The heart and the blood vessels provide the body with a pump and a closed system of about 100,000 miles (160,000 kilometers) of tubing through which some 5 quarts (4.7 liters) of blood circulate continuously. In this way, blood reaches every living tissue, bringing oxygen and nutrients from lungs and digestive tract, respectively, and removing wastes to be disposed of by the lungs and kidneys.
The heart is a fist-sized bag of tough-walled cardiac muscle protectively encased by the thin, tough tissue of the pericardium, and shaped roughly like a pear. It lies in the chest cavity, slightly to the left of center.
The heart is an electrically-activated pump, contracting rhythmically to keep blood coursing through the arteries and veins. More precisely, it works as two connected pumps, separated by a muscular wall, the septum. Each side contains an upper and lower chamber called the atrium (or auricle) and the ventricle, respectively. The two thin-walled atria receive blood from the veins, and the two thick-walled ventricles pump this blood away through arteries. One-way valves control this blood flow through the heart.
The heart’s pump action works in the following sequence. Oxygenated blood from the lungs fills the left atrium. The atrium contracts, driving blood past the mitral valve into the left ventricle. Then the ventricle contracts, and the blood pressure inside it shuts the mitral valve, forcing blood out through the aortic valve (also called the semilunar valve) to that huge artery, the aorta, which supplies oxygenated blood to all parts of the body. Meanwhile, deoxygenated blood from body tissues fills the right atrium. It contracts, driving blood past the tricuspid valve into the right ventricle, which then contracts. The resulting pressure shuts the tricuspid valve and forces blood out via the pulmonary artery to the lungs, where it is reoxygenated.
Corresponding valves of the heart relax and fill the chambers simultaneously, then contract and empty a fraction of a second later. The brief phase of ventricular relaxation is called diastole, the contracting phase, systole. Each systole can be felt as a heartbeat. Each normal beat expels one-third of a pint (17 centiliters) of blood. In one day, an adult’s heart may pump out altogether 3,475 gallons (13,637 liters). The heart beats about 70 times a minute in resting individuals, but the rate can double if activity demands a greatly increased oxygen supply.
Although cardiac muscle tends to contract rhythmically, coordination comes from the sinoatrial node (also known as the pacemaker), an area of special tissue in the wall of the right atrium. Impulses from this node control the rate of contraction of the heart muscles. The actual impulse for cardiac contraction is transferred from the atria to the ventricles by the atrioventricular node, located between the atria and ventricles.
Arteries and veins
Blood pumped out from the heart travels under pressure through large, strong-walled arteries. The aorta, the largest artery, lies deep inside the body, but you can feel blood pulsing through other arteries such as the radial artery in the wrist and the carotid artery in the neck as their elastic walls expand with each heartbeat.
Branching off from the main arteries are smaller arteries, which in turn branch into minute arterioles, about a tenth of a millimeter or less in diameter. These have a simpler structure than arteries, but have some smooth muscle and elastic tissue similar to the internal elastic lamina of the larger vessels. Arterioles expand or contract, allowing more or less blood to pass as the autonomic nervous system dictates changes in the tissues’ need for blood.
Arterioles themselves give rise to even smaller tubes capillaries, with walls just one cell thick. These microscopic tubules infiltrate all parts of the body, bringing blood to every living tissue.
When the tissues have absorbed oxygen and nutrients from the blood, and have given up waste products and carbon dioxide to it, blood begins its return journey to the heart. First it passes through venules tubules made of endothelium with muscle fiber and connective tissue. These lead on to the larger, stronger veins. By now, the pressure pushing blood along is very weak, so many of the larger veins have valves to combat the effects of gravity. In the legs, contracting calf muscles provide extra pumps that squeeze the veins, thus driving blood on up toward the heart.