The average adult has about 12 pints of blood in his/her body. Suspended in the blood are billions of tiny cells that can only be seen under a microscope. Most of these are red blood cells (also called erythrocytes) that contain hemoglobin. Hemoglobin carries oxygen from the lungs to tissues, such as muscle, where it is needed. Red blood cells take up about half the volume of the blood.
Blood also contains a large number of white blood cells (also called leukocytes), which do not have hemoglobin. Their job is to attack bacteria and viruses that invade the body.These white blood cells are often the ones that become cancerous.
Another type of blood cell is called a platelet. Platelets help to control bleeding and form clots.
The rest of the blood is straw-colored liquid called plasma. The plasma contains nutrients and hormones.
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