Porth's Essentials of Pathophysiology, 4e

394

Circulatory Function

U N I T 5

Artery

Vein

SUMMARY CONCEPTS (continued)

Tunica intima

The Systemic Circulation and Control of Blood Flow The systemic vascular system is a closed system of ves- sels that distributes blood from the heart to the tis- sues and returns blood to the heart. Its three divisions include the arterial system, which delivers blood to the tissues; the venous system, which returns blood to the heart; and the capillaries of the microcirculation, which separate the arterial and venous systems and is the site where nutrients and gas exchange take place. Blood Vessels All blood vessels, except the capillaries, have walls composed of three layers, or coats, called tunicae (Fig. 17-17). The outermost layer of a vessel, called the tunica externa or tunica adventitia , is composed pri- marily of loosely woven collagen fibers that protect the blood vessel and anchor it to the surrounding structures. The middle layer, the tunica media , is largely a smooth muscle layer that constricts to regulate and control the diameter of the vessel. Larger arteries have an external elastic lamina that separates the tunica media from the tunica externa. The innermost layer, the tunica intima , consists of a single layer of flattened endothelial cells with minimal underlying subendothelial connective tissue. The endothelial layer provides a smooth and ■■ The cardiac cycle, which describes the pumping action of the heart, is divided into two parts: systole, during which the ventricles contract and blood is ejected from the heart; and diastole, during which the ventricles relax and allow for filling to occur.The cardiac output, or amount of blood that the heart pumps each minute, represents the stroke volume, or amount of blood pumped with each beat; and the heart rate, the number of times the heart beats each minute. Cardiac reserve refers to the maximum percentage of increase in cardiac output that can be achieved above the normal resting level. ■■ The heart’s ability to increase its output according to body needs depends on: (1) the preload, or filling of the ventricles (i.e., end- diastolic volume); (2) the afterload, or resistance to ejection of blood from the heart; (3) cardiac contractility, which is determined by the interaction of the actin and myosin filaments of cardiac muscle fibers; and (4) the heart rate, which determines the frequency with which blood is ejected from the heart.

slippery inner surface for the vessel that prevents plate- let adherence and blood clotting. The layers of the different types of blood vessels vary with vessel function. The walls of the arterioles, which control blood pressure, have large amounts of smooth muscle. Veins are thin-walled, distensible, and collapsible vessels. Capillaries are single-cell–thick vessels designed for the exchange of gases, nutrients, and waste materials. Vascular smooth muscle cells, which form the pre- dominant cellular layer in the tunica media, produce vasoconstriction or dilation of blood vessels. Smooth muscle contracts slowly and generates high forces for long periods with low energy requirements; using only 1/10 to 1/300 the energy of skeletal muscle. These char- acteristics are important in blood vessels that must maintain their tone day in and day out. Compared with skeletal and cardiac muscle, smooth muscle has a less–well-developed sarcoplasmic reticu- lum for storing intracellular calcium, and it has very few fast sodium channels. Therefore, depolarization of smooth muscle relies largely on extracellular calcium, which enters through calcium channels in the muscle membrane. Sympathetic nervous system control of vas- cular smooth muscle tone occurs by way of receptor- activated opening and closing of the calcium channels. In general, α -adrenergic receptors are excitatory, in that they causing the channels to open and produce vaso- constriction, and β -adrenergic receptors are inhibitory, in that they causing the channels to close and produce vasodilation. Calcium channel–blocking drugs cause vasodilation by blocking calcium entry through the cal- cium channels. Smooth muscle contraction and relaxation also occur in response to local tissue factors such as lack of oxygen, increased hydrogen ion concentrations, and Tunica externa FIGURE 17-17. Medium-sized artery and vein, showing the relative thickness of the three layers. Tunica media Tunica externa Tunica media