Joanna Bristow, age 9, of Thomasville, N.C., for her question:
HOW DOES BLOOD CIRCULATE?
About 55 to 65 percent of your blood is made up of a liquid material called plasma. The red and white blood cells and the platelets are solid substances that are actually suspended in the plasma. Although plasma is made up mostly of water, it also contains hundreds of other substances including proteins, digested food, waste products and minerals.
Blood is the fluid that circulates in the body through a vast network of blood vessels. It supplies the cells of the body with food and oxygen that is needed for work and growth, and it also carries waste products to special organs that remove them from the body.
The heart pumps blood that is rich in oxygen and food into the body's arteries. The arteries then carry the blood to all parts of the body. The blood then returns to the heart through the veins.
Tiny blood vessels called capillaries make up an exchange system for the blood. The vital fluid passes from the arteries to smaller arterioles and then into the capillaries.
Most capillaries are so minute that blood cells must pass through them in single file. Food and oxygen needed by the cells of the body ooze out through the thin capillary walls. Carbon dioxide and other waste materials from the body cells squeeze back through the walls into the blood stream.
From the capillaries the blood makes its way back to the heart by way of venules and veins.
Blood circulates through a vast network of vessels. If all of the blood vessels in your body could be laid end to end, they would total about 100,000 miles in length. That would be enough vessels to go around the earth at the equator about four times.
The heart pumps the blood which circulates through the body.
There are two sides to the heart, divided by a wall called the septum. Each side has two chambers, the upper called the atrium and the lower called ventricle.
Blood from the body returns through veins and goes into the right atrium. Moving through valves, it enters the right ventricle before it moves to the lungs. Oxygen laden blood from the lungs enters the heart through the left atrium and passes into the left ventricle on its way to the body.
The part of the circulatory system which carries blood from the heart to the lungs and back again is called pulmonary circulation. The system that carries the blood from the heart to the other parts of the body is called systemic circulation.
Portal circulation is a part of the systemic circulation that carries blood from the stomach, pancreas, small intestine and spleen to the liver.