Porth's Essentials of Pathophysiology, 4e

422

Circulatory Function

U N I T 5

U N D E R S T A N D I N G  Determinants of The arterial blood pressure, which is the force that moves blood through the arterial system, reflects the intermittent contraction and relaxation of the left ventricle. It is determined by (1) the properties of the arterial system and the factors that maintain (2) the systolic and (3) the diastolic components of the blood pressure.These factors include the blood volume, elastic properties of the blood vessels, cardiac output, and peripheral vascular resistance.

1

Arterial Pressure Regulation. The arterial blood pressure represents the force that distributes blood throughout the cap- illaries of the body. The aorta and large arteries have large amounts of elastin in their walls that allow them to stretch and store energy during ventricular systole and recoil during diastole. The arterioles, which are the terminal components of the arterial system, serve as resistance vessels that regulate blood pressure by control- ling the distribution of blood to the cap- illary beds. The elastic properties of the aorta and large arteries, coupled with the high resistance properties of the arterioles, allow the arterial system to act as a filter that converts the intermittent flow gener- ated by the heart to virtually steady flow in the capillaries. The low-pressure venous system collects blood from the capillaries and returns it to the heart as a means of maintaining the cardiac output needed to sustain the arterial pressure. Systolic Pressure. The systolic blood pressure reflects the amount of blood (stroke volume) that is ejected from the heart with each beat, the rate and force with which it is ejected, and the elastic- ity or compliance of the aorta and large arteries. The blood that is ejected from the heart during systole does not move directly through the circulation. Instead, a substan- tial fraction of the stroke volume is stored in large arteries. Because the walls of these vessels are elastic, they can be stretched to accommodate a large volume of blood without an appreciable change in pressure. The systolic pressure often increases with aging as the aorta and large arteries lose their elasticity and become more rigid.

120

Systolic

80

Diastolic

40

Arterial pressure (mm Hg)

0

Aorta

Veins

Arteries

Venules

Arterioles

Capillaries

Vena cava

Peripheral resistance

2

Aorta

Systolic

240 220 200 180 160 140 120 100

250 230 210 170 150 130 110

Left atrium

90 70 50 30 10

80 60 40 20

Left ventricle