Why is blood red?
Blood is red because hemoglobin, a protein in red blood cells that binds oxygen and carbon dioxide, contains chemical compounds called hemes, and a heme is a blood pigment that contains iron which is reddish in color. There are about thirty-five trillion red blood cells-tiny, round, flat disks-circulating in our blood at any one time-that is thirty-five followed by twelve zeros. And each red blood cell typically has more than 250 million hemoglobin molecules, each with four heme groups.
Blood is pumped by the heart and circulated around the body through blood vessels. Blood is bright red when the hemoglobin picks up oxygen in the lungs. The red blood cells carry the oxygen, bound to their hemoglobin, to the rest of the body through arteries and capillaries. Carbon dioxide from the body’s cells returns to the heart through capillaries and veins. The darker venous blood carries the carbon dioxide from he tissues to the lungs, which expel them.
Circulation of Blood
The blood coursing through our body’s plumbing of arteries, veins, and capillaries contains many different materials and cells. Plasma, the liquid part of blood, is a light yellow color, denser than water, and carries proteins, antibodies to fight diseases, and fibrinogen, which helps the blood clot. Plasma also has carbohydrates, fats, and salts. Young red blood cells mature in the marrow of the bone. Red blood cells have life expectancy of about four months. Then they are broken up in the spleen and replaced by new blood cells. New cells are constantly replacing old cells. Our blood also contains several types of white blood cells. When a germ infects the body, some white blood cells race to the scene and produce protective antibodies that overpower the germs, while other white blood cells surround and devour them.
The average adult has between eight and twelve pints, or four to six quarts of blood. If a person loses a significant portion of their blood supply, they go into shock and die. This can be prevented by transfusing blood from another person with a matching blood type. The first blood transfusion on record took place in 1665. Richard Lower of Oxford, England, took blood from one dog and put it in another dog. The known human-to-human blood transfusion took place in 1795 in Philadelphia.