Section 8: Cardiovascular System

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

The cardiovascular system is composed of the heart and blood vessels.

The heart wall can be divided into three layers: (1) The endocardium (the innermost layer, in contact with blood) is an endothelial cell-lined layer continuous with the tunica intima of those blood vessels that join and leave the heart. (2) The myocardium is composed of cardiac muscle and corresponds to the tunica media of the blood vessel wall. (3) The epicardium (the outermost layer) is covered by a reflection of the mesothelial-lined (serous or visceral) pericardium, contains coronary blood vessels and nerves, and corresponds to the tunica adventitia of blood vessels (Plates 149 and 152).

The mammalian heart has four chambers, two thin-walled atria and two thicker-walled ventricles. The central supporting structure is the "cardiac skeleton," composed of dense white fibrous (collagenous) connective tissue into which the cardiac muscle fibers of the atria and ventricles insert and to which the heart valves are attached. The orifices of the four chambers are guarded by valves, which are endocardial folds supported by internal plates of dense collagenous and elastic connective tissue continuous with the cardiac skeleton. The right atrioventricular valve has three cusps; hence, it is called the tricuspid valve. The left atrioventricular valve has two cusps and is called the bicuspid, or mitral (for the bishop's hat or miter) valve. Semilunar valves located at the ventricular entrance to the aorta and pulmonary arteries have three cusps each. The valves are arranged to prevent retrograde or reverse blood flow.

The heart is a four-chambered pump, which moves blood throughout the vascular system. We will trace the sequence of flow and begin with the atria. Blood enters the right atrium from the great veins (inferior and superior venae cavae) and coronary veins, which carry blood, poor in oxygen and rich in carbon dioxide, returning from the entire body. Blood, rich in oxygen and poor in carbon dioxide, enters the left atrium from the lungs via pulmonary veins. This is the only instance in which oxygen- rich blood is carried in vessels called veins.

Contraction of the right and left atria forces blood past the right tricuspid and left bicuspid valves into the right and left ventricles, respectively. At the end of their contraction, the right and left atria begin to fill once again with blood. Contraction of the ventricles forces oxygen-poor blood from the right ventricle past the right semilunar valve into the pulmonary artery, and the oxygen-rich blood past the left semilunar valve into the aorta to supply the entire body and the heart itself. Note that the pulmonary artery contains oxygen-poor blood, a situation opposite to that of the pulmonary vein and the only time oxygen-poor blood is carried in a vessel called an artery. A red corpuscle, for example, moves through the heart in the following way: right atrium, right ventricle, pulmonary artery, lung capillaries, pulmonary vein, left atrium, left ventricle, and leaves the heart to enter the aorta and systemic or coronary arteries. The contractile force required to move blood through the pulmonary system is less than that required to force blood throughout the entire body. This fact is reflected in the thickness of the myocardium of the right and left ventricles (Plates 148 and 149).

The mammalian heart possesses a special system of cardiac fibers, which function to determine heart rate and to coordinate contraction of the heart. These modified cardiac fibers lie beneath the endocardium. Two pacemakers are recognized: (1) the sinoatrial node (SA node) cardiac muscle fibers, in continuity with all other atrial cardiac fibers, lie at the junction of the superior vena cava and the right atrium, and (2) the atrioventricular node (AV node), a mass of irregularly arranged highly branched specialized cardiac fibers located in the right ventricle near the opening of the coronary sinus. Extending from the AV node is a bundle of small, unbranched cardiac muscle fibers called the atrioventricular bundle, which passes to the midline of the heart to branch and form two larger bundles beneath the endocardium on either side of the interventricular septum. The small striated cardiac fibers are continuous with the large, vacuous, glycogen-filled fibers, with few myofibrils, called Purkinje fibers. Purkinje fibers are functionally continuous with ordinary cardiac muscle fibers (Plates 78, 79, and 150). The sequence of events is as follows: The SA node fibers are spontaneously active and transmit electrical signals in the form of muscle action potentials to all other atrial muscle fibers, which are functionally linked to each other by intercalated discs (Plate 76), resulting in atrial contraction. The electrical signal then impinges on AV nodal fibers (Plate 150), which transmit the signal via small unbranched bundle fibers to Purkinje fibers (Plate 78) and finally to the ordinary ventricular cardiac fibers around the apex of the heart, where ventricular contraction begins and spreads upward to end at the midline skeleton between the atria and ventricles.

