The cardiovascular system and the blood contained in its vessels are the body’s transport system. The heart pumps blood around two circuits of blood vessels. The main (systemic) circuit carries blood that contains oxygen, vital nutrients, and hormones to every cell. The second (pulmonary) circuit takes blood to the lungs, where oxygen is absorbed and the waste product carbon dioxide is eliminated. Other waste products are taken to the liver for processing and finally eliminated by the kidneys.
In this section
The heart pumps the body’s total volume of blood (about 5 litres or 9 pints) around the entire body about once a minute. In the systemic circulation, blood containing oxygen and vital nutrients is pumped to tissues and organs through blood vessels called arteries. Body cells absorb the oxygen and nutrients, while the blood absorbs waste products from the cells before returning to the heart through blood vessels called veins. The deoxygenated blood is then pumped to the lungs in the pulmonary circulation. After oxygen has been absorbed and carbon dioxide eliminated, the blood returns to the heart. During exercise, the rate of circulation may increase several times to meet the body’s demand for oxygen. The blood supply to some muscles may increase twelvefold while that to the digestive system falls by a third.
The heart and its chambers
About the size of a clenched fist, the heart is a muscular organ that lies in the centre of the chest, slightly to the left. It is divided into two halves, each of which contains an upper chamber (the atrium) and a lower chamber (the ventricle). The atria collect blood from various parts of the body, while the ventricles pump blood out of the heart. Each of the four chambers is joined to one or more blood vessels. The largest of these vessels, the aorta, is about the diameter of a garden hose. Forceful contractions of the ventricles pump blood out of the heart about 70 times per minute at rest. This pumping rate is called the heart rate and is measured in beats per minute. With each beat, a pressure wave travels along the arteries, causing their walls to expand. This wave, or pulse, can be felt where the arteries are close to the skin’s surface.
Heart muscle (myocardium) must work continuously for 24 hours a day without rest. Therefore, myocardial cells contain more and larger energy-producing units (mitochondria) than other types of body cell.
Heart rate and blood pressure
Heart rate is regulated by electrical impulses from the heart’s pacemaker, the sinoatrial node, which is a small area of nervous tissue in the wall of the right atrium. Each impulse causes a rapid sequence of contractions, first in the atria and then in the ventricles, that corresponds to one heartbeat.
Blood pressure depends on the rate and force of the heart’s contractions, the volume of blood pumped out, and the resistance to blood flow in the blood vessels, which varies with their size. Heart rate and blood pressure are controlled by the nervous system in the short term and by hormones, which act over a longer period.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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