The heart is a wonderfully reliable mechanical pump, beating about 100,000 times a day without our ever having to think about it. Its sole purpose is to circulate blood throughout the body. It takes oxygen-poor blood from the body and pumps it to the lungs, where carbon dioxide can be released and more oxygen absorbed. The heart then collects the oxygenated blood as it comes from the lungs and pumps it to every part of the body, delivering oxygen to the body's cells, which need it to survive. Along the way, blood also passes through the abdomen, past cells that line the intestines and stomach, where it picks up nutrients that it will distribute to other parts of the body.

As important as it is, the heart would be useless without blood to pump. Often called "the elixir of life," blood is the fluid that carries the precious oxygen and nutrients to each of our cells. The average-sized human body contains about six quarts of blood, which is circulated completely every one to three minutes. Blood is made up primarily (about 55 percent by volume) of an amber-colored liquid called plasma. This fluid is more than 90 percent water, but it also contains fibrinogen, a protein that helps blood clot, as well as nutrients, hormones, enzymes, antibodies, and the body's waste products.

One of the most important components of blood is the oxygen-carrying red blood cells. There are about 25 trillion of these cells in the entire body, and each one contains hundreds of millions of molecules of a substance called hemoglobin, which enables the cell to collect oxygen in the lungs and release it in the tissues. Red blood cells have a life span of only about four months and so must be replaced constantly. Your bone marrow produces replacement red blood cells at a rate of about 1.5 million every second.

Another important component of blood is the white blood cells. Although red blood cells outnumbered them by about 750 to 1, white blood cells are no less important. Their function is to defend against bacteria, viruses, and other foreign intruders. They do this by active consumption of the intruders -- a process called phagocytosis -- or by producing antibodies, which are proteins that kill or otherwise interfere with the intruder's normal functions. White blood cells are different from most cells in the human body in their ability to move throughout the body. They can travel against the flow of blood within the bloodstream and, despite their large size, can pass through the walls of capillaries and into damaged and diseased tissue in search of infection.

Platelets are a third important component of blood. These tiny disks are not actually cells but fragments of very large cells found in the bone marrow. When blood vessels are damaged, the platelets release a substance that initiates the process of blood clotting to inhibit the loss of blood.