Porth's Essentials of Pathophysiology, 4e

376

Circulatory Function

U N I T 5

vessels in the lungs, and the left heart, which propels blood through the vessels that supply all the other tis- sues in the body. Both sides of the heart are further divided into two chambers, an atrium and a ventricle . The atria function as collection chambers for blood returning to the heart and as auxiliary pumps that assist in filling the ventricles. The ventricles are the main pumping chambers of the heart. The right ventricle pumps blood through the pulmonary artery to the lungs and the left ventricle pumps blood through the aorta into the systemic circulation. The ventricular chambers of the right and left heart have inlet and outlet valves that act reciprocally (i.e., one set of valves is open while the other is closed) to control the direction of blood flow through the cardiac chambers and out into the arteries. Because the circulatory system is a closed system, both sides of the heart must pump the same amount of blood over time. If the output of the left heart were to fall below that of the right heart, blood would accu- mulate in the pulmonary circulation. Likewise, if the right heart were to pump less effectively than the left heart, blood would accumulate in the systemic circu- lation. However, the left and right heart seldom eject exactly the same amount of blood with each beat. This is because blood return to the heart is affected by activi- ties such as taking a deep breath or moving from the seated to standing position. These beat-by-beat varia- tions in stroke volume (amount of blood pumped with each beat) are accommodated by the large storage capa- bilities of the venous system that allow for temporary changes in blood volume. The accumulation of blood occurs only when the storage capacity of the venous sys- tem has been exceeded. Volume and Pressure Distribution Blood flow in the systemic circulatory system depends on a blood volume that is sufficient to fill the blood ves- sels in the systemic circulation and a pressure difference that provides the force needed to move blood forward. As shown in Figure 17-2, approximately 4% of the blood at any given time is in the left heart, 16% is in the arteries and arterioles, 4% is in the capillaries, 64% is in the venules and veins, and 4% is in the right heart. The arteries and arterioles, which have thick, elastic walls and function as a distribution system, have the highest pressure. The capillaries are small, thin-walled vessels that link the arterial and venous sides of the circulation. They serve as an exchange system where transfer of gases, nutrients, and wastes take place. Because of their small size and large surface area, the capillaries contain the smallest amount of blood. The venules and veins, which contain the largest amount of blood, are thin- walled, distensible vessels that function as a reservoir to collect blood from the capillaries and return it to the right heart. Because the pulmonary and systemic circulations are connected and function as a closed system, blood can be shifted from one circulation to the other.

Head and upper limbs

Systemic circuit

Lungs

Pulmonary circuit

Heart

Digestive tract

Systemic circuit

Kidneys

Trunk and lower limbs

FIGURE 17-1. Systemic and pulmonary circulations.The right side of the heart pumps blood to the lungs, and the left side of the heart pumps blood to the systemic circulation.

pulmonary artery is the only artery that carries deoxy- genated venous blood and the pulmonary veins are the only veins that carry oxygenated arterial blood. The systemic circulation consists of the left side of the heart, the aorta and its branches, the capillaries that sup- ply the brain and peripheral tissues, and the systemic venous system and the vena cava. The veins from the lower portion of the body merge to form the inferior vena cava and those from the head and upper extremi- ties form the superior vena cava, both of which empty into the right heart. Although the pulmonary and systemic circulations function similarly, they have some important differences. The pulmonary circulation, which is the smaller of the two, is located in the chest near the heart and func- tions as a low-pressure system (mean arterial pressure of approximately 12 mm Hg). This low pressure allows blood to move through the lungs more slowly, provid- ing more time for gas exchange. Because the systemic circulation must transport blood to distant parts of the body, often against the effects of gravity, it functions as a high-pressure system, with a mean arterial pressure of 90 to 100 mm Hg. The heart, which propels blood through the circula- tory system, consists of two pumps in series—the right heart, which propels blood through the gas exchange