12 CARDIOVASCULAR SYSTEM

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.

HEART

The heart is a folded endothelial tube whose wall is thickened to act as a regulated pump. The heart is the major determinant of systemic blood pressure.

The cardiac wall consists of three layers:

1. Endocardium, consisting of an endothelial lining and subendothelial connective tissue.

2. Myocardium, a functional syncytium of striated cardiac muscle fibers forming three major types of cardiac muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers.

3. Pericardium. The epicardium, the visceral layer of the pericardium, is a low-friction surface lined by a mesothelium in contact with the parietal pericardial space.

The heart is composed of two syncytia of muscle fibers: (1) the atrial syncytium, forming the walls of the two atria; and (2) the ventricular syncytium, forming the wall of the two ventricles. Atria and ventricles are separated by fibrous connective tissue surrounding the valvular openings between the atria and the ventricles.

Conductive system of the heart

The heart has two specialized conductive systems:

1. The sinus node, or sinoatrial (S-A) node, which generates impulses to cause rhythmic contractions of the cardiac muscle.

2. A specialized conductive system, consisting of the internodal pathway, which conducts the impulse from the S-A node to the atrioventricular (A-V) node); the A-V node, in which the atrial impulse is delayed before reaching the ventricles; the atrioventricular bundle, which conducts the impulse from the atria to the ventricles; and the left and right bundles of Purkinje fibers, which conduct the impulse to all parts of the ventricles (Figure 12-2).

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.

ARTERIES

Arteries conduct blood from the heart to the capillaries. They store some of the pumped blood during each cardiac systole to ensure continued flow through the capillaries during cardiac diastole.

Arteries are organized in three major tunics or layers (Figure 12-4):

1. The tunica intima is the innermost coat. It consists of an endothelial lining continuous with the endocardium, the inner lining of the heart; an intermediate layer of loose connective tissue, the subendothelium; and an external layer of elastic fibers, the internal elastic lamina.

2. The tunica media is the middle coat. It consists mainly of smooth muscle cells surrounded by a variable number of collagen fibers, extracellular matrix, and elastic sheaths with irregular gaps (fenestrated elastic membranes).

Figure 12-4 Structure of a muscular artery

Collagen fibers provide a supporting framework for smooth muscle cells and limit the distensibility of the wall of the vessel. Veins have a higher content of collagen than arteries.

3. The tunica externa, or adventitia, is the outer coat and consists mainly of connective tissue. An external elastic lamina can be seen separating the tunica media from the adventitia. The adventitia of large vessels (arteries and veins) contains small vessels (vasa vasorum) that penetrate the outer portion of the tunica media to supply oxygen and nutrients.

From the heart to the capillaries, arteries can be classified into three major groups: (1) large elastic arteries, (2) medium-sized muscular arteries (see Figure 12-4), and (3) small arteries and arterioles.

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.