into or away from tissues according to their need.



Blood in the arteries must be put into motion to circulate throughout the body. In the digestive system, the body uses peristaltic contractions to move food along the alimentary canal. A rhythmic peristalsis might work for arteries, but probably could not generate the force necessary to overcome the resistance presented by the smallest capillaries. Thus a larger pump, the heart, is needed. The simplest heart is a muscular chamber that can contract and push blood along the artery. The introduction of valves controls the direction in which the blood will move. The complexity of the heart is increased by its division into chambers that initially served to increase the effective force of contraction.

Cardiac Tissue

It is simple to imagine a heart created by strengthening the muscle already in the walls of the vessels. However, the muscle of the heart has diverged into a unique tissue, different from both skeletal and visceral muscle. Cardiac muscle is striated &emdash; that is, its contractile proteins are aligned in bands within the fibers. Individual cells are stacked end to end to form long muscle fibers. Cardiac cells do not required individual innervation to stimulate contraction. Instead, excitation is initiated rhythmically by a small cluster of specialized cardiac cells and then spreads directly from one cell to another.

In its striation and formation of long fibers, cardiac tissue resembles skeletal muscle. By its composition of individual cells and its ability to spread rhythmic waves of excitation across the tissue, it more resembles smooth muscle. Clearly this specialized tissue has adopted the desirable characteristics of both of its relatives.


Valves maintain the direction of the blood flow. It is important for the integrity and efficiency of the circulatory system that the valves operate passively. Each set of valves, whether in the veins or in the heart, is opened by the pressure of blood flowing in the proper direction. Each set of valves is closed by any tendency of the blood beyond it to flow backwards.

Valves separate the chambers of the heart and have specialized forms corresponding to the size of the passages and the blood pressure generated by the different chambers.

The Chambered Heart

The force with which a muscular chamber can contract is proportional to the initial stretch of its walls. If sufficient blood enters a chamber of the heart under pressure, it will distend the walls and permit the fibers to generate a stronger contraction. This relationship of stretch and force of contraction has been formalized as Starling's Law.

How does the body create an initial pressure to fill the heart? The solution is to create a series of chambers. The first chamber fills with blood returning from the veins that has very little pressure. When it contracts, it fills a second chamber with blood under a greater pressure. The second chamber is thus able to produce a stronger force. The third chamber contracts more strongly yet, and so on with each successive chamber.

Primitive vertebrates, if we are to make inferences from fish, had hearts with four successive chambers, the sinus venosus, atrium, ventricle, and conus arteriosus. The sinus venosus is simply the confluence of the major veins of the body. It pumps blood through a valve into the atrium. Pressure builds as the blood successively passes through the other chambers.



The human (and, more generally, mammalian) heart is an elaboration of this primitive vertebrate pattern. It has been reduced to two pumping chambers, atrium and ventricle, but each of these has been divided into right and left chambers so that there are two independent streams of blood passing through the heart at all times.

Blood from the veins of the body is collected into the superior and inferior venae cavae, which empty into the right atrium. The right atrium pumps into the right ventricle, enabling the right ventricle to pump blood more forcefully out of the heart. From the right ventricle, blood travels through the pulmonary trunk and arteries to the lungs to be oxygenated. Blood returns from the lungs via the pulmonary veins into the left atrium. From the left atrium it passes through the left ventricle and out the aorta to be distributed to body tissues.

These eight large arteries and veins entering and leaving the heart &emdash; superior and inferior venae cavae, four pulmonary veins, pulmonary trunk, and aorta &emdash; are termed the great vessels of the heart.


The heart is surrounded by a serous membrane derived from the peritoneum and called the pericardium. The pericardium forms its own closed, double-walled sac. It has a visceral layer, the epicardium, that adheres to the heart, and a parietal layer that anchors to adjacent organs. The two layers come together at the bases of the great vessels. Between them is a potential space, the pericardial cavity. A small amount of serous fluid lubricates the cavity to permit the heart to move as it beats.

The parietal layer of the pericardium is fused with the central tendon of the diaphragm below the heart around the inferior vena cava as it rises through its hiatus. This anchors the heart in place. On its right and left sides, the pericardium adheres to the pleural membranes of the lungs. Anteriorly it faces the sternum and ribcage; posteriorly the esophagus and aorta.

Gross Construction

The wall of the heart consists of three tissue layers. The external layer, the epicardium, is the visceral layer of pericardium. The middle layer, the myocardium, consists of the cardiac muscle. The inner layer, or endocardium, is an epithelium that is continuous with the endothelium lining the blood vessels. Like the endothelium, it is smooth and tends to repel blood cells.

There is normally a layer of lipid between the epicardium and myocardium. This lipid is liquid at body temperature, but becomes solidified fat at room temperature. It is concentrated along the pathways of the coronary arteries and probably enhances their ability to stretch and contract.

The ventricles have much thicker and stronger walls than the atrium. The left ventricle is thicker and more rounded than the right. These proportions indicate the force of contraction needed to move the blood to its immediate destination. The left ventricle must work harder to propel blood throughout the body than the right does to move it to the lungs. Nonetheless, all of the chambers must move the same volume of blood in the same number of strokes.Thus their capacities are of comparable size, expelling approximately 70-90 ml. (two or three ounces) of blood with each contraction.

The fibers of the myocardium encircle the chambers of the heart so as to compress them. A single fiber will weave around both right and left chambers. This ensures that the two will contract simultaneously.


The atria are formed embryologically by the merging of the sinus venosus and the atrium. These two parts are distinguishable in the adult heart by the structure of their walls. The walls derived from the sinus are smooth and regular, resembling the lining of large veins. The true atrial walls have conspicuous muscular bands, called the pectinate muscles. The abrupt line along which the two parts fuse is called the crista terminalis.

The septum that divides the atria into right and left chambers formed later in the fetal period by growth inward from all sides of the chamber. The point of the last communication between the two sides is marked by a small depression, the fossa ovalis (see below).

The right atrium receives the superior and inferior venae cavae. These are large vessels, vertically aligned with one another and blending into the right wall of the chamber. A smaller opening into the atrium permits the flow of blood from the coronary sinus. The right atrium empties inferiorly into the right ventricle by way of the tricuspid valve.

The left atrium receives four pulmonary veins, two from each of the lungs. These enter the back of the heart running horizontally. Blood from the left atrium passes through the bicuspid valve into the left ventricle.


The ventricles have thicker and more muscular walls than the atria. The muscle fibers that form them encircle the chambers and terminate in pronounced ridges on the inner walls. These ridges are called trabeculae carnae and give the ventricles a much different appearance than the atria.

In the right ventricle, the three flaps of the tricuspid valve open into the ventricle. Each of the flaps is extended by a few fibers called the chordae tendiniae, which in turn are anchored to the wall or floor of the ventricle by muscular processes called the papillary muscles. The papillary muscles are extensions of bundles of muscle fibers that make up the ventricular walls. As the ventricle contracts, so do the papillary muscles. At first glance, it might appear that they function to draw open the flaps of the valve. This is not the case, and it may be emphasized that opening and closing the valves is a passive operation. Because the muscles contract as the ventricle is contracting, they are working just as rising blood pressure in the ventricle is closing the valve. Their critical role is to provide tension on the chordae tendiniae to stabilize the flaps of the valve and prevent them from being swept upwards into the atrium.

In the left ventricle, a similar arrangement ensues. The atrioventricular valve on the left has two flaps. Hence it is called the bicuspid valve (or mitral valve after its fancied resemblance to a bishop's hat, or mitre). The flaps of the bicuspid valve also have chordae tendiniae and papillary muscles.

