DK Human Body: Circulation
The heart, BLOOD, and blood vessels make up the circulatory system, which supplies the body with oxygen and nutrients, removes waste, distributes heat, and fights disease.
The heart pumps blood continually around the circulatory system. Arteries and veins divide into ever finer branches as they reach throughout the body.
Arteries carry blood from the heart, while veins bring it back. Arteries have thicker walls than veins, to withstand the force of the blood pumping directly out of the heart.
About 98 percent of blood vessels are microscopically thin vessels, called capillaries, that form a network between arteries and veins. Their walls are only one cell thick, so that chemicals can pass between the blood and the body’s tissues.
A liquid tissue, blood consists of trillions of cells suspended in a watery liquid called plasma. Blood is the body’s transportation system, keeping all tissues and organs supplied with the chemicals needed for life and removing waste. Plasma makes up 55 percent of blood, and red cells around 44 percent. White cells account for less than 1 percent of blood.
Red blood cells pick up the life-giving gas oxygen in the lungs and release it throughout the body. A single drop of blood contains about five million of these tiny cells. Each cell is packed with the bright red protein hemoglobin, which binds with oxygen and then releases it where it is needed.
White blood cells destroy germs and damaged tissue. A drop of blood contains about 7,000 cells, and there are many different types. Some patrol the body like soldiers, swallowing germs. Others produce chemicals, called antibodies, that stick to germs and so make them easier to kill.
