Just about everyone knows that we can't live without blood. And that the 10 pints or so of blood in a human adult are pumped by the heart through a network of arteries and veins.

But exactly what does blood do, where is blood made, and what's in it? Why does blood appear red when you cut yourself, but seem blue or purple when you look at the veins on your wrist? How does blood clot?

Those questions and more are explained in this Body Basics article about the mysterious, life-sustaining fluid called blood.

How Is Blood Important for Living?
Our survival depends on a steady supply of oxygen and fuel (especially blood sugar, or glucose) reaching all of our billions of body cells. At the same time, carbon dioxide and other waste materials produced by these cells have to be removed. As the heart pumps, it forces blood through a network of tube-like blood vessels that branch out through every part of our bodies.

The blood, the network of blood vessels, and the heart make up the circulatory system. By nourishing and supporting the body's other systems, the circulatory system plays a key role in sustaining life.

Here's how.

Our cells need oxygen, fuel, and other nutrients so that the body can function, grow, and repair itself. But much of the fuel and other nutrients that come from the foods we eat have to be processed first before going to our body cells. The digestive system does this by breaking down complex foods containing proteins, fats, carbohydrates (starches and sugars), vitamins, and minerals into simpler forms. Along with water, these nutrients are then absorbed through the intestines into the bloodstream where they are transported to cells throughout the body.

Blood also transports the oxygen our cells need to carry out their chemical processes. The respiratory system makes sure there is a supply of oxygen and a way to breathe out carbon dioxide. Carbon dioxide is a gaseous waste product created by the body's cells as part of their normal functioning. Red blood cells carry the oxygen to every tissue in our bodies. Blood also picks up the body's waste products (extra acids and other substances produced by chemical processes within the cells) and transports them to the kidneys and lungs where these waste products are released from the body.

But blood does a lot more than just carry oxygen, carbon dioxide, and nutrients. Blood plays an important role in the way the body fights infections and defends itself.

White blood cells fight infection-causing germs and other invaders of the body. White blood cells called granulocytes (pronounced: gran-yuh-low-sites) and lymphocytes (pronounced: lim-fuh-sites) travel along the walls of blood vessels, fighting germs such as bacteria and viruses. They also attempt to destroy cells that have become infected or have changed into cancer cells.

Certain types of lymphocytes produce antibodies, special proteins that recognize germs and foreign materials and help to stop them from causing damage. When a person has an infection, his white cell count usually becomes much higher than normal because his white blood cells multiply to battle the infection. After an infection has passed, lymphocytes "remember" how to make antibodies against the particular germ and will quickly attack it if it enters the body again.

Blood is a liquid - at least before it clots. Have you ever wondered why when we get cut all the blood doesn't drain out of our bodies? The reason is that the body has a natural defense mechanism against blood loss - substances in the blood form a seal, called a clot. Clotting is a complex series of chemical reactions in which blood forms patch-like lumps to close off leaks from wounds, cuts, and scratches, as well as bleeding inside our bodies.

If our blood didn't clot, we'd lose too much of it - even a simple cut could continue to bleed and make us sick. "Black-and-blue" marks (bruises) are the result of blood leaking from injured blood vessels under the skin. As bruises heal, substances in the blood are broken down by the body. The red or bluish color on the skin changes to green, brown, and yellow as the bruise fades and disappears.

When large blood vessels like arteries are torn or severed, the body may not be able to repair itself through clotting alone. In these cases, pressure bandages or stitches may be needed to help control bleeding.

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