The passages out of the ventricles utilize a different set of valves. The base of the pulmonary trunk, emerging from the right ventricle, contains a pulmonary semilunar valve. This also has three flaps. Because the pressure experienced in the arteries between heart beats is so much less than that emerging from the heart, it is not necessary to strengthen them with papillary muscles. On the left side, the aorta is separated from the left ventricle by an aortic semilunar valve of similar construction.

One additional feature is unique to the right ventricle. The septomarginal band (formerly called the "moderator band") spans the chamber from its septum to its floor. This band appears to be one of the trabeculae of the wall, but it contains a highly specialized bundle of cardiac fibers conducting impulses to initiate contraction of the walls (see below).

The Intrinsic Conduction System

It is essential to human life that the heart contract in a regular rhythm, about 70 times per minute. The heart does not rely on the brain for this rhythm, but is self stimulating. Specialized cardiac muscle fibers have assumed the role of initiating, propagating, and synchronizing excitation across the walls of the heart.

The "pacemaker" of the heart where a heartbeat is initiated, is the sinuatrial node, a small clump of cells in the wall of the right atrium. These cells are self-exciting, the frequency of heartbeat determined by their intrinsic properties. The membrane activity they generate spreads from one cardiac cell to the next across the walls of the atria.

As the atria are contracting, the impulse is prevented from engulfing the ventricles by an insulating layer of connective tissue. Instead, the excitation is detected by the atrioventricular node, near the junction of the four chambers. After a calculated delay, the signal is passed on to the ventricular walls, dispersing on the right and left atrioventricular bundles. The delay ensures that the contractions of the atria and ventricles are sequential, rather than overlapping. The arrangement of cardiac fibers, coiling about both ventricles at once, assures that the right and left sides contract synchronously.

The rhythm of sinuatrial stimulation can be altered by the autonomic nervous system. Sympathetic fibers terminating in the wall of the heart and releasing the neurotransmitter norepinephrine increase the rate and force of contraction. The hormones norepinephrine and epinephrine secreted into the bloodstream by the suprarenal medulla during sympathetic arousal enhance this effect. Acetylcholine, released by parasympathetic fibers terminating in the heart, slows the heart rate.



The evolutionary transition from the primitive vertebrate four-chambered heart to the modern mammalian four-chambered heart is elegantly described by comparative anatomy and reiterated in human embryology. So lucid is this model that we must remind ourselves that living vertebrates do not themselves form an evolutionary sequence, but represent only the end products of many evolutionary lineages.

Circulation through Gills

The four chambered heart of a fish described above may be understood as a primitive vertebrate form. Blood leaving the conus arteriosus enters a ventral aorta and runs anteriorly to the branchial arches. These arches are specialized for respiration as internal gills in modern fishes. The branchial arches contain arteries known as the aortic arches. Each of the aortic arches produces a rich capillary bed from which gases are exchanged with the passing water. The aortic arches are reformed and the oxygenated blood is collected into a dorsal aorta that runs posteriorly. Branches of the dorsal aorta supply the body and its visceral organs.

This circulatory system loses most of its blood pressure at the capillaries of the aortic arches. Blood runs through other body arteries because of fluid pressure and skeletal muscular contractions. Obviously the oxygen supply must be adequate for the needs of the fish, but the flow of arterial blood must be described as sluggish by mammalian standards. The evolution of the lung as a respiratory organ provided an opportunity to improve on this pattern.

Circulation through Lungs

The lung/swim bladder organ system that appears in bony fish is, like the branchial arches themselves, a derivative of the pharynx. Its blood supply, the pulmonary artery, is a branch of the sixth aortic arch. Blood draining from the primitive lung joins other venous blood in returning to the heart. As the lungs became increasingly important in respiration for fishes and amphibians, the pulmonary veins became more important as carriers of oxygen. Any alterations of circulation or heart structure that might keep this oxygenated blood from being diluted before it reaches other body tissues would increase the efficiency of oxygen supply to those tissues and would be evolutionarily favored. Changes in this direction appear in modern lungfish and amphibians.

Air-breathing terrestrial vertebrates should be able to by-pass the capillary beds of the aortic arches with much of their blood flow. This is accomplished in amphibians and all derived tetrapods by directing that flow primarily through the fourth aortic arch, as a direct link between the ventral and dorsal aortas. This shunt thus elevates the pressure in the dorsal aorta.

At the same time, lower tetrapods display modifications of the heart that prevent blending of oxygenated blood from the pulmonary veins with deoxygenated blood from the other veins of the body. Incomplete septa within the atria and ventricle and a flap of tissue within the conus arteriosus called the spiral fold direct parallel streams of venous blood through these chambers. Mixing of the streams is incomplete. More importantly, the two streams of blood are targeted separately. The more oxygenated blood is directed to the more anterior aortic arches, supplying the head and body via carotid arteries and dorsal aorta. Less oxygenated blood enters the fifth and sixth aortic arches to the lungs.

The design, as described, is clearly a successful evolutionary strategy, as it has been maintained for hundreds of millions of years. It has the advantage, among others, of varying the proportion of blood entering the lungs or passing directly to the body. For a fish that can carry on respiration alternately with gills or lungs, or for an aquatic amphibian or vertebrate that ceases to breathe for periods of time, this option is essential.

Complete separation of the two circuits occurs in birds and mammals. The mammalian pattern, of which humans are typical, reduces the aortic arches to one (the left fourth arch) and converts the sixth arch into the pulmonary arteries. The septa divide the atria and ventricles each into two chambers. The spiral fold of the conus arteriosus divides that into the entwined bases of the two great vessels, the pulmonary trunk and the ascending aorta. Birds follow an almost identical strategy except that the right fourth aortic arch is retained and the left one lost.

Human Ontogeny Recapitulating Phylogeny

The evolutionary sequence inferred from comparative anatomy is replayed in every human fetus. In the human embryo six pairs of branchial and aortic arches are created. The fate of each one can be traced into the adult form. The first two arches are lost. The termination of the ventral aorta persists as the internal carotid artery to the brain, while the third arch becomes the external carotid artery to the neck and face. Of the two fourth arches, the right one is lost (except as the base of the right subclavian artery) while the left arch persists. The fifth aortic arch is lost and the sixth becomes the pulmonary artery. The connection of the sixth arch with the dorsal aorta is retained until birth.

The Fetal Circulation Pattern

This developmental sequence accomodates the two different patterns of circulation that must serve mammals before and after birth. A fetus obtains its oxygen from the maternal bloodstream at the placenta. Its lungs are non-functional. Worse, the lungs are filled with fluid, which applies a pressure to pulmonary vessels and greatly increases the resistance to blood flow there. If the right side of the heart were strong enough to pump a full stream of blood through the lungs before birth, it would be hypertrophied and critically overpowered for its role after birth.

Fetal circulation resolves these problems with several structural modifications from the adult pattern. Oxygenated blood from the placenta enters the body via the umbilical vein. It passes through the liver via a shunt called the ductus venosus and enters the inferior vena cava. The inferior vena cava and superior vena cava both drain into the right atrium of the heart. However, the two streams do not completely mix. The parts of the atrial septum do not fuse and leave a passage, the foramen ovale, between the right and left atria. Oxygenated blood from the inferior vena cava is preferentially directed through the foramen ovale, while deoxygenated blood from the superior vena cava preferentially passes into the right ventricle. The transfer of blood from the right to the left atrium reduces the volume and pressure of blood pumped by the right ventricle.

