Page 16, Turbulence & Rheology, Dr. D. Penney

Circulatory Design (continued..)

First, transporting hemoglobin in erythrocytes provides non-Newtonian characteristics to the blood, conferring on it the advantages discussed earlier. Keeping hemoglobin confined in the erythrocyte means that it adds little to the osmotic pressure of the plasma. If hemoglobin were dissolved in the plasma, as is the case for albumin, the osmotic pressure would be much greater and a much greater transmural (blood) pressure would be required to offset its tendency to pull water from the interstitial space. This of course would require greater work by the heart, and hence a larger heart.

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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