General characteristics of the cardiovascular system

The cardiovascular system is a continuous, completely closed system of endothelial tubes. The general purpose of the cardiovascular system is the perfusion of capillary beds permeating all organs with fresh blood over a narrow range of hydrostatic pressures. Local functional demands determine the structural nature of the wall surrounding the endothelial tubes.

The circulation is divided into the systemic or peripheral circulation and the pulmonary circulation.

Arteries transport blood under high pressure and their muscular walls are thick (Figure 12-1). The veins are conduits for transport of the blood from tissues back to the heart. The pressure in the venous system is very low and the walls of the veins are thin.

Figure 12-1 Blood pressure and vascular anatomy

There are variations in blood pressure in various parts of the cardiovascular system (see Figure 12-1). Because the heart pumps blood continuously in a pulsatile fashion into the aorta, the pressure in the aorta is high (about 100 mm Hg) and the arterial pressure fluctuates between a systolic level of 120 mm Hg and a diastolic level of 80 mm Hg.

As the blood flows through the systemic circulation, its pressure reaches the lowest value (0 mm Hg) when it returns to the right atrium of the heart through the terminal vena cava. In the capillaries, the pressure is about 35 mm Hg at the arteriolar end and lower (10 mm Hg) at the venous end. Although the pressure in the pulmonary arteries is pulsatile, as in the aorta, the systolic pressure is less (about 25 mm Hg), and the diastolic pressure is 8 mm Hg. The pressure in the pulmonary capillaries is only 7 mm Hg, as compared with an average pressure of 17 mm Hg in the capillary bed of the systemic circulation.

Conductive system of the heart

The heart has two specialized conductive systems:

Figure 12-2 Heart: Purkinje fibers

When stretched, cardiac muscle cells of the atrium (atrial cardiocytes) secrete a peptide called atrial natriuretic factor (ANF) (Figure 12-3) that stimulates both diuresis and excretion of sodium in urine (natriuresis) by increasing the glomerular filtration rate. By this mechanism, the blood volume is reduced.

Figure 12-3 Atrial natriuretic factor

Histologically (see Figure 7-18 in Chapter 7, Muscle Tissue), individual cardiac muscle cells have a central nucleus and are linked to each other by intercalated disks. The presence of gap junctions in the longitudinal segment of the intercalated disks between connected cardiac muscle cells allows free diffusion of ions and the rapid spread of the action potential from cell to cell. The electrical resistance is low because gap junctions bypass the transverse components of the intercalated disk (fasciae adherentes and desmosomes).

Differences between cardiac muscle fibers and Purkinje fibers

The Purkinje fibers lie beneath the endocardium lining the two sides of the interventricular septum (see Figure 12-2). They can be distinguished from cardiac muscle fibers because they contain a reduced number of myofibrils located at the periphery of the fiber and the diameter of the fiber is larger. In addition, they give a positive reaction for acetylcholinesterase, and they contain abundant glycogen. Purkinje fibers lose these specific characteristics when they merge with cardiac muscle fibers. Like cardiac muscle fibers, Purkinje fibers are striated and are linked to each other by atypical intercalated disks.

Large elastic arteries are conducting vessels

The aorta and its largest branches (the brachiocephalic, common carotid, subclavian, and common iliac arteries) are elastic arteries (Figure 12-5). They are conducting arteries because they conduct blood from the heart to the medium-sized distributing arteries.

Figure 12-5 Structure of an elastic artery (aorta)

Large elastic arteries have two major characteristics: (1) They receive blood from the heart under high pressure. (2) They keep blood circulating continuously while the heart is pumping intermittently. Because they distend during systole and recoil during diastole, elastic arteries can sustain a continuous blood flow despite the intermittent pumping action of the heart.

The tunica intima of the elastic arteries consists of the endothelium and the subendothelial connective tissue.

Large amounts of fenestrated elastic sheaths are found in the tunica media, with bundles of smooth muscle cells permeating the narrow gaps between the elastic lamellae. Collagen fibers are present in all tunics, but especially in the adventitia. We have seen in Chapter 4, Connective Tissue, that smooth muscle cells can synthesize both elastic and collagen fibers. Blood vessels (vasa vasorum), nerves (nervi vasorum), and lymphatics can be recognized in the tunica adventitia of large elastic arteries.

Clinical significance: Aortic aneurysms

The two major types of aortic aneurysms are the syphilitic aneurysm (relatively rare because syphilis is no longer common) and the abdominal aneurysm. The latter is caused by a weakening of the aortic wall produced by atherosclerosis (see Figure 12-14). Aortic aneurysms generate murmurs caused by blood turbulence in the dilated aortic segment. A severe complication is rupture of the aneurysm followed by immediate death.

