The two upper chambers of the heart are the
A) atria.
B) septa.
C) ventricles.
D) bundle of His.

Question

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The heart consists of two side-by-side pumps, each divided into two chambers: two upper chambers called atria, and two lower chambers called ventricles. As venous blood returns to the heart from the body, it enters the right atrium, passes through the tricuspid valve, and with atrial contraction, enters the right ventricle. Heart valves prevent the blood from flowing backward. From the right ventricle, blood is pumped through the pulmonary valve and, with ventricular contraction, into the pulmonary arteries and on to the lungs. In the lungs, carbon dioxide is removed and oxygen is added to the blood. Freshly oxygenated blood then returns to the heart via the pulmonary veins. It enters the left atrium, moves through the mitral (bicuspid) valve with atrial contraction, and enters the left ventricle. As the left ventricle contracts, the blood is forced through the aortic valve, into the aorta, and on to the rest of the body (see Figures 10-3 and 10-4). This process is called the cardiac cycle (see Figure 10-5). Refer to Chapter 2 for fetal circulation.

The Two Upper Chambers of the Heart Are the