[ 2008-01-25 15:53 ]
The blood flowing through thearteries,capillaries, andveinsof our bodies contains many different materials and cells, each with a different function.Plasma, the liquid portion of the blood, comprises more than half of the blood. Plasma is light yellow in color, and is thicker than water, because it contains many substances, in addition to the actual blood cells. These substances include proteins, antibodies that combat disease,fibrinogenthat helps blood clot,carbohydrates, fats, salts, and others.
Red blood cells, orcorpuscles, encased in blood vessels, color the blood. Since there are about 35 trillion of these tiny, round, flat discs circulating in one's body at any one time, their sheer number necessarily lends their red color to the blood.
As the young red blood cell matures, and takes on an adult form in the marrow of the bone, it loses its nucleus, and it increases its production ofhemoglobin. Hemoglobin is the red pigment, or color of blood, and contains iron, combined with protein.
When blood passes through the lungs, oxygen piggybacks on the hemoglobin of the red cells. From there, the red cells carry the oxygen through the arteries and the capillaries to all other cells of the body. Carbon dioxide from the body cells returns to the lungs through the veins in the same manner, by attaching to the hemoglobin.
Red blood cells have a life expectancy of approximately four months, before they are broken up, primarily in the spleen, and are replaced by new red blood cells. New cells are continuously generated to replace the old cells that have past their prime, and have been destroyed to make room for the younger generation.
Let's not forget that, in addition to red blood cells, we also have several types of white blood cells!
（来源：coolquiz.com 英语点津 Annabel 编辑）