Blood Circulation Routes

A. Each half of the heart (atrium and ventricle) functions as pump in either the pulmonary (right) or systemic (left) circuits.

B. Pulmonary Circuit

1. The path of blood flow: blood from the body tissues enters (via the superior and inferior vena cavae) the right atrium >>> tricuspid valve >>> right ventricle >>>right semilunar valve >>> pulmonary arteries >>> lungs >>> pulmonary veins >>> left atrium.

2. Blood returning from the body tissues is high in carbon dioxide and low in oxygen; these contents are reversed after passage through the lung capillaries.

C. Systemic Circuit

1. Blood moves from the left atrium >>> bicuspid valve >>> left ventricle >>>left semilunar valve >>> aorta >>> body tissues.

2. Capillary beds are of two types: in true capillaries (with blood flow controlled by precapillary sphincters) exchange of substances occurs, but in "thoroughfare channels" blood passes directly from arterioles to venules.

3. Usually a given volume of blood in either circuit passes through only one capillary bed; one exception is blood from the digestive tract (via the hepatic portal vein) that passes through the liver capillary beds before leaving via the hepatic vein to return to the general circulation.

D. Heartbeat: The Cardiac Cycle:

1. The cardiac cycle is a sequence of contraction (systole) and relaxation (diastole).

a. As the atria fill, the ventricles are relaxed.

b. Pressure of the blood in the atria forces the atrioventricular valves open, and the ventricles fill as the atria contract.

c. The ventricles contract, the atrioventricular valves close, and blood flows out through the semilunar valves.

2. The heart sound "lub" is made by the closing of the AV valves; the "dup" sound is the closure of the semilunar valves.

Pulmonary circuit

The systemic circuit

The cardiac cycle

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