The fetal pulmonary trunk remains connected to the aortic arch by the ductus arteriosus. This passage represents the primitive connection of the sixth aortic arch with the dorsal aorta. Because the lungs still have a high resistance to blood flow, the ductus arteriosus functions as a relief valve permitting even more blood to be diverted away from the lungs and into the aorta. Relatively little blood trickles through the lungs and returns to the left atrium; thus blood in the left side of the heart is predominantly that which was received from the inferior vena cava through the foramen ovale.

Note that the most highly oxygenated blood flows to the coronary arteries (to the heart tissue) and the carotid arteries (to the head), while other parts of the body are supplied with blood that has been diluted by the ductus arteriosus. Since other visceral systems of the fetus are not performing vital functions and the skeletal muscles do little work in the nearly weightless environment of the womb, the heart and brain are the only tissues that demand a large supply of oxygen.

Changes at Birth

At birth, the newborn circulation must immediately switch to an adult circulation pattern. The groundwork for this transformation has already been laid, and only the stimulus of birth itself is required to trigger the changes. The following changes happen almost simultaneously.

The vessels of the umbilical cord are emptied of their contents and collapse. The umbilical arteries constrict, permanently shutting off the flow of blood out of the fetus to the placenta. The now unneeded umbilical vein also collapses. Its remnants persist as the ligamentum teres, a strand of connective tissue running from the umbilicus to the liver on the inside of the abdominal wall.

Ductus venosus in the liver is constricted and closed. Its remnants persist as the ligamentum venosum between the right and left lobes of the liver. Before birth, the ductus venosus carried blood from both the umbilical vein and the hepatic portal vein to the inferior vena cava. The hepatic portal vein drained the non-functioning digestive tract. After birth, the portal vein carries newly absorbed nutrients. When the ductus closes, these are forced to pass through sinusoids of the liver for reprocessing and storage.

The lungs empty of fluid and resistance to blood flow drops. Ductus arteriosus constricts and all of the pulmonary blood is now directed into the lungs. Ductus arteriosus persists as a non-functioning connection between the pulmonary trunk and the aortic arch, the ligamentum arteriosum.

The increased flow of blood through the lungs causes an increase in the return of blood to the left atrium. As pressure rises in the left atrium, it equals the pressure in the right. The overlapping flaps of the septum are now pushed together to effectively close the foramen ovale. The final shunt of blood away from the lungs is now eliminated and the full volume of blood passes through the right ventricle and pulmonary trunk. A depression in the wall of the septum, the fossa ovalis, marks the site of the foramen ovale. In a substantial number of individuals, the parts of the septum never fuse completely, but are held together by the balanced pressure.



The circulatory system constantly moves blood about the body, but the needs of the body and of individual tissues are constantly changing. It is necessary for the body to be able to monitor its effectiveness and to adjust the flow of blood both for the body as a whole and for individual tissues.

Regulation of Pressure

Blood pressure is a measure of the effectiveness of the heart. Although absolute pressure differs in various parts of the body, pressure may be considered a property of the system as a whole because changes in pressure will register in all open blood vessels. Three factors are important in determining blood pressure: the force of the heartbeat, the volume of the blood, and the volume of the blood vessels. Each of these is subject to regulation.

Pressure of Ventricular Contraction

The heart is able to regulate the force of its contraction. This is effected by the autonomic nervous system. Sympathetic arousal and its associated hormones epinephrine and norepinephrine increase both the rate and force of contraction. Parasympathetic stimulation slows the heart and lessens its force.

Blood pressure rises and falls in a cyclic manner following the action of the heart. As the ventricle contracts, pressure leaving the blood peaks. This is called systolic pressure. The pressure wave advances through the arteries, but this takes time. Thus different arteries may experience systole (the moment of maximum pressure) at different times, depending on their distance from the heart. Diastolic pressure is a minimum pressure corresponding to the interval between ventricular contraction.

The pressure wave traveling through the arteries is dissipated by a number of factors that absorb its energy. The friction between blood and the walls of the arteries produces a resistance to flow that is very sensitive to vessel size and length. Resistance is proportionate to the inverse of the fourth power of the diameter. Thus resistance experiences a sudden increase whenever vessels diverge or when blood enters a narrow branch of an arterial trunk. The change in resistance is enough to reflect a small wave of pressure upstream, back to the heart. The elasticity of arterial walls also absorbs energy from the pressure wave. Vessel walls stretch at systole, momemtarily reducing the pressure. This energy is partly returned to the bloodstream during diastole as the walls rebound.

These factors reduce blood pressure to nearly zero in the capillaries and veins. The slow passage of blood in the capillaries is adaptive in that it facilitates diffusion of gases and nutrients. Movement of blood in the veins is accomplished primarily from external sources, such as the contraction of skeletal muscles in the limbs and wall of the trunk.

The elasticity of vessel walls provides an immediate feedback mechanism for resistance. When high pressure forces the vessel to stretch and increase its diameter, the small increase in diameter leads to a much greater drop in resistance and pressure and an increase in flow.

Blood Volume and Viscosity

The volume of blood is regulated primarily by long term mechanisms. Water retention in the body increases fluid volume. This may be adjusted by the mechanisms of thirst and urine production. Adjustments in the proportion of body sodium and potassium by the kidneys shifts the balance of fluid between intracellular and extracellular spaces. Albumin in the plasma can be used to regulate osmotic pressure and to draw fluid specifically into the blood vessels. These mechanisms may be regulated through hormones secreted by a number of endocrine glands, including the pituitary and suprarenal glands. The heart itself secretes a hormone, atrial natriuretic factor, to fine-tune volume and pressure.

The viscosity of the blood corresponds to its resistance to flow. A high viscosity requires more pressure to move blood. Changes in the albumin content or the hematocrit affect viscosity. Red blood cell count can be changed quickly by the release of more cells from storage in the spleen or more slowly by adjusting the rate of their manufacture.

Blood Vessel Volume

The volume of the blood vessels relative to the volume of blood helps to determine pressure. Blood pressure can be manipulated in the short term by constricting or dilating the walls of the arteries. These mechanisms depend on sensors that report blood pressure to the brain. Specialized pressure sensors called baroreceptors are strategically located in the walls of the atria and the aorta, and in the carotid bodies at the base of the internal carotid artery. These report to the vasomotor center in the medulla. The vasomotor center responds to low pressure by stimulating vasoconstriction along sympathetic pathways. High pressure in the atria cause a dilation of the arteries by an inhibition of their walls. The center is also sensitive to blood chemistry and may elevate pressure when carbon dioxide levels rise or oxygen levels drop.

Veins also have the capacity to adjust their volume. When their walls are relaxed, they may serve as reservoir for blood. Venous constriction takes up the slack in blood vessel volume, speeds up the passage time of blood in the venous system, and effectively places the sluggish venous blood back into circulation.

Longer term changes in vessel volume may be accomplished through hormones. A sudden drop in blood pressure, as from a loss of blood, results in the secretion of vasopressin (= anti-diuretic hormone) by the neurohypophysis. In addition to causing water retention in the kidneys, vasopressin increases the tone of the smooth muscle in vessel walls and elevates blood pressure. Another hormone, angiotensin released by the kidneys, is also a vasoconstrictor.

Additional changes in blood vessel volume may represent pathological changes, such as the clogging of arteries with plaque.

Regulation of Blood Flow

The ability to shunt blood to the tissues that need it most enables the circulatory system to provide for the body's requirements while conserving on the bulk of blood and the energy needed to move it through the tissues. More importantly, perhaps, the dynamic changes in circulatory patterns enables the body to respond in the shorter moments of extreme demand. This capacity requires the ability of the nervous system to anticipate changing demands or a mechanism by which the tissues communicate their need to the circulatory system. Both systems of controls exist.

