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Human Body

31 30 vEIns And ArtErIEs two trees unite The fact that the blood circulates endlessly was not discovered until 1628 by the nononsense Englishman, William Harvey. The arterial tree from the left heart branches to supply the body with blood, carrying oxygen, food, and signals. Between the blood and tissues is the micromesh of capillaries, tiny tubes that a red cell can just squeeze through, where gases, molecules, and ions soak out through the thin capillary walls. Here the blood picks up cell produce, and waste, including CO2, before a twin tree converges into veins, returning old blue blood to the right heart. Blood cells are bred in the bone marrow. Red blood cells have no nucleus, being disposable discs that maximise surface area for gas exchange. The molecule that carries gases inside red cells, and gives blood its colour, is haemoglobin, whose four protein pigment subunits each contain one iron atom. Each subunit carries a fiery oxygen in its ironclad cage, and cleverly swaps O2 for CO2 according to local conditions in the tissues and lungs. Red cells are loaded with ATPgenerating systems that dont use oxygen, so dont use up their cargo. Unlike nearly all other cells, their outer surface is electronegative, attracting them to the cells they supply through the extracellular fluid. So, an electric current flows in your blood vessels, with the heart as the battery. Arteryvein pairs often run with a nerve, the three sharing a name. One red cell lasts about four months before it is chomped by a macrophage, in the spleen or liver, at the same rate as your marrow makes new red cells, which is about three million per second.
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