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CHAPTER 10: The Circulatory System


The human body is a well-constructed collection of trillions of cells. As indicated in Chapter 1, each of those cells needs essential raw materials such as food, water, and oxygen to survive. Because most of the body's cells are distant from food and air, these materials must be brought to them, and waste products generated by the cells must be carried away.

In keeping with our oceanic origins, human cells, like those of all Metazoa, are made mostly of water. Consequently, they are surrounded by extracellular tissue fluids that, in turn, are water based. As a result, the major task of the digestive and respiratory systems is to take in food and gases, respectively, and put them into solution or suspension in a watery medium. That aqueous medium, the blood, is enclosed within a hydraulic system of tubes and pumps designed to transport fluid to and from all the body's cells and tissues. This blood vascular system is subdivided into two major circuits: the pulmonary circulation, which takes blood to and from the lungs, and the systemic circulation, which carries blood to and from the rest of the body.

The circulatory system is a closed hydraulic system powered by a pump-the highly efficient heart-that weighs less than a pound. At rest, it pulses at around 70 beats per minute - a rate that results in approximately 40 million heartbeats each year. Exercise increases this number of dramatically: for example, during strenuous exercise, the pulse rate can rise to 180 heartbeats per minute, and the blood can flow out of the, aorta to the tissues at a volume in excess of 30 L (7.5 gallons) per minute. In a highly trained athlete, that number can reach 12 gallons per minute. Given a total blood volume of 6 L, the heart can cycle all of the blood through the body many times per minute. Consequently, blood coming out of the heart exits the ventricles at high pressure. This high pressue is handled by a series of thick-walled, resilient distribution tubes called arteries. The arteries lead to thin-walled exchange vessels called capillaries, in which the crucial exchange of materials between blood and tissue fluids occurs. As illustrated in Plate 10-4, the capillaries are surrounded by extremely thin walls that facilitate the passage of materials across them. The capillaries, in turn, lead to a series of relatively thin-walled collecting tubes called veins, which carry the blood back to the heart at moderately low pressure.

The microarchitecture of the heart-arteries, veins, and capillaries-is reflected in their functions. The close and strikingly obvious correlation between structure and function in the heart and vessels of the blood vascular system greatly simplifies understanding their histologic organization. The distribution vessels, the arteries, handle large volumes of blood at high pressure. Consequently, one would predict that the arteries closest to the heart would be big bored and thick walled, and those farthest from the heart, small bored and thin walled. This prediction holds true not only for arteries but also for veins: veins closest to capillaries are small and thin walled, and those near the heart, large and thick walled.

No discussion of the circulatory system would be complete without emphasizing that it is composed of the blood and lymph vascular systems. Unlike the blood vascular system, the lymphatic circulation has no heart of its own. Instead, lymph - a viscous, creamy fluid devoid of red blood cells - picks up materials from the tissue fluids via blind-ended, thin-walled lymph capillaries. These lymphatic capillaries flow into larger vessels that pass lymph through lymph nodes. Lymph, which is picked up from the tissue spaces, is delivered into the blood vascular system at the subclavian vein; close to the neck, a large lymphatic vessel enters the wall of the subclavian vein and empties its contents into the lumen of that blood vessel. As a result, the blood and lymph vascular systems, although made up of a separate system of tubes, eventually connect with one another.


The Aorta And Vena Cava

Many of the body's blood vessels have a common pattern of organization, exemplified by the body's largest artery and vein - the aorta and the vena cava. Figure A is a light micrograph of a cross section taken through the wall of the aorta of the squirrel monkey. Figure B is a light micrograph of a similar section through the vena cava of the same animal. Figure C is a low-magnification electron micrograph of the inner portion of the wall of the same vena cava shown in Figure B.

The aorta like all large blood vessels, has a wall composed of three layers - the Tunica Intima, the tunica media, and the tunica Adventitia. As shown in Figure A, the innermost layer, the Tunica Intima (TI), borders on the lumen of the vessel (L) and is lined by a single layer of flattened endothelial cells (arrow). The endothelial cells are supported by a thin bed of subendothelial connective tissue that, in turn, rests upon a thick sheet of elastic tissue called the Internal Elastic Membrane (EM). The Internal Elastic Membrane forms a boundary between the thin Tunica Intima and the middle layer, the thick Tunica Media (TM).