The myocytes of the right (and to a lesser extent the left) atrium contain specific, membrane-bound, granules 0.3 to 0.4 µm in diameter. Two polypeptide hormones have been extracted from atrial muscle: (1) cardionatrim, which has both diuretic and natriuretic effects, and (2) cardiodilatin, which acts on vascular smooth muscle. Hence, it has been suggested that the atria be considered endocrine organs containing endocrine fibers. These fibers have functional properties also seen in peptide- secreting cells of the gastrointestinal and respiratory tracts.

Blood vessels that originate from the right and left ventricles are designated as arteries and have a distinctive structure. The wall of an artery usually has three tunics or coats: (1) the innermost layer, or tunica intima, consists of an endothelial cell layer in contact with blood, a delicate subendothelial connective tissue layer, and an elastic tissue layer, the internal elastic membrane; (2) the middle coat, or tunica media, consists of smooth muscle fibers and variable amounts of elastic and collagenous tissues; and (3) the outer coat, or tunica adventitia, composed primarily of loose collagenous connective tissue (Plates 152 and 153). The exact structure and relative thickness of the three coats vary with the size of the artery. Be aware that there are regional structural variations.

In general, three different "types" of arterial vessels may be distinguished, but it must not be forgotten that these so-called types occur as part of a continuous, gradually changing, vascular morphology based on functional requirements. The three types are: (1) large elastic (conducting) arteries, which leave the heart and are continuous with (2) medium and small muscular (distributing) arteries, which join (3) arterioles, which are continuous with capillary vessels. From the structural/functional point of view, elastic tissue is the most important component in the larger vessels, whereas smooth muscle is the most important in the smaller vessels. Blood is ejected from the heart in a pulsating manner, and the aorta and the pulmonary arteries must expand to receive the bolus-type output (systole) of the right and left ventricles. The passive, elastic recoil between systoles (diastole) maintains the blood pressure, smooths the flow of blood, and forces blood through the coronary arteries while the ventricles are filling. Muscular (distributing) arteries (Plate 157) regulate the blood flow to different parts of the body depending upon need, that is, when exercising to skeletal muscles, and during and after dining to the gastrointestinal tract. Hence, the "old-saw," never engage in vigorous exercise after eating a hearty meal unless you wish to experience painful skeletal muscle cramps. Arterioles (Plates 154 and 155) are small arteries that vary in size from 0.02 to 0.3 mm in diameter, with one to five layers of smooth muscle fibers. Arterioles have a relatively thick muscular wall in comparison to their luminal diameter; the lumen of the smallest arterioles can accommodate about three to four red blood corpuscles. Arterioles determine local blood flow with their precapillary sphincters (Plate 158) located at the origin of capillary beds. Blood pressure falls sharply, and blood flow slows in arterioles.

Capillaries (Plates 157 and 159) are endothelial cell tubes whose walls appear as thin lines with bulging nuclei. Capillaries, because of their intimate relationship with the cells of the body and their special permeability characteristics, are functionally the most interesting of the blood vessels. Their thin walls and slow blood flow favor the exchange of nutrients and oxygen for metabolic wastes and carbon dioxide. In addition, hormones from endocrine glands enter and leave the vascular system through regionally specialized capillaries. The detailed "submicroscopic" structure of capillaries can be found in comprehensive textbooks of histology. In brief, four types of capillaries have been recognized based on endothelial structure and the presence or absence of a basal lamina (basement membrane). The four types are described as follows:

