Close to 50% of blood is made up of red cells, white cells and platelets, all of which incubate in the bones. Each of these life-sustaining agents begins as a stem cell, a sort of hematological embryo that idles in the marrow until ordered to develop into one of the three discrete blood cell types.
Red cells ferry oxygen throughout the body. Alerted by a chemical alarm from the kidneys, which continuously monitor the blood for dropping oxygen levels, a hormone called erythropoietin directs the marrow to build a fresh supply of red cells.
White blood cells protect the body from disease and infection by ingesting or ousting microscopic intruders. They are found in a number of forms and produced in a number of ways. For example, a hormone called granulopoietin signals the marrow to produce the white cells of the disease fighting variety. Foreign substance called antigens trigger the manufacture of the others that specialize in battling infection. As a precaution, the marrow houses a standby force of fully developed white cells. So vigilant is this reserve platoon that it may interpret even routine exercise as a sign of potential bodily injury and flood the system to tend to illusory wounds.
For legitimate emergencies the body also keeps on hand a supply of platelets, disk-shaped cells whose job is to facilitate clotting. Platelets mature from stem cells whenever the body nourishes them with the hormone thrombopoietin. Only a bit of the hormone is manufactured at a time, which prevents platelet production from running amuck. When the thrombopoietin is totally consumed, the platelet population stops growing.
Red cells, white cells and platelets are transported in the body by plasma, a thin liquid that makes up the other half of the blood. Though complex, plasma is only 10% organic substances. The remainder is water, a component which must always be present in precise concentrations. Should the blood become either too diluted or too viscous, the kidneys retain or excrete surplus water until the proper balance is reached.
A single blood cell will travel through its plasma medium for as many as 120 days until it is ultimately washed ashore in the spleen, the junkyard of the circulatory system. Although trillions of cells meet this fate daily, all are replaced as effortlessly as they are discarded. In fact, the cellular contents of the blood are entirely destroyed and renewed 300 times over the course of 80 year life span. But despite such upheaval, the river of life flows on.
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