The aorta, the largest blood vessel in the human body, can deliver 12 gal of blood per minute to the general circulation under conditions of strenuous exercise. The aorta's ability to handle that large volume of blood, forcefully pumped in pulsatile bursts from the left ventricle of the heart, resides in the great strength and resilience of its Tunica Media. The strength of the tunica media comes from its Collagen fibers; the resilience, from large sheets of elastic fibers. These elastic sheets (E), each flat and perforated by holes like a slice of Swiss cheese, appear as black, wavy lines when viewed in cross section in Figure A. The elasticity of the aorta permits it to expand when it receives the large bolus of blood from the heart delivered by ventricular contraction (Systole), and to recoil when the left ventricle relaxes (Diastole). In addition to being quite elastic, the aorta is very muscular; Figure A reveals large numbers of Smooth Muscle fibers (S) in the Tunica Media. The aorta's Collagen fibers give it great strength and set limits to its ability to stretch under pressure.

The tunica Adventitia (TA) forms the outermost layer of the aorta. Only partially included in the field of view in Figure A, the Adventitia is a loosely woven envelope made up of Collagen fibers, a few elastic fibers, and some Smooth Muscle fibers. Small autonomic nerves and blood vessels that serve the aorta course through the Adventitia.

The vena cava, shown in Figure B, returns circulating blood to the heart. Because blood pressure within the vena cava is much lower than in the aorta, the vena cava's wall is much thinner (compare Figures A and B, photographed at the same magnification). The tunica media of the vena cava contains far less elastic tissue than that of the aorta.

The wall of the vena cava is shown by electron microscopy in Figure C. Here, a few blood cells remain in the lumen (L), stuck to the Endothelium that lines the vessel. The inner surface of the vena cava depicted here is thrown into folds. The Tunica Intima (TI) contains surface endothelial cells (arrow), subendothelial connective tissue (CT), and a few Smooth Muscle fibers (S). Beneath the wavy profile of the innermost elastic sheets lies the Tunica Media, which consists of connective tissue, numerous Smooth Muscle fibers (S), and a few strands of elastic tissue (E).

Plate 10-1

Plate 10-1, Figure A. Light micrograph of cross section through the aorta of the squirrel monkey. 330 X

Figure B. Light micrograph of cross section through the vena cava of the squirrel monkey. 330 X

Figure C. Electron micrograph through inner portion of the same vena cava shown in Figure B. 1,150 X CT, subendothelial connective tissue; E, elastic sheets; EM, Internal Elastic Membrane; L, lumen; S, Smooth Muscle; TA, tunica Adventitia; TI, tunica intima; TM, Tunica Media; arrow, Endothelium.


Medium-Sized Artery And Vein

One of the most common and frustrating obstacles faced by students of microanatomy is the differential identification of arteries and veins in sectioned material. Careful study of Figures A, B, and C at right should help solve that problem.

Figure A is a light micrograph of a cross section through a medium-sized artery (A) and vein (V) from the Testis of the squirrel monkey. Because the tissue was fixed by intravascular perfusion, the vessels contain no blood and their walls are dilated - unlike the constricted walls of the aorta and vena cava shown in Plate 10-1. The artery has a much thicker wall in proportion to its lumen than does the neighboring vein. In addition, the profile of the artery is almost perfectly circular, whereas that of the vein is somewhat irregular. These characteristics, which provide key visual cues to the histologic identification of artery and vein, originate in the comparative ultrastructures of the vessels' walls. This difference is evident in Figures B and C, which are electron micrographs of the walls of the same artery and vein shown in Figure A.

In Figure B, the wall of the medium-sized artery is seen to consist of the three basic layers previously described in the aorta-the Tunica Intima (TI), the Tunica Media (TM), and the tunica Adventitia (TA). Here, the tunica intima consists of a single layer of endothelial cells (E) and is therefore quite thin. A well-developed internal elastic membrane (EM) lies between the intima and media. The Tunica Media contains seven concentric layers of Smooth Muscle fibers (SM) whose fusiform nuclei are cut in longitudinal section. The Smooth Muscle fibers are spiralled in a tight helix; consequently, their contraction can dramatically reduce the caliber of the vessel, thereby restricting blood-flow through the artery itself. Hence, medium-sized arteries such as this one are commonly called distributing arteries. Their capacity to rapidly change in diameter - to open and close under control of autonomic nerves (N) - can distribute blood-flow differently to various organs in the body as needed.