The competition between the parasympathetic and sympathetic nervous systems to direct energy either to storage or to skeletal muscle is also played out in the circulatory system. During times of relaxation, the visceral systems are most active in maintaining the state of the body and in processing new energy stores. The parasympathetic dominates. The blood flow to the visceral organs is relatively abundant. Sympathetic arousal changes the priority of distributing energy in favor of skeletal muscle. Blood vessels to the viscera are constricted and an increased proportion of blood flows to the body wall and limbs.

Sympathetic arousal is a body-wide response and cannot identify the specific muscles that are being used. It is therefore desirable that individual tissues have the ability to signal their own needs for increased blood flow. The messenger of this signal is the molecule nitric oxide (NO). NO is synthesized by body tissues in need of oxygen and diffuses across cell membranes. It causes the smooth muscle of the arteriole walls and precapillary sphincters to relax and the vessels to dilate. As oxygen supply increases or need drops, the rate of synthesis of NO should decrease. This interplay of oxygen and NO represents a negative feedback system that can be specific to individual capillary beds and small areas of tissue.



The systemic arteries arise from the aorta and its branches. The aorta emerges from the left ventricle of the heart and arches asymmetrically to the left before descending through the thoracic and abdominal cavities. For convenience, therefore, we are able formally to distinguish the ascending aorta, aortic arch, and descending aorta. From the ascending aorta arise the coronary arteries that supply the heart. The aortic arch gives rise to three large vessels, the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery, which together supply blood to the head, neck, and upper limbs. The descending aorta produces all the branches to the trunk and its organs. The aorta terminates by dividing into the right and left common iliac arteries. Each of these in turn divides into an internal iliac artery to the organs of the pelvic cavity and an external iliac artery supplying the lower limb. The major arteries of the body are summarized on Table 2.

The names of the vessels reproduce much of the Latin vocabulary for body parts. Learning the names of the vessels helps to reinforce that learning. The terminology will make more sense if some other usages are understood. An arterial trunk is a short artery of substantial diameter that quickly divides into two or more vessels. A common artery divides into a pair of vessels with similar names (e.g., the common carotid divides into the internal carotid and external carotid arteries). An artery whose name bears a relative term (e.g., superior epigastric, anterior interosseous) will have a counterpart with the opposite term (inferior epigastric, posterior interosseous).

Arteries of the Heart

The coronary arteries are the first branches of the aorta. They emerge from its base, at the level of the semilunar valve and before the aorta leaves the pericardial sac. The right and left coronary arteries form an anastomosing ring about the heart, between the atria and the ventricles. (Latin corona = ring, hence the name "coronary.") In addition, there is a second loop in the interventricular sulcus on both the anterior and posterior surfaces. These major branches are constant, although the points of anastomoses vary greatly in different individuals.

These arteries and their larger branches mostly lie in the external grooves on the heart, corresponding to the edges of the walls that partition the chambers. Their smaller branches are thus well placed to supply the muscle tissue of those internal walls.

The positioning of the coronary arteries near the semilunar valve is also strategic. Most arteries of the body receive a pulse of blood during or immediately following contraction of the ventricle. This pulse provides the force to push blood through body tissues against resistance. However, this is the worst time to supply blood to the heart itself because the contracting cardiac muscle is compressing its capillaries and providing maximal resistance to blood flow. To resolve this problem, the flaps of the semilunar valve partly cover and block the entrances to the coronary arteries as they are forced open by blood leaving the ventricle. Between the ventricular contractions, backflow of blood in the aorta closes the valve and fills the coronary arteries. Thus blood is supplied to the heart muscle when it is relaxed.

The left coronary artery branches quickly into circumflex and descending branches. The left circumflex branch occupies the atrioventricular groove on the left side of the heart, circling to the posterior side. The anterior descending branch (= anterior interventricular branch) occupies the interventricular sulcus on the anterior surface of the heart. This sulcus and the artery descend to the apex and continue onto the dorsal surface.

The right coronary artery runs in the atrioventricular sulcus on the right side of the heart. As it passes to the dorsal side, it meets and anatomoses with the left circumflex branch. The right artery gives off a marginal branch, which travels toward the apex along the inferior surface of the heart. It also usually produces a posterior descending branch that runs toward the apex on the dorsal surface to meet with the anterior descending branch.

Arteries of the Head and Neck

The head and neck are supplied by branches of the common carotid arteries. The right common carotid is a branch of the brachiocephalic trunk from the aortic arch. The left common carotid is an independent branch of the aortic arch. The common carotid artery ascends in the neck within a fascial wrapping called the carotid sheath. Near the level of the larynx, each of the common carotid arteries branches into an external carotid and an internal carotid.

The external carotid artery supplies tissues of most of the neck, the face, and the outside of the head. Among its major branches are the superior thyroid artery, to the thyroid gland; the lingual artery to the tongue and floor of the mouth; the facial artery, to the skin and superficial muscles of the face; the maxillary artery to deep structures of the face; and the superficial temporal arteries to the scalp.

The internal carotid artery continues to ascend within the carotid sheath. It enters the skull through the carotid canal. Its first branch, the ophthalmic artery, enters and supplies the orbit. The major part of the internal carotid then joins the circle of Willis on the underside of the brain.

The circle of Willis is an anastomotic ring of vessels that encircles the infundibulum. It gives off several vital paired arteries to the brain. The anterior cerebral artery runs between the cerebral hemispheres and supplies medial cortex. The middle cerebral artery passes between the temporal and frontal lobes and supplies lateral cortex, including the primary motor and somatic sensory areas.

Blood to the brain is supplemented by a branch from the subclavian artery. The vertebral artery ascends the neck within the transverse foramina of the cervical vertebrae and enters the skull through the foramen magnum. On the surface of the brain stem, the right and left vertebral arteries give rise to spinal arteries, which descend along the spinal cord to supply that part of the central nervous system. The vertebral arteries join to create an unpaired basilar artery that ascends on the midline of the brainstem and gives off branches to supply the medulla, pons, and cerebellum. The basilar divides into right and left posterior cerebral arteries, each supplying the occipital lobe and visual cortex. The posterior cerebral arteries communicate with the circle of Willis.

Arteries of the Upper Limb

Blood for tissues of the upper limb leaves the trunk between the clavicle and the first rib in the subclavian artery. On the right side, the subclavian is a branch of the brachiocephalic trunk. On the left, it is an independent branch from the aortic arch. As this vessel descends down the limb, it changes names. "Subclavian" describes its course inferior to the clavicle. As it crosses the armpit (axilla), it becomes the axillary artery. In the upper arm (brachium) it is the brachial artery.

Each of these sections of the artery has several smaller branches. Among the branches of the subclavian are the vertebral artery, which ascends to the brain (above), the internal thoracic artery, descending in the thorax (described below), and other branches to the neck and upper thorax.

The axillary artery gives rise to the thoraco-acromial trunk, supplying the pectoral muscles and anterior shoulder, and the subscapular artery to the muscles lying deep to the scapula. In the axilla, the artery lies deep but relatively unprotected. A penetrating wound from below that passes between the two folds of muscles may open the vessel fatally.

The brachial artery descends along the medial side of the humerus in the intermuscular septum. This fascial space separates the flexors from the triceps muscle. A pulse may often be felt in the artery in this region. The brachial artery produces the deep brachial artery, which spirals around the humerus and supplies triceps muscles.

The brachial artery terminates at the elbow as it divides into the radial and ulnar arteries.These two branches descend along the forearm on the radial and ulnar sides. The ulnar artery crosses the wrist ventrally and produces the superficial palmar arch that gives rise to arteries of the fingers. The radial artery passes posterior to the base of the thumb and enters the palm between the first and second metacarpal. As it passes on the radial side of the carpal bones, the artery is fairly exposed and the pulse is readily palpated.

