Ella McCardle, age 13, of Prescott, Ariz., for her question:
WHAT MAKES BLOOD CLOT?
A person would bleed to death from even a small cut if blood did not clot or coagulate. A variety of substances in blood called factors cause the clotting.
When the blood is circulating normally, the clotting factors are inactive. But if a blood vessel is damaged, a clot forms as a result of a chain reaction that involves platelets and clotting factors.
Clotting starts as platelets pile up in a damaged blood vessel. Platelets are dislike structures that develop in the marrow.
The platelets and the injured tissue of the blood vessel give off certain chemicals. These chemicals react with clotting factors in the plasma to form a substance called thromboplastin. Thromboplastin, along with other substances, changes the inactive blood chemical prothrombin to thrombin.
Thrombin, in turn, causes fibrinogen to form long, sticky threads of fibrin. The fibrin threads crisscross each other to form a "net" that catches and holds platelets and blood cells.
All the liquid is squeezed out, and a solid plug a clot ¬forms. A blood clot at the surface of the skin is called a scab.
Some people have blood that does not clot properly. This serious condition, called hemophilia, happens because of a defect in, or a shortage of, one of the clotting factors.
In other individuals, a clot may form in an undamaged blood vessel.. Such a clot, called a thrombus, may block the flow of blood completely.
A thrombus cuts off the supply of food and oxygen to the tissues nourished by the vessel. A serious heart condition called coronary thrombosis occurs when a thrombus blocks the arteries that nourish the heart muscle.
Blood is the life stream of the human body. No part of the body can live without it.
The amount of blood in a person's body depends on the size and the environment of the individual. An adult who weighs 160 pounds has about five quarts of blood.
An 80 pound child has about two and a half quarts of blood in his body while a 10 ounce infant has only about 10 ounces.
People who live in high altitudes, where there is less oxygen, may have up to two quarts more blood than those who live in low regions. This extra blood helps capture the oxygen needed by the body.
The heart pumps the blood through the body. Blood flows from the heart through the arteries and it returns to the heart through the veins.
The large arteries that leave the heart carry blood into smaller and smaller vessels. The smallest of these vessels are the narrow, thin walled capillaries. Oxygen, food and wastes pass between the blood and the body cells through the capillary walls.