The cardiovascular system is composed of the heart, blood vessels and blood. In simple terms, its main functions are:

Blood is composed of plasma, an aqueous solution containing electrolytes, proteins and other molecules, in which cells are suspended. The cells comprise 40–45% of blood volume and are mainly erythrocytes, but also white blood cells and platelets. Blood volume is about 5.5 L in an ‘average’ 70-kg man.

Figure 1 illustrates the ‘plumbing’ of the cardiovascular system.

Blood is driven through the cardiovascular system by the heart, a muscular pump divided into left and right sides. Each side contains two chambers, an atrium and a ventricle, composed mainly of cardiac muscle cells. The thin-walled atria serve to fill or ‘prime’ the thick-walled ventricles, which when full constrict forcefully, creating a pressure head that drives the blood out into the body. Blood enters and leaves each chamber of the heart through separate one-way valves, which open and close reciprocally (i.e. one closes before the other opens) to ensure that flow is unidirectional.

Consider the flow of blood, starting with its exit from the left ventricle.

When the ventricles contract, the left ventricular internal pressure rises from 0 to 120 mmHg (atmospheric pressure = 0). As the pressure rises, the aortic valve opens and blood is expelled into the aorta, the first and largest artery of the systemic circulation. This period of ventricular contraction is termed systole. The maximal pressure during systole is called the systolic pressure

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