Arteries of the Body Wall

The arteries of the thoracic and abdominal walls reflect the segmentation of those regions of the body. Each segment contains a pair of arteries that circle the body wall, accompanying spinal nerves, and following similar branching patterns to those of the nerves.

Intercostal arteries lie within the eleven pairs of intercostal spaces between ribs. Below these is a pair of subcostal arteries, representing segment T12, that run inferior to the twelfth rib. The first two arise from a branch of the subclavian artery. The others arise directly from the descending aorta in the thorax. These arteries give off dorsal rami to the epaxial region of the back and continue to the front of the trunk. The lower intercostal arteries descend to supply the abdominal wall, as well. In the lumbar region are a series of short paired lumbar arteries that mostly supply muscles along the spine.

Anteriorly is a vertical chain of anastomosing arteries. The internal thoracic artery arises from the subclavian and descends deep to the ribs just lateral to the sternum. Its segmental branches anastomose with each of the intercostal arteries in turn. As the internal thoracic pierces the diaphragm, it changes its name to the superior epigastric artery. In the lower abdominal wall, the inferior epigastric artery ascends from the external iliac artery. Superior and inferior epigastric arteries run within the rectus sheath deep to the rectus abdominis muscle.

Arteries of the Visceral Organs

The body wall and limbs are supplied by paired arteries that mostly emerge symmetrically from the right and left sides of the aorta. The major thoracic visceral organs &emdash; heart and lungs &emdash; have their own special relationships with the circulatory system. Arteries to the urinary and reproductive systems are paired, as are the organs they supply. In contrast, the organs of the digestive tract are supplied by unpaired branches emerging from the front of the aorta.

Near the proximal end of the abdominal aorta are a pair of short renal arteries to the kidneys. They carry a large volume of blood to be cleansed by the urinary system. Near this level also are paired gonadal arteries (ovarian arteries in the female; testicular arteries in the male). These vessels are long and slender, descending to the pelvic area to their organs. The rest of the urogenital system is supplied primarily by branches of the internal iliac artery in the pelvic cavity. Among the divisions of the internal iliac are the internal pudendal artery that supplies most of the perineum, and the gluteal arteries to the gluteal muscles.

The digestive tract, from the esophagus to the rectum, is supplied by unpaired branches of the aorta. In the thorax, there are four or five small esophageal branches. In the abdomen, however, there are three large trunks emerging from the aorta. These are the celiac trunk, the superior mesenteric artery, and the inferior mesenteric artery .

The celiac trunk emerges just inferior to the diaphragm and immediately divides into three smaller arteries. The left gastric artery courses along the lesser curvature of the stomach starting at the esophagus. The large splenic artery runs along the superior border of the pancreas to the spleen, supplying both of those organs. It also sends off several branches to the stomach. The common hepatic artery is responsible for the right side of the stomach, the upper duodenum, and the accessory glands &emdash; liver, gall bladder, and head of the pancreas.

The superior mesenteric artery supplies the next portion of the digestive tract from the lower duodenum to the transverse colon. Many of its numerous branches lie within the mesentery of the small intestine. Others run within the peritoneal folds to the cecum and ascending colon.

The inferior mesenteric artery, smaller than the other two visceral trunks from the aorta, supplies the transverse, descending, and sigmoid colon and the rectum. This vessel runs to the left after leaving the aorta and distributes its branches within the large question mark described by this portion of the large intestine.

Arteries of the Pelvis and Lower Limb

The abdominal aorta terminates at the level of the lumbar spine by dividing into the right and left common iliac arteries. Each of these quickly divides again into the internal and external iliac arteries.

The internal iliac artery supplies the visceral contents of the pelvic cavity, including the bladder and internal reproductive organs. Some of its branches leave the pelvis through the greater sciatic foramen and supply muscles of the gluteal region and also much of the perineum.

The external iliac artery gives off the inferior epigastric artery (see above) and leaves the body cavity under the inguinal ligament. At this landmark, it changes its name to become the femoral artery. The femoral artery descends the thigh anterior to the femur. Its largest branch is the deep femoral artery, which more directly supplies muscles on all sides of the thigh.

Just above the knee, the femoral artery circles on the medial side of the femur to the popliteal fossa, or pit of the knee. Within that space it lies deep as the popliteal artery. The popliteal artery gives off a number of small geniculate branches that encircle the knee joint. Behind the head of the tibia the popliteal artery divides into anterior and posterior tibial arteries. The anterior tibial artery passes through a foramen at the top of the interosseous membrane between the tibia and fibula. It descends the leg deep to the anterior tibial muscles and crosses the ankle. The last portion of this artery is called the dorsalis pedis, as it distributes small branches on the dorsum of of the foot. The posterior tibial artery descends deep to the triceps surae, supplying posterior muscles. As it enters the sole of the foot, it divides into the medial and lateral plantar arteries.



Pulmonary arteries


Branches of the ascending aorta

right coronary artery

marginal branch

left coronary artery

left circumflex branch

anterior interventricular branch


Branches of the aortic arch

brachiocephalic trunk

right common carotid artery (branches similar to left side)

right subclavian artery (branches similar to left side)

left common carotid artery

external carotid artery

superior thyroid artery

lingual artery

facial artery

maxillary artery

superficial temporal artery

internal carotid artery

ophthalmic artery

circle of Willis (anastomoses with branches of basilar artery)

anterior cerebral artery

middle cerebral artery

left subclavian artery

vertebral artery

anterior spinal artery

posterior spinal artery (unpaired)

basilar artery (unpaired)

posterior cerebral artery

internal thoracic artery

superior epigastric artery

axillary artery

thoraco-acromial trunk

brachial artery

deep brachial artery

radial artery

ulnar artery

superficial palmar arch


Branches of the descending thoracic aorta

intercostal arteries (9 pairs)

subcostal artery

esophageal branches


Branches of the abdominal aorta

renal artery

gonadal artery

lumbar arteries

celiac trunk (unpaired)

left gastric artery

splenic artery

common hepatic artery

superior mesenteric artery (unpaired)

inferior mesenteric artery (unpaired)


Common iliac artery

internal iliac artery

internal pudendal artery

superior and inferior gluteal arteries

external iliac artery

inferior epigastric artery

femoral artery

deep femoral artery

popliteal artery

anterior tibial artery

dorsalis pedis

posterior tibial artery

medial plantar artery

lateral plantar artery



The veins of the body mostly follow the pathways of the arteries and are given comparable names (Table 3). In each region of the body, there are a few notable exceptions to this pattern, as described in the following section. It is also common for veins to be doubled as they run alongside the artery, with such pairs tightly bound to the artery and anastomosing frequently with one another. These veins are sometimes referred to as venae comitantes.

In addition to being more numerous than arteries, veins are also more variable in their presence or absence, branching patterns, and size. This is consistent with the more numerous anatomoses that provide redundant channels by which venous blood can return to the heart.

Venous Drainage of the Heart

The branches of the coronary arteries drain into cardiac veins. These veins mostly occupy the sulci of the heart and course with the arteries. The largest is the great cardiac vein, which ascends in the anterior interventricular sulcus and then runs with the left circumflex artery to the posterior side of the heart. The cardiac veins drain into a chamber in the posterior atrioventricular sulcus called the coronary sinus. The sinus drains into the right atrium by a small formen in the wall of that chamber. A few small veins from the internal walls of the heart open independently into the right atrium.

Venous Drainage of the Head and Neck

Smaller veins of the face and neck closely parallel the arteries. These collect in the external jugular vein that descends superficial to the sternocleidomastoid muscle to empty into the subclavian vein. A smaller anterior jugular vein carries blood from the anterior neck.

