THE HEART

The heart consists of two pumps in series: one pump propels blood through the lungs for exchange of O₂ and CO₂ (the pulmonary circulation) and the other pump propels blood to all other tissues of the body (the systemic circulation). Flow of blood through the heart is one way (unidirectional). Unidirectional flow through the heart is achieved by the appropriate arrangement of flap valves. Although cardiac output is intermittent, continuous flow to body tissues (periphery) occurs by distention of the aorta and its branches during ventricular contraction (systole) and by elastic recoil of the walls of the large arteries with forward propulsion of the blood during ventricular relaxation (diastole).

THE CARDIOVASCULAR CIRCUIT

In the normal intact circulation the total volume of blood is constant, and an increase in the volume of blood in one area must be accompanied by a decrease in another. However, the distribution of blood circulating to the different regions of the body is determined by the output of the left ventricle and by the contractile state of the resistance vessels (arterioles) of these regions. The circulatory system is composed of conduits arranged in series and in parallel (Fig. 15-1). This arrangement, which is discussed in subsequent chapters, has important implications in terms of resistance, flow, and pressure in blood vessels.

Figure 15-1 Schematic diagram of the parallel and series arrangement of the vessels composing the circulatory system. The capillary beds are represented by thin lines connecting the arteries (on the right) with the veins (on the left). The crescent-shaped thickenings proximal to the capillary beds represent the arterioles (resistance vessels).

(Redrawn from Green HD: In Glasser O [ed]: Medical Physics, vol 1. Chicago, Year Book, 1944.)

Blood entering the right ventricle via the right atrium is pumped through the pulmonary arterial system at a mean pressure about one seventh that in the systemic arteries. The blood then passes through the lung capillaries, where CO₂ in the blood is released and O₂ is taken up. The O₂-rich blood returns via the pulmonary veins to the left atrium, where it is pumped from the ventricle to the periphery, thus completing the cycle.