Outside the muscular Tunica Media lies the tunica Adventitia, a loosely woven meshwork of collagenous connective tissue (Co) secreted by a sparse population of fibroblasts (F). In this section, the profiles of several small, unmyelinated autonomic nerves are evident as they course through the loose connective tissue of the Adventitia.

The ultrastructure of the wall of a medium-sized vein is illustrated in Figure C. As in the artery just described, the Tunica Intima (TI) consists of a single layer of endothelial cells (E). A thin Internal Elastic Membrane (EM) is present, beneath which lies the Tunica Media (TM) In this vein, the Tunica Media is seen to consist of only two concentric layers of Smooth Muscle fibers (SM). As is evident in Figures A, B, and C, the thin, somewhat flaccid wall of the vein has a much less massive Tunica Media than does the artery. Because the Tunica Media and tunica Adventitia of the artery and vein are similar, the different relative properties of the walls of the two kinds of vessels, which allow for their identification in sectioned material, originate in the ultrastructure of the Tunica Media.

Plate 10-2

Plate 10-2, Figure A. Light micrograph of a cross section through a medium-sized artery and vein from the Testis of the squirrel monkey. A, artery; V, vein. 438 X

Figures B and C. Electron micrographs of cross sections taken through the walls of the same artery (Figure B) and vein (Figure C shown in Figure A. Co, Collagen fibrils; E, endothelial cell; EM, Internal Elastic Membrane; F, Fibroblast; N, nerve; SM, smooth muscle; TA, tunica Adventitia; TI, Tunica Intima; TM, Tunica Media. Figure B, 3,300X; Figure C, 8,900 X


Arterioles, Capillaries, And Lymphatic Vessels

Whereas medium-sized arteries serve to distribute blood to organs, arterioles move blood within organs. Arterioles, having an outer diameter of 0.1 mm or less, are the smallest of all arteries. Because the limit of resolution of the human eye is about 0.2 mm, arterioles are too small to be seen with the naked eye. Like the medium-sized (distributing) arteries described in Plate 10-2, the walls of arterioles are capable of graded contraction during vasoconstriction, permitting them to direct bloodflow to different regions within the organ they serve.

Figure A is a low-power electron micrograph of a longitudinal section through a small Arteriole within the colon. A chain of red blood cells (R) are lined up in single file within its lumen. The lumen is lined by a monolayer of endothelial cells (E). A thin sheet of elastic tissue, the Internal Elastic Membrane (EM), separates the Tunica Intima (TI) from the Tunica Media (TM). In this Arteriole, the Tunica Media consists of a single layer of Smooth Muscle fibers (SM) that are wrapped about the Arteriole's long axis like the coils of a spring. Since the Arteriole is embedded within an organ, no true tunica Adventitia is present. Instead, a thin layer of collagenous connective tissue (Co) binds the vessel to surrounding tissues.

Arterioles eventually lead to capillaries, in which the major functions of the circulatory system are carried out - the vital exchange of nutrients, gases, and metabolites between blood and tissue. Figure B, an electron micrograph of a cross section through a continuous capillary, shows that the capillary is ideally suited for the diffusion of gases and exchange of materials across its wall. First, the ultra-thin walls favor gas diffusion. Second, hundreds of micropinocytotic vesicles (arrow), shown at high magnification in the inset, serve to shuttle nutrients and waste products back and forth between the circulating blood and the cells and tissue spaces next to the capillary. The wall of the capillary shown in Figure B consists of a single endothelial cell (E) rolled into a tube. The endothelial cell's Nucleus (N) is evident, as is a tangentially sectioned red blood cell (R) in the lumen. The capillary is surrounded by a thin, delicate basal lamina (arrowhead).

When viewed with the light microscope, small blood vessels and small lymph vessels can easily be mistaken for one another. Under the electron microscope, however, their ultrastructural differences become readily apparent. Compare, for example, the images in Figures B and C. Figure B is the capillary we have just described; Figure C is a lymph capillary. Its wall is extremely thin, consisting of the attenuated Cytoplasm of a single thin, flattened endothelial cell (E). The shape of the lymph capillary is irregular, in part because the lymphatic circulation has no heart. Instead, lymph is passively propelled through its vessels by the movement of neighboring muscles. Consequently, lymph circulates at very low pressure, and its flaccid, thin-walled vessels present irregular-and often collapsed-profiles in sectioned material. The lumen (L) is quite large compared to the thin wall; the wall is held in place by a delicate skein of Collagen fibrils (Co). Red blood cells are never seen in healthy lymphatic vessels. Lymphocytes, however, are often evident, especially in inflamed or infected regions of the body.


