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Red Blood Corpuscles

From the outset, nature went in for creating very large molecules, from ones with a molecular weight twice that of an atom of hydrogen, the lightest substance, to ones occasionally even ten million times greater. Such proteins cannot pass through the cell membranes. They ‘get stuck’ even in quite large pores, and this is why they are retained in the blood for a long time and used over and over again. In higher animals the problem was solved by haemo­globin, which has a molecular weight more than 16 thousand times that of hydrogen. Moreover, so that the haemoglobin does not pass into the surrounding tissues, it was placed in special containers, erythrocytes, which circulate in our blood stream.

In most animals these red blood corpuscles are round, but in some, for instance camels and llamas, they are sometimes oval, for which, as yet, no explanation has been found.

In early animals erythrocytes were large and cumbersome. In one extinct cave-dwelling amphibian they were 35 to 38 microns in diameter. In most amphibians they are much smaller, but occasionally may be as much as 1100 cubic microns in volume. This proved inconvenient since the larger the cell, the smaller, relatively, is the surface area through which the oxygen passes on both sides. There is too much haemoglobin per unit of surface area and this prevents it from working to the full. Once convinced of this, Nature set about decreasing the size of the erythro­cytes to 150 cubic microns for birds and 70 for mammals. In man they are 8 microns in diameter and 90 cubic microns in volume.

In many mammals the red blood corpuscles are even smaller. In goats they are barely 4, and in musk deer 2.5 microns in diameter. It is easy to understand why goats have such small erythrocytes. Domestic goats are descended from mountain animals, which live in a highly rarefied atmosphere. It is not without reason that they have a large number of erythrocytes, as many as 14.5 mil­lion per cubic millimetre of blood, while in amphibians whose metabolism is low there are only 40 to 170 thousand erythrocytes.

In order to reduce their volume, in vertebrates the red blood cells became flat discs, thus minimizing the depth to which the oxygen molecules diffuse in them. In man the disc is biconcave. The volume of the cell is thus reduced even more, and, at the same time, the surface area increased.

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