The average-sized adult has about 10 pints of blood, called "units" by blood banks. At rest, that entire quantity is pumped by the heart via the arteries to the lungs and all other tissues every minute. Almost half the volume of blood consists of cells, which include red blood cells, white blood cells and platelets. The remainder is a watery fluid called serum, which contains dissolved proteins, sugars, fats, minerals and other chemicals.
Trillions of red blood cells (erythrocytes) carry oxygen from the lungs to the tissues, where it's exchanged for the waste product carbon dioxide. Red blood cells get their color from the iron-containing protein called hemoglobin, which carts oxygen and carbon dioxide through the blood. The flexibility of red cells, which are thin in the center and thick at the edges, makes it possible for them to fit through even the narrowest blood vessel. Red cells survive about three to four months but are rapidly replaced.
White blood cells (leukocytes) play an important role in defending the body against infection by viruses, bacteria, fungi and parasites. The five types of white cells, which multiply as needed, can pass through the capillary walls to pursue invading organisms within the body's tissues.
Platelets (thrombocytes) are essential for arresting bleeding and repairing damaged blood vessels. They are formed in the bone marrow and survive for about 10 days in the blood.
Blood plasma is a straw-colored fluid, consisting mainly of water (95 percent) with a salt content similar to seawater. Plasma also serves to transport nutrients, waste products, proteins and hormones throughout the body.
Clotting is the process by which blood becomes solid. Clotting starts almost immediately at the site of a cut and helps limit blood loss by sealing damaged blood vessels. However, if abnormal clotting occurs in a major blood vessel, it may trigger a heart attack, stroke or other major disorder.
The clotting process has two main parts: platelet activation and the formation of fibrin filaments.
Platelets are activated by coming into contact with damaged blood vessel walls, where they become sticky and then clump together to block small holes. These clumps release chemicals that begin the process of clotting by forming filaments of fibrin at the site of injury. The fibrin filaments enmesh the platelets along with red and white blood cells. Once the cut blood vessel is plugged by the mass of fibrin, platelets and red and white blood cells, the fibrin filaments contract to form a solid clot.