1. Non-fenestrated (continuous or somatic) capillaries. The endothelium does not have fenestrae or open channels. This most common type is found in connective, muscle, nervous, and endocrine tissues. Transport of macromolecules back and forth across the cell is via pinocytotic vesicles approximately 70 nm in diameter. These capillaries have a basal lamina. It is of interest that the capillaries of the central nervous system do not transport by this mechanism, because pinocytotic activity is greatly reduced or absent in these vessels (cf, the blood-brain barrier).
2. Fenestrated (visceral) capillaries possess large (60 to 80 nm in diameter) fenestrae or "openings," which are closed by a very thin diaphragm devoid of the typical trilaminar ultrastructure of other cellular membranes. These capillaries have a continuous basal lamina. The structural organization of these vessels is believed to favor rapid exchanges between blood and tissue spaces. This type of capillary is found in the kidney, intestine, and endocrine organs.
3. Fenestrated capillaries that do not possess diaphragms. They possess a thick basal lamina that separates the endothelium from the overlying epithelium (podocytes). These capillaries are type specific for the renal corpuscle (glomerulus).
4. Sinusoidal capillaries are enlarged capillaries about 30 to 40 µm in diameter. They also possess open fenestrations through their endothelium. This type of capillary normally has phagocytic cells attached to the endothelium within its lumen. The basal lamina characteristically is discontinuous. Sinusoids are found in the liver, spleen, and bone marrow.

Capillaries have several important functions, including (1) selective control of what is exchanged, at what rate, and what size between blood and tissue spaces; (2) production of "substances" that convert angiotensin I to angiotensin II and that can inactivate bradykinin, serotonin, prostaglandins, norepinephrine, and thrombin; (3) breakdown of lipoproteins to produce energy-yielding triglycerides and cholesterol used in membrane formation and hormone synthesis; and (4) production of arachidonic prostacyclin (prostaglandin I₂), a significant inhibitor of platelet aggregation (blood clot).

Pericytes are located along the external surface of capillaries and small venules. Because they possess contractile proteins (myosin and actin), it has been suggested that these cells are contractile and may assist the movement of blood through sluggish, non- or poorly contractile, small blood vessels.

As with the arterial system, veins can be divided into three "types" according to size: (1) venules, (2) small and medium-sized veins, and (3) large veins. Venules can be recognized when they are about 20 µm in diameter (about three red corpuscles across) and possess an endothelial lining, a thin layer of collagenous fibers with some fibroblasts. They have neither muscle fibers nor elastic fibers. With increasing size (about 45 µm), some elastic fibers appear in the tunica intima along with collagenous fibers, and smooth muscle begins to appear between the endothelium and the outer fibrous coats. With still greater increases in caliber, a distinct intima, media, and adventitia become recognizable. The largest veins possess some longitudinal smooth muscle and a delicate internal elastic membrane in the tunica intima; a thin smooth muscle coat (which may be absent) forms the tunica media; and prominent bundles of smooth muscle separated by collagenous fibers appear in the thickest of the three coats, the tunica adventitia (Plates 157 and 161).

Many small and medium-sized veins contain valves that prevent retrograde blood flow and the pooling of blood in the limbs where they are especially frequent. The erect posture of man, in particular, necessitates this structural specialization in veins. Valves are paired folds of intima, which are commonly located just distal to the entry of a communicating vein. Some veins do not possess smooth muscle fibers and, as a result, do not have a tunica media. These veins are found in the maternal part of the placenta, the spinal cord pia mater, retina, sinuses of the dura mater, most cerebral veins, trabecular veins of the spleen, and veins of the nail bed. Other interesting veins are found in the penis (Plate 160); these veins possess specializations of their intima called polsters. Polsters are local accumulations of fibroblasts and smooth muscle cells located beneath the endothelium that form conspicuous longitudinal thickenings or ridges. They are believed to play a role in retarding venous outflow during erection.

Blood flow against gravity and toward the heart in the thin-walled veins is aided by the contraction of skeletal muscle and the system of valves. Blood pressure in the venous system is less than one tenth that in the aorta, and blood travels slowly and smoothly through relatively large, thin-walled vessels. In spite of the differences in blood pressure and velocity of flow, the venous return to the heart must equal the ventricular output. The vascular system contains approximately 5 liters of blood, which is pumped and circulated throughout the body about 3200 times daily.

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