ARTERIAL PATHWAYS OF THE BODY

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; thefacial artery, to the skin and superficial muscles of the face; the maxillary artery to deep structures of the face; and thesuperficial 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 thesubscapular 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 archthat 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.

TABLE 2. MAJOR ARTERIES OF THE BODY

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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

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VENOUS PATHWAYS OF THE BODY

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.

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TABLE 3. MAJOR VEINS OF THE BODY

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

 

CIRCULATION AND THERMOREGULATION

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.

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THE LYMPHATIC SYSTEM

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.

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. Thepalatine 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.

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SUMMARY OF IMPORTANT POINTS

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.
19. 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.

 

1. 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.
2. Quantity of blood flow to individual tissues may be regulated by systemic or local mechanisms to respond to need for oxygen.
3. The heart is supplied by the coronary arteries and drained by the cardiac veins.
4. The head and neck are supplied with blood by the external and internal carotid arteries and drained by the jugular veins.
5. The upper limb is supplied by the subclavian artery and its branches.
6. The wall of the trunk is supplied by segmental intercostal arteries. Venous blood returns asymmetrically via the azygos vein.
7. Most visceral organs are supplied by unpaired branches from the aorta.
8. Blood from the abdominal viscera is carried by the hepatic portal vein to sinusoids in the liver before reaching the inferior vena cava.
9. Pelvic structures are supplied by branches of the internal iliac artery.
10. The lower limb is supplied by the external iliac and femoral arteries and their branches.
11. Venous return of blood from the limbs is supplemented by networks of subcutaneous veins.
12. The circulatory system conserves heat in the core of the body with countercurrent exchanges between arteries and veins.
13. Cutaneous circulation is able to direct excess heat to the skin to be dumped.
14. 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.
15. Extracellular fluid constitutes lymph.
16. Lymph is collected in the lymphatic vessels, is filtered, and is returned to the blood stream.
17. The thoracic duct is the largest of the lymphatic vessels and empties into the left subclavian vein.
18. Lymph nodes house lymphocytes and macrophages and other cells that filter lymph and perform immune surveillance for the body.
19. 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.
20. The thymus houses maturing lymphocytes and secretes hormones to regulate the body's defenses.
21. Other lymphatic tissues, including tonsils and Peyer's patches, perform filtering and surveillance operations within mucous membranes.

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REFERENCES

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 Obstetrics131(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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