References

Tables

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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 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.

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.

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.

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 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.

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.

Spleen

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.

Thymus

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

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.

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.

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.

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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.

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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.