This chapter reviews the normal structure and function of the vascular component of the cardiovascular system and then considers the pathophysiology of three common conditions frequently seen by practicing physicians: atherosclerosis, hypertension, and shock.
Normal Vascular Structure & Function
The blood vessels are a closed system of conduits that carry blood from the heart to the tissues and back to the heart. All of the blood flows through the lungs, but the systemic circulation is made up of many different circuits in parallel (Figure 11–1). This permits wide variation in regional systemic blood flow without changing the total systemic flow.
Diagram of the circulation in the adult. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 24th ed. McGraw-Hill, 2012).
The characteristics of the various types of blood vessels in humans are summarized in Figure 11–2. Note that as the diameter of the vessels decreases, their number in the body increases so that the total cross-sectional area increases.
Characteristics of systemic blood vessels. Cross sections of the vessels are not drawn to scale because of the huge range in size from aorta and vena cava to capillaries. (Redrawn from Burton AC. Relation of structure to function of the tissues of the wall of blood vessels. Physiol Rev. 1954;34:619.)
All the blood vessels are lined by a single layer of endothelial cells. Collectively, the endothelial cells constitute a remarkable organ that secretes substances that affect the diameter of the vessels and provide for their growth, their repair when injured, and the formation of new vessels that carry blood to growing tissues.
The aorta, the large arteries, and the arterioles are made up of an outer layer of connective tissue, the adventitia; a middle layer of smooth muscle, the media; and an inner layer, the intima, containing the layer of endothelial cells and some subendothelial connective tissue. The walls of the aorta and the large arteries contain abundant elastic tissue, much of it concentrated in the internal elastic lamina, a prominent band between the intima and the media, and another band, the external elastic lamina, between the media and the adventitia (Figure 11–3). The vessels are stretched by the force of cardiac ejection during systole, and the elastic tissue permits them to recoil during diastole. This maintains diastolic pressure and aids the forward motion of the blood. The walls of the arterioles contain less elastic tissue than the arteries but proportionately more smooth muscle (Figure 11–2). The muscle is extensively innervated by noradrenergic nerve fibers, ...