Blood is composed of cells suspended in a liquid. These cells -- red cells, white cells and platelets -- account for 50% of the volume of blood. The remaining liquid portion is plasma.

Red cells are as important to life as breathing in and out. They transport oxygen throughout the body cells. The oxygen provides the fuel, or energy, for all the work your body does. Red cells need foods rich in iron, such as meat, liver, eggs, green leafy vegetables and whole-grain bread to make them healthy. Red cells are produced in the bone marrow at a normal rate of about 17 million cells per second.

White cells, also known as leukocytes, are the protective cells in the blood stream. They attack bacteria by squeezing through capillary walls to reach the area of infection where they destroy bacteria. White blood cells are also made in the bone marrow and are produced at twice the rate of red blood cells.

Platelets are colorless cells or fragments produced in the bone marrow. They control bleeding by helping to form a blood clot. Platelets also assure that blood vessels stay "leakproof" in daily life by acting like an internal band-aid.
Plasma contains minerals, proteins, sugars and hormones and is the liquid through which all blood cells "swim".