The Circulatory System, the Heart, and Blood - Blood cell manufacture and turnover
Most types of blood cells, both red and white, are manufactured in the red marrow of bones. The rate and quantity of total production is staggering; estimates range from one to five million red blood cells per second. This prodigious output is necessary because blood cells are disintegrating, having served their useful lives, in the same enormous numbers every second. The normal life span of a red blood cell is about four months, which means that four months from now every blood cell in your body will have died and been replaced with new cells.
The Liver As a Producer of Red Blood Cells
The red bone marrow is backed up by several other tissues that can, if called upon, turn out blood cells in quantity or serve as specialized producers of certain blood cells and blood elements. One such organ is the liver, which, before and after birth and into childhood, is a site of red blood cell production. In an emergency, such as severe internal hemorrhage, the liver sometimes reverts to its earlier function of manufacturing red cells. The liver also serves as a kind of salvage yard for the iron from dead red cells. It stores the iron for later combination into hemoglobin and passes off the rest of the red blood cell fragments as part of the bile pigments that empty into the duodenum of the small intestine.
The Spleen As a Producer of Blood Cells
Certain white blood cells, in particular the lymphocytes, are produced at a variety of locations in the body—for example, by the lymph nodes, by little clumps of tissue called Peyer's patches in the intestinal tract, and by the spleen.
The spleen plays a number of interesting secondary roles in blood cell production. Like the liver, it can be pressed into service as a manufacturer of red blood cells and serve as a salvage yard for iron reclaimed from worn-out red blood cells. A newborn baby is almost totally dependent on its spleen for the production of red blood cells, with a little help from the liver. In an adult, however, a damaged or diseased spleen can be surgically removed, with little or no apparent effect on the health or life span of the person, provided the patient's bone marrow is in good functioning order.
Movement of Blood Cells through the Capillaries
A blood cell must be able to slip through the microscopic, twisting and turning tunnels of the capillaries that mark the turn-about point in the blood cell's round-trip voyage from the heart. Blood cells, therefore, must be small (the point of a pin could hold dozens of red blood cells), and they must be jellylike in order to navigate the tight tortuous, capillary channels without either blocking the channel or breaking apart themselves. A red blood cell is further adapted to sneaking through the capillaries by its concave-disk shape, which allows it to bend and fold around itself. Nevertheless, so narrow are the passageways within some of the capillaries that blood cells must move through them in single file. If a substantial number of the cells are misshapen, as in sickle-cell anemia, they tend to move sluggishly or clog up the passageway—a condition that can have serious consequences. See also Ch. 9, Diseases of the Circulatory System .