The cerebrum has a network of veins on its surface, but these empty into a series of passive chambers, the dural sinuses, formed between layers of the dura mater around the brain. The sinuses leave imprints on the internal surfaces of the cranial fossa. The paired cavernous sinus lies on the side of the pituitary gland. It receives blood from the upper face, via the ophthalmic vein. The mixing of blood from the face and brain in this sinus represents a potentially dangerous pathway for skin infections to gain access to the brain.

Dural sinuses are drained via the internal jugular vein. The internal jugular exits the cranium via the jugular foramen and descends within the carotid sheath. As it joins the subclavian vein, the two form the brachiocephalic vein. Right and left cephalic veins join to make the superior vena cava.

Venous Drainage of the Upper Limb

A series of veins accompany the arteries of the upper limb and bear corresponding names. In addition to these deep veins, there is a network of superficial veins lying at the base of the superficial fascia. These superficial veins are often visible on the surface of the skin, depending on their size and the depth of overlying adipose tissue.

Superficial veins are many in number and the smaller ones are quite variable. Numerous anastomoses interconnect them. The largest is the cephalic vein, which ascends the limb from the radial side of the hand and continues across the shoulder to drain into the axillary vein. The basilic vein ascends on the ulnar side of the hand and forearm. After crossing the elbow, the basilic passes deep to drain into the brachial vein. Anterior to the elbow, a venous shunt called the median cubital vein often connects the cephalic with the basilic. This short vessel is a preferred clinical site for drawing blood.

Venous Drainage of the Body Wall

The pattern of venous drainage of the body wall mirrors that of arterial distribution and includes the epigastric veins, internal thoracic vein, and the intercostal veins. Blood from the intercostal veins of the right side of the body collect into an ascending vessel called the azygos vein (azygos = "without a twin"). The azygos empties directly into the superior vena cava. On the left side, intercostal veins of the lower thoracic segments drain into the hemiazygos vein, while those of the upper segments empty into the accessory hemiazygos vein. Both the hemiazygos and accessory hemiazygos drain across the spine into the azygos vein.

Venous Drainage of the Visceral Organs

Blood from the kidneys returns to the heart via the large renal veins and the inferior vena cava. Blood from the other organs within the peritoneal cavity is routed through the liver in the hepatic portal vein before entering the inferior vena cava. The portal vein is formed by the junction of the superior mesenteric vein, from the small intestine and proximal colon, and the splenic vein, from the spleen, stomach, and nearby organs. The splenic vein also receives blood from the inferior mesenteric vein, draining the distal colon. The splenic, superior mesenteric, and inferior mesenteric veins are the approximate counterparts of the celiac trunk and superior and inferior mesenteric arteries.

The venous blood in the hepatic portal vein carries the newly digested nutrients from the digestive tract to the liver, where these molecules may be extracted and processed or stored. To facilitate this, the portal vein divides repeatedly into capillary-sized vessels called sinusoids. Sinusoids flow past individual hepatocytes, which can extract nutrients or secrete the proteins, hormones, and other products of the liver. Venous blood in the sinusoids mingles with that of the hepatic arteries and is collected in the right and left hepatic veins. The hepatic veins enter directly into the inferior vena cava as it passes on the posterior side of the liver.

Venous Drainage of the Pelvis and Lower Limb

The veins of the lower limb and pelvis have similar pathways and names as the arteries, with the femoral vein ascending the thigh to become the external iliac vein. The external and internal iliac veins form the common iliac and the right and left vessels join to create the inferior vena cava.

This pattern of deep veins of the limb is supplemented by superficial veins lying at the base of the layer of superficial fascia. The two largest of these veins are the great and small saphenous veins. The great saphenous vein begins on the medial side of the foot and ankle and ascends the length of the limb to plunge deep in the femoral triangle and empty into the femoral vein. The long exposed course of this vessel and its redundancy with alternate pathways of venous return permit surgeons to utilize it as a graft to replaced blocked coronary arteries (coronary bypass operation). The small saphenous vein arises on the lateral side of the ankle and ascends on the posterior aspect of the leg as far as the popliteal fossa. There it runs deep to drain into the popliteal vein.





Coronary sinus

cardiac veins


Superior vena cava

azygos vein

right intercostal veins

hemiazygos vein

left intercostal veins

accessory hemiazygos vein

left intercostal veins

brachiocephalic veins

internal jugular vein

dural sinuses

cerebral veins

subclavian vein

external jugular vein

anterior jugular vein

axillary vein

cephalic vein

brachial vein

basilic vein

median cubital vein

ulnar vein (branches similar to artery)

radial vein (branches similar to artery)


Inferior vena cava

hepatic veins

hepatic portal vein (unpaired)

splenic vein

inferior mesenteric vein

superior mesenteric vein

renal vein

common iliac vein

internal iliac vein

external iliac vein

femoral vein (branches similar to artery)

great saphenous vein

popliteal vein (branches similar to artery)

small saphenous vein



As the circulatory system transports blood, oxygen, nutrients, and wastes within the body, it also distributes heat. Blood is warmed as it passes through metabolically active tissues, and it transfers that heat to superficial structures cooled by contact with the environment. Several specific mechanisms have evolved to increase or decrease the efficiency of this heat exchange. The circulatory system thus plays an important role in thermoregulation of the body.

The effectiveness of heat exchange depends on the rate at which blood passes through a tissue and the surface area of the vessels in contact with that tissue. Exchange is therefore more complete in a capillary bed, where fluid moves slowly and the surface to volume ratio of the vessels is greatest.

Countercurrent Exchanges: Maintaining a Temperature Differential

A countercurrent exchange may occur where venous and arterial pathways pass one another in opposite directions. Generally the arterial blood carries warmth from the core of the body, while venous blood is returning from the cooler periphery. In such a case, venous blood may be warmed so as not to chill the center of the body. Arterial blood may be cooled to reduce heat loss from the skin. In this way a countercurrent exchange helps to conserve body heat.

Countercurrent exchanges commonly occur in the limbs. Deep veins lie in contact with the arteries. Often two small veins exist for each artery. This facilitates heat transfer by increasing the area of contact. In the spermaticord, an exchange occurs between the testicular artery and a plexus of veins returning to the body. This mechanism maintains the testis and epididymis below body temperature.

Cooling the Body: Regulating Cutaneous Blood Flow

A second vascular mechanism for thermoregulation is an adjustment of the volume of blood flow to the skin. Major cutaneous vessels lie deep in the superficial fascia, insulated by adipose tissue from the surface. Smaller vessels carry blood through the fascia closer to the surface. From the more superficial capillaries, heat is more readily lost and fluids are more readily available to sweat glands.

By dilating or constricting the vessels to the skin, the body can influence the rate of heat loss. When the body is hot or is producing excessive heat, as by exercise, the arteries open and blood comes more to the surface. This accounts for the reddening or flushing of the skin. If the body is cold, vessels are closed down and the skin becomes visibly more pale.

Cooling the Brain

The brain is extremely sensitive to temperature fluctuations. Thermoregulation in the head may be expected to be more elaborate than elsewhere in the body. The most important factor determining brain temperature is the temperature of blood ascending in the internal carotid arteries (Baker 1982). Additionally, an active brain may produce a significant amount of waste energy.

Many mammals, including carnivores and ungulates, keep the brain cool by an elaborate countercurrent exchange mechanism. The internal carotid artery typically passes through the wall of the cavernous sinus, one of the dural sinuses surrounding the brain. In these animals, the artery briefly divides into a network of small arterioles, called a rete mirabile ("marvelous network"), as it passes the sinus and then reforms into a large artery once more. This network increases the effectiveness of heat exchange. As the cavernous sinus receives blood from the face via the ophthalmic vein, its ability to cool the carotid artery is that much greater.