Marfan syndrome (see Chapter 4, Connective Tissue) is an autosomal dominant defect associated with aortic dissecting aneurysm and skeletal and ocular abnormalities due to mutations in the fibrillin 1 gene. Fibrillins are major components of the elastic fibers found in the aorta, periosteum, and suspensory ligament of the lens.

Medium-sized muscular arteries are distributing vessels

There is a gradual transition from large arteries, to medium-sized arteries, to small arteries and arterioles. Medium-sized arteries are distributing vessels, allowing a selective distribution of blood to different organs in response to functional needs. Examples of medium-sized arteries include the radial, tibial, popliteal, axillary, splenic, mesenteric, and intercostal arteries. The diameter of medium-sized muscular arteries is about 3 mm or greater.

The tunica intima consists of three layers: (1) the endothelium, (2) the sub-endothelium, and (3) the internal elastic lamina (see Figure 12-4).

The internal elastic lamina is a fenestrated band of elastic fibers that often shows folds in sections of fixed tissue owing to contraction of the smooth muscle cell layer (tunica media).

The tunica media shows a significant reduction in elastic components and an increase in smooth muscle fibers. In the larger vessels of this group, a fenestrated external elastic lamina can be seen at the junction of the tunica media and the adventitia.

Arterioles are resistance vessels

Arterioles are the final branches of the arterial system. Arterioles regulate the distribution of blood to different capillary beds by vasoconstriction and vasodilation in localized regions. Partial contraction (known as tone) of the vascular smooth muscle exists in arterioles. Arterioles are structurally adapted for vasoconstriction and vasodilation because their walls contain circularly arranged smooth muscle fibers. Arterioles are regarded as resistance vessels and are the major determinants of systemic blood pressure (Figure 12-6).

Figure 12-6 Arterioles: Resistance vessels

The diameter of arterioles and small arteries ranges from 20 to 130 μm. Because the lumen is small, these blood vessels can be closed down to generate high resistance to blood flow. The tunica intima has an endothelium, subendothelium, and internal elastic lamina. The tunica media consists of two to five concentric layers of smooth muscle cells. The tunica adventitia, or tunica externa, contains slight collagenous tissue, binding the vessel to its surroundings.

The segment beyond the arteriole proper is the metarteriole, the terminal branch of the arterial system. It consists of one layer of muscle cells, often discontinuous, and represents an important local regulator of blood flow.

Capillaries are exchange vessels

Capillaries are extremely thin tubes formed by a single layer of highly permeable endothelial cells surrounded by a basal lamina. The diameter range of a capillary is about 5 to 10 μm, large enough to accommodate one red blood cell, and thin enough (0.5 μm) for gas diffusion.

The microvascular bed, the site of the microcirculation (Figure 12-7), is composed of the terminal arteriole (and metarteriole), the capillary bed, and the postcapillary venules. The capillary bed consists of slightly large capillaries (called preferential or thoroughfare channels), where blood flow is continuous, and small capillaries, called the true capillaries, where blood flow is intermittent.

Figure 12-7 Microcirculation: Components and function

The amount of blood entering the microvascular bed is regulated by the contraction of smooth muscle fibers of the precapillary sphincters located where true capillaries arise from the arteriole or metarteriole. The capillary circulation can be bypassed by channels (through channels) connecting terminal arterioles to postcapillary venules.

When functional demands decrease, most precapillary sphincters are closed, forcing the flow of blood into thoroughfare channels. Arteriovenous shunts, or anastomoses, are direct connections between arterioles and postcapillary venules and bypass the microvascular bed.

The three-dimensional design of the microvasculature varies from organ to organ. The local conditions of the tissues (concentration of nutrients and metabolites and other substances) can control local blood flow in small portions of a tissue area.

Three types of capillaries: Continuous, fenestrated, and discontinuous

Three morphologic types of capillaries are recognized (Figures 12-8 and 12-9): continuous, fenestrated, and discontinuous (sinusoids).

Figure 12-8 Structure of capillaries

Figure 12-9 Types of capillaries

Continuous capillaries are lined by a complete simple squamous endothelium and a basal lamina. Pericytes can occur between the endothelium and the basal lamina. Pericytes are undifferentiated cells that resemble modified smooth muscle cells and are distributed at random intervals in close contact with the basal lamina. Endothelial cells are linked by tight junctions and transport fluids and solutes by caveolae and pinocytotic vesicles. Continuous capillaries occur in the brain, muscle, skin, thymus, and lungs.

Fenestrated capillaries have pores, or fenestrae, with or without diaphragms. Fenestrated capillaries with a diaphragm are found in intestines, endocrine glands, and around kidney tubules. Fenestrated capillaries without a diaphragm are characteristic of the renal glomerulus. In this particular case, the basal lamina constitutes an important permeability barrier, as we will analyze in Chapter 14, Urinary System.

Discontinuous capillaries are characterized by an incomplete endothelial lining and basal lamina, with gaps or holes