General Definitions

The circulatory apparatus is a closed system filled with blood, consisting of the heart and blood vessels; it is the principal supply line between various organs and parts of the body. The circulation delivers fuel to the body, namely oxygen and other essential substances; it also removes carbon dioxide and other products of metabolism. Blood is circulated in two separate circuits: the smaller circuit is known as the lesser circulation or the pulmonary circulation ; and the larger circuit, the greater circulation or the systemic circulation . The former supplies the lungs, the latter all other organs of the body. Figure 1 is a diagrammatic outline of the circulation, showing the lesser circulation above the heart and the greater circulation below the heart.

The central organ of the circulatory system is the heart . It consists of two separate pumps, one for each circuit, simultaneously ejecting an equal quantity of blood into the greater and the lesser circulation. The heart is a muscular organ weighing approximately 300 gm, which contracts rhythmically about 70 times a minute and with each beat expels about 75 cc of blood into each circuit. Each system of blood vessels into which the blood is pumped consists of three parts: the arterial system , the capillary system , and the venous system , as shown in figure 1. The objective of the lesser circulation is to send blood through the vessels of the lungs and there to bring it into close contact with the air, so that oxygen can

Figure 1. General diagram of the circulation showing the
pulmonary circuit above the heart and the systemic circuit
below the heart. Oxygenated blood is shown in white
and deoxygenated blood in black.

be replenished and carbon dioxide removed; thus the blood in the pulmonary artery (leading to the lungs) has a low oxygen content and high carbon dioxide content, whereas blood returning from the lungs in the pulmonary veins has a high oxygen content and low carbon dioxide content. The objectives of the greater circulation are as follows: to deliver oxygen to the tissues; to pick up and deliver all nourishing substances, vitamins, hormones, and other vital compounds; to carry carbon dioxide, the "exhaust" of the tissues, to the right side of the heart and hence to the lungs for elimination; and to pick up other waste products and deliver them to the points of their excretion or elimination (kidneys, liver, etc.).

Oxygen is the most essential fuel for every tissue of the body. Its utilization is intimately connected with carbon dioxide, the principal waste product of tissues. Thus each cell in the body "breathes" by extracting oxygen from the blood and depositing carbon dioxide in its place. Blood destined for the tissues, fully saturated with oxygen and containing a lower quantity of carbon dioxide, is bright red. It is ordinarily referred to as arterial blood , as it is contained in the arteries of the greater circulation. Blood returning from the tissues has a lower oxygen content and is high in carbon dioxide; such blood, dark red in color, is termed venous blood , as it is contained in the veins of the greater circulation. It shines through the superficial veins under the skin, making them appear blue. It is clear from figure 1 that the terms "arterial blood" (indicated as white in the chart) and "venous blood" (indicated as black) apply only to the greater circulation. In the pulmonary circulation, as I have said, the role of arteries and veins is reversed. Figure 1 emphasizes the fact that the pulmonary circuit consists of a simple system of vessels supplying a single organ; the greater circulation, by contrast, consists of a great number of semiautonomous systems connected parallel to each other. Each organ of the body receives blood from arterial branches, which then divide into capillary branches. The actual exchange of all substances between the blood and tissues occurs within the capillary system.