Confining hemoglobin in red blood cells prevents its loss from the body by the kidney, and its seepage into the lymphatics and intercellular spaces. It also makes possible the use of a hemoglobin of relatively small molecular weight (64,000 Daltons) and size. The lobster, for example, has the respiratory protein hemocyanin in plasma solution. It is so big (M.W. = several million), however, that it does not leak out of the vascular compartment. Its oxygen carrying capacity is not nearly as great as is our hemoglobin, but this is acceptable since the lobster is poikilothermic (i.e. cold blooded).
Finally, keeping hemoglobin in erythrocytes provides enhanced central control of oxygen affinity, such as that by 2,3-diphosphoglycerate. Clearly there are many advantages to confining hemoglobin to erythrocytes.
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