Blood is a mixture of cells and plasma. The heart pumps blood through arteries, capillaries and veins providing oxygen and nutrients to the body's cells. Blood also carries away waste products.  

An adult human body contains approximately five liters of blood. Blood makes up about 7 or 8 percent of a person's body weight. About 2.75 to 3 liters of blood is plasma. The remaining portion is cellular.  

Plasma is the liquid portion of blood. The blood cells float in the plasma. Several other materials are dissolved in plasma including electrolytes, nutrients, vitamins, hormones, clotting factors, and proteins such as albumin and immunoglobulins. Plasma distributes these substances as it circulates throughout the body.  

In the cellular portion of blood we find red blood cells, white blood cells and platelets. The red blood cells carry oxygen from the lungs T white blood cells help to fight infection. Platelets are used to clot blood. All blood cells are produced in the bone marrow. When we are children, most of our bodies bones produce blood. As we age fewer bones are producing blood until blood is only produced by the bones of the spine, breastbone, ribs, pelvis and parts of the upper arm and leg.