Primates do not possess a rete at the carotid sinus, although their increased brain size requires effective cooling. Other mechanisms contribute to this function. Blood from the upper respiratory tract may drain to the sinus. Inspiration especially cools this blood in order to warm air going to the lungs. The evaporation of human perspiration from the face and scalp provides another mechanism for cooling blood. This surface blood then may enter the cranial fossa to mingle with blood in the cavernous sinus and other dural sinuses instead of draining via the external jugular vein (Baker 1982; Cabanac 1986; Dean 1988). This alternate pathway is possible because of the absence of valves in these connections. The variable communications between the dural sinuses and external veins are called emissary veins. One of the more consistent pairs of these pass through the parietal foramina on either side of the sagittal suture. Other conspicuous canals for emissary veins are sometimes found at the base of the occipital bone.

Human upright posture may use gravity to facilitate the flow of blood from the brain. In addition to the internal jugular vein, a plexus of veins in the vertebral canal drains the cranial fossa (Eckenhoff 1970). The combination of emissary connections to the perspiration-cooled skin with the gravity-driven siphon of the vertebral plexus is highly effective in cooling the human brain. It has been argued that the achievement of this "radiator" mechanism overcame thermal barriers that limited brain size and permitted its further expansion in hominids (Falk 1990). As critics have observed, however, the logic may be reversed. The evolution of a large brain may have required the secondary development of a mechanism to cool it.



The fluids that escape from the capillaries enter the extravascular extracellular spaces. Although some of this fluid along with small electrolytes reenter the vessels through osmotic pressure, the remaining fluid, along with leukocytes and blood proteins must be gathered and returned to the bloodstream. The anatomical structures tht accomplish this constitute a second circulatory system, the lymphatic system.

Lymph Production

Extracellular fluid, regardless of its composition, constitutes lymph. The majority of this fluid derives from escaped plasma from the capillaries. It thus has a similar composition to blood, lacking the red blood cells. White blood cells, able to move of their own accord, are able to leave the capillaries and circulate in the lymph. Lymph also gathers debris and waste materials from the extracellular space. This might include remnants of broken down cells, blood clots, metabolic by-products, and invading bacteria. The flow of lymph thus serves the purpose of cleansing body tissues.

The rate of lymph production is related to blood pressure. As pressure rises, more plasma is forced through the gaps in the capillary walls. For the most part, extra lymph production is of little consequence because it is continuously returned to the blood stream. Certain tissues, however, are more sensitive. The accumulation of fluid (edema) in the lungs, for example, would interfere with respiration. Therefore pressure in the pulmonary arteries is limited to far below that in the systemic arteries. Likewise, edema in the lower limbs is a concern because filtration of blood under pressure is enhanced by gravity.

Lymph Vessels and Circulation

In contrast to the closed system that carries blood, the lymphatic system is open. Lymph is gathered from outside the vessels in the extracellular spaces and dumped into the subclavian veins. Thus the pathway has a clear beginning and end.

The smallest lymphatic vessels are called capillaries. Each of these lymphatic capillaries has a blind end. The epithelial cells that form this terminal are not attached firmly together, but they overlap in a way that they act as valves. If the fluid pressure outside the capillaries is greater than that inside, the cells are pushed apart and lymph enters the vessels. If the pressure difference is the opposite, the cells are pushed together and the fluid is kept within. This mechanism underscores the fact that it is fluid pressure which initiates lymphatic circulation.

Capillaries converge into larger and larger vessels, although most are no larger than one millimeter in diameter. Lymph moves from the extremities toward the center of the body by the combined effects of fluid pressure, valves within lymphatic vessels, and skeletal muscle contraction. A failure of these mechanisms to move adequate quantities of lymph results in edema, most commonly in the lower extremities.

As the vessels converge, they encounter lymph nodes, which filter particles from the lymph. Lymph from the lower limbs and the abdominal cavity is collected in a small sac called the cisterna chyli. The cisterna lies near the midline immediately inferior to the diaphragm. It represents the beginning of the only substantial lymphatic vessel, the thoracic duct. The thoracic duct is a slender vessel that runs along the ventral surface of the vertebrae, ascending from the cisterna chyli to the proximal end of the left subclavian vein. As it ascends, it also receives lymph from the thoracic cavity, the left upper limb, and the left side of the head and neck. At the subclavian vein, lymph is returned to the bloodstream.

The remaining lymph &emdash; from the right side of the head and neck, the right upper limb, and the right pectoral area &emdash; converges on a much smaller vessel, the right lymphatic duct, which empties into the right subclavian vein.

The direction of lymphatic flow has clinical importance. Germs and metastisizing cancer cells may be carried along with the lymph and lodge in lymph nodes. These nodes may then become secondary centers of infection or tumors.

Lymphatic Tissues

"Lymphatic tissues" are a diverse assortment of structures within the body that are capable of filtering its fluids and detecting foreign invaders. These include the lymph nodes filtering lymph itself, the spleen filtering blood, the tonsils filtering mucus of the upper respiratory membranes, the thymus housing lymphocytes, and isolated tissues of the digestive tract. These tissues share a common structure. They are considered reticular connective tissue. This is a form of loose connective tissue in which a network of short collagen fibers anchor large numbers of lymphocytes and other cells. The cells are able to interact with fluid passing among them and to extract, identify, and destroy particles in that fluid.

Lymph Nodes

Lymph nodes are small clusters of tissue placed along the lymph vessels. There are hundreds of lymph nodes varying in size up to two or three centimeters. The nodes are not evenly distributed, but are found in especially high concentrations in the neck, at the axilla, and in the inguinal regions. In these areas, the lymphatic vessels are converging and entering the trunk itself. Other nodes lie in likely sites of entry for pathogens &emdash; in the mesenteries of the intestine and at the roots of the lungs.

Each lymph node consists of discrete clusters of lymphocytes and macrophages enclosed in a capsule. Lymph entering the node flows around the cell clusters. The cells are able to monitor the contents of the fluid and to cleanse it of particles. Lymphocytes may increase in numbers during certain types of infections and the nodes may become conspicuously swollen.


The spleen has a structure and function analogous to that of a lymph node. However, instead of contributing directly to the lymphatic circulation, it sits astride a major artery, the splenic artery, and filters blood. Like lymph nodes, the spleen consists of reticular tissue with numerous lymphocytes and macrophages whose job it is to conduct immune surveillance. In addition, the spleen is responsible for removing worn out blood cells from circulation. The fragments of these cells are broken down and stored for later recycling. In particular, the iron compounds are salvaged and transported to bone marrow for resynthesis of hemoglobin.

The spleen is an important source of new blood cells in a fetus. This ability is normally lost after birth. Platelets are stored within the spleen, to be released into the bloodstream as needed.


The thymus was classified with the endocrine system before its primary role in the immune system was recognized. It is a mass of reticular tissue immediately deep to the superior part of the sternum. The thymus is large and active in children and degenerates into a fibrous and fatty tissue in adults.

The thymus houses large numbers of immature lymphocytes. It is here that their abilities to produce antibodies are screened. Lymphocytes producing antibodies against the body's own proteins are destroyed before or shortly after birth. Those producing antibodies to antigens encountered later in life are encouraged to clone (make copies of) themselves. Some of these copies circulate and are active in other parts of the body; others remain dormant as "memory cells" to be activated quickly upon a later enocunter with the same antigen.

The thymus also secretes the hormone thymosin and other factors that regulate the behavior of the lymphocytes and other body defenses.