Plate 10-3

Plate 10-3, Figure A. Electron micrograph of longitudinal section through an Arteriole. Co, Collagen fibrils; E, endothelial cell; EM, internal elastic membrane; R, red blood cell; SM, Smooth Muscle; TI, Tunica Intima; TM, Tunica Media. 9,140 X

Figure B. Electron micrograph of cross section through a continuous capillary. E, endothelial cell; N, Nucleus of endothelial cell; R, red blood cell; arrows, micropinocytotic vesicles; arrowhead, Basal Lamina. Inset, same capillary wall at higher magnification. 15,000 X; inset, 35,500 X

Figure C. Electron micrograph of cross section through a lymphatic capillary. Co, Collagen fibrils; E, endothelial cell; L, lumen. 5,100


The Heart: The Wall Of The Atrium

The heart is a truly amazing organ. Although it beats cyclically and constantly throughout life, the heart is so quietly efficient that we often go about our business quite unaware of its vigorous activity. The human four-chambered heart consists of two atria, which receive venous blood, and two ventricles, which pump blood independently to the body and lungs. The microanatomy of the heart reflects its function; it is a highly muscular pump, held together by connective tissue, that receives blood from the venous circulation and pumps it into the arterial circulation at high pressure.

The microanatomy of the monkey heart is illustrated at right. Figures A and B are a matched pair of light and electron micrographs of serial sections taken through a thin portion of the Atrium, chosen because other parts of the heart are far too thick and bulky to fit into the confines of a single thin section. In these illustrations, the lumen (L) of the heart is at the top of the photomicrograph; the outside of the heart, or epicardiurn (Ep), is at the bottom. The microanatomic organization of the heart parallels that of the larger blood vessels; it is built up of three major layers. On the inside of the heart, a thin inner lining called the Endocardium (E) corresponds to the Tunica Intima of a blood vessel. A thick, muscular middle layer, the myocardiurn (M), corresponds to the Tunica Media of a blood vessel. The Myocardium, a powerful mass of cardiac muscle, does the active pumping of blood. An outer layer of connective tissue the epicardiurn (Ep), is homologous to the tunica Adventitia of a blood vessel. The similarity in structure between heart and blood vessel is no accident; its origin is in vertebrate evolution, during which the heart is thought to have originated as a well-placed expansion of a muscular blood vessel.

Because Figures A and B are serial sections, much can be learned about the microanatomy of the heart by first finding specific structures as seen by light microscopy (Figure A) and then looking at precisely the same structures shown in greater detail by electron microscopy (Figure B). In these images, the endocardiurn (E) consists of a thin layer of endothelial cells underlain by a small amount of connective tissue. The Myocardium, by contrast, is massive and consists of many cardiac muscle fibers running in a variety of different directions. Intercalated disks (arrowhead), characteristic of cardiac muscle, are visible even at this comparatively low magnification. A vein (V) is evident running through the Myocardium, as are many small capillaries (C). The heart is well supplied with blood, and its high degree of vascularity is consistent with its continuous activity. At the point where the Myocardium. and the epicardiurn (Ep) meet, a bundle of small nerves (N) is evident. Difficult to identify by light microscopy (Figure A), their nature is obvious when viewed by electron microscopy (N, Figure B). Deeper in the Epicardium (Ep) may be seen an Arteriole (A), cut in longitudinal section. In Figure A, the elastic tissue of the Internal Elastic Membrane (arrow) shows as a dark-staining line; that same structure, when viewed by electron microscopy in Figure B, appears as an electron-lucent, clear line (arrow).


Cirlitory System


Plate 10-4

Plate 10-4, Figures A and B. Matched pair of light and electron micrographs of serial thick and thin sections taken through the Atrium of the heart of the squirrel monkey. A, Arteriole; C, capillary; E, Endocardium; Ep, Epicardium; L, lumen of Atrium; M, Myocardium; N, nerve; V, vein; arrow, Internal Elastic Membrane of Arteriole; arrowhead, Intercalated Disk, 1,000 X

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