Aorta

The body contains three types of muscles, one of which is cardiac muscle, or myocardium. The heart is mainly composed of myocardium, which has unique properties that make it able to meet the demands placed on it.

Unlike other types of muscle, myocardial muscle is relatively fatigue-resistant. In an adult, the heart beats about seventy times a minute, or more than one hundred thousand times in a single day. Incredibly, it never gets tired. The heart pumps about five to seven quarts of blood a minute. Over a lifetime, the heart of a person at rest pumps enough fluid to fill a supertanker ship with a million barrels. Since the heart pumps much more blood when a person is active, the actual amount of fluid pumped during a life would be even greater.

through the pulmonary valve into the pulmonary artery. Pulmonary means "lung related," and arteries are responsible for carrying blood away from the heart. The pulmonary artery carries blood into the lungs. The pulmonary arteries are a unique element of the circulatory system because the pulmonary arteries carry un-oxygenated blood, whereas the rest of our arteries carry oxygenated blood.

In the lung, carbon dioxide molecules are given off by the red blood cells. These tiny molecules travel through the capillary wall into small air sacs called alveoli (Fig. 2.4). In turn, the oxygen that we breathe in moves through the alveoli wall and is taken up by the blood. The newly oxygenated blood next passes into the pulmonary veins, which carry the oxygenated blood back to the heart. This oxygenated blood is bright red.

Pulmonary Artery

Pulmonary Artery

Pulmonary Vein

Alveolus

Pulmonary Valve

Aortic Valve

Capillary

Alveolus

Pulmonary Valve

Aortic Valve

The four heart valves direct the flow of blood through the heart and into the major arteries and veins. They are shown here in two phases of the cardiac cycle.

Pulmonary Vein

Capillary

Oxygen transfer takes place in the lungs through the thin membranes of the alveoli. When the blood vessels release carbon dioxide, it passes back through the alveoli and is exhaled during respiration.

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