Tonsils are small bodies of lymphatic tissues within the mucous membranes of the pharynx. Three pairs are most conspicuous. The pharyngeal tonsils ("adenoids") lie on the lateral wall of the nasopharynx, near the entrance to the auditory tube. The palatine tonsils, visible in a child's open mouth, lie at the junction of the oral cavity and the pharynx. These are frequently enlarged during throat infections. The lingual tonsils lie on the surface of the posterior third of the tongue.

The tonsils screen the mucus of the pharynx for trapped bacteria, alerting the immune system and stimulating appropriate responses. They actually provide a pathway for bacteria to cross the membrane and gain access to the lymphatic circulation; but this is a calculated breach of the barrier to facilitate identification of potential pathogens.

Typical of lymphatic tissues, the tonsils are well developed and conspicuous in children and participate in the exploration and priming of the immune system. As the immune system matures, the tonsils are reduced in size and the frequency of inflammation is reduced. They are relatively inconspicuous in adults. In the recent past, the oral and pharyngeal tonsils were commonly removed surgically under the mistaken impression that their frequent causation of discomfort outweighed any positive function.

Abdominal Lymphatic Tissues

Nodules of lymphatic tissue are present in the walls of the intestines as a defense against bacteria that might invade from the alimentary canal. In structure and function, these resemble the tonsils. Clusters of lymphocytes along the small intestine are called Peyer's patches. Additional nodules surround the appendix, and probably justify its persistance in the human species.



1. Increase in body size in early vertebrates led to the development of a more elaborate circulatory system that included closed circulation and a chambered heart.

2. Blood is a specialized connective tissue whose components include plasma and blood cells.

3. Plasma contains serum with its small molecular nutrients and ions plus larger proteins, including albumins, globulins, and clotting factors.

4. Erythrocytes exist primarily to transport hemoglobin. They are short-lived and easily replaced or increased in number upon demand.

5. Leukocytes are diverse in form and function. They perform several roles relating to the bodies defenses and scavenging operations.

6. Platelets are cell fragments that participate in hemostasis, or the control of bleeding from ruptured vessels.

7. Arteries and veins have similar structures with walls of three layers. Both have elastic tissue and smooth muscle to adjust their diameters.

8. Capillaries have thin walls of simple squamous epithelium that facilitates exchange of gases and fluids with body tissues.

9. Circulation in veins is maintained by externally generated pressure and by valves.

10. Arterial anastomoses provide alternate pathways, or collateral circulation, for blood to reach a given tissue.

11. Venous anastomoses provide alternative pathways by which blood may return to the heart.

12. Arteriovenous anastomoses permit blood to be shunted into or away from given tissues of the body, according to need.

13. Cardiac tissue contains fibers made from stacks of short cells capable of spreading a wave of excitation to contract from one cell to another.

14. Valves of the heart operate passively to maintain the direction of flow of the blood from one chamber to the next.

15. The chambered heart permits the cumulative increase in force of contraction by successive chambers in accordance with Starling's Law.

16. The primitive vertebrate heart had four chambers aligned in a series: sinus venosus, atrium, ventricle, and conus arteriosus.

17. The mammalian heart has two sets of two chambers pumping in synchrony to provide separate pulmonary and systemic circulation.

18. The heart is surrounded by a double layer of serous membrane called the pericardium.

7. The wall of the heart consists of three tissue layers: epicardium, myocardium, and endocardium.

20. The right atrium receives blood from the superior and inferior venae cavae and empties into the right venticle.

21. The right ventricle pumps blood through the pulmonary trunk to the lungs.

22. The left atrium receives blood from the pulmonary veins and empties into the left venticle.

23. The left ventricle pumps blood through the aorta to the body tissues.

24. Flaps of the atrioventricular valves are reinforced by chordae tendiniae and papillary muscles.

25. Cardiac tissue is self-exciting through a specialized body of cells called the sinuatrial node. Impulses are propagated from one cardiac cell to another across the walls of the chambers.

26. Primitive vertebrate circulation directed blood out of the heart, through the ventral aorta; through the aortic arches in the gills, and to the body tissues via the dorsal aorta.

27. The transition to air-breathing required a separation of pulonary and aortic circuits and the elimination of the gills.

28. This evolutionary sequence is reproduced in every human fetus.

29. Fetal circulation and adult circulation have different patterns due to the non-functioning of the lungs. Numerous traces of the fetal pattern of circulation are preserved in adult anatomy.


30. Three factors are important in determining blood pressure: the force of the heartbeat, the volume of the blood, and the volume of the blood vessels. These are each subject to regulation through long and short term strategies.

31. Quantity of blood flow to individual tissues may be regulated by systemic or local mechanisms to respond to need for oxygen.

32. The heart is supplied by the coronary arteries and drained by the cardiac veins.

33. The head and neck are supplied with blood by the external and internal carotid arteries and drained by the jugular veins.

34. The upper limb is supplied by the subclavian artery and its branches.

35. The wall of the trunk is supplied by segmental intercostal arteries. Venous blood returns asymmetrically via the azygos vein.

36. Most visceral organs are supplied by unpaired branches from the aorta.

37. Blood from the abdominal viscera is carried by the hepatic portal vein to sinusoids in the liver before reaching the inferior vena cava.

38. Pelvic structures are supplied by branches of the internal iliac artery.

39. The lower limb is supplied by the external iliac and femoral arteries and their branches.

40. Venous return of blood from the limbs is supplemented by networks of subcutaneous veins.

41. The circulatory system conserves heat in the core of the body with countercurrent exchanges between arteries and veins.

42. Cutaneous circulation is able to direct excess heat to the skin to be dumped.

43. The temperature of the brain and its blood supply is critical for brain function. Several mechanisms have evolved to prevent the brain from becoming overheated.

44. Extracellular fluid constitutes lymph.

45. Lymph is collected in the lymphatic vessels, is filtered, and is returned to the blood stream.

46. The thoracic duct is the largest of the lymphatic vessels and empties into the left subclavian vein.

47. Lymph nodes house lymphocytes and macrophages and other cells that filter lymph and perform immune surveillance for the body.

48. The spleen houses lymphocytes and macrophages and other cells that filter blood and perform immune surveillance for the body. In addition, the spleen removes and recycles worn out blood cells.

49. The thymus houses maturing lymphocytes and secretes hormones to regulate the body's defenses.

50. Other lymphatic tissues, including tonsils and Peyer's patches, perform filtering and surveillance operations within mucous membranes.



Baker, Mary Ann 1979. A brain-cooling system in mammals. Scientific American 240(5):130-139.

Baker, Mary Ann 1982. Brain cooling in endotherms in heat and exercise. Annual Review of Physiology 44:85-96.

Cabanac, Michel 1986. Keeping a cool head. News in Physiological Sciences 1(4):41-44.

Dean, M. Christopher 1988. Another look at the nose and the functional significance of the face and nasal mucous membrane for cooling the brain in fossil hominids. Journal of Human Evolution 17:715-718.

Eckenhoff, James E. 1970. The physiologic significance of the vertebral venous plexus. Surgery, Gynecology, and Obstetrics 131(1):72-78.

Falk, Dean 1990. Brain evolution in Homo: the "radiator" theory. Behavior and Brain Sciences 13(2):333-381.

Mayerson, H.S. 1963. The lymphatic system. In Vertebrate Adaptations: Readings from Scientific American. Pp. 114-124. (Reprinted from Scientific American, June, 1963.)

Vogel, Steven 1992. Vital Circuits: On Pumps, Pipes, and the Workings of Circulatory Systems. New York: Oxford University Press.




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