The most important part of any cell is its nucleus. If this is carefully removed (an ultramicroscopic operation within the power of modern scientists), then the denucleated cell, although still living, will become non-viable, its main functions will stop and metabolism will be drastically reduced. This is the very phenomenon which nature decided to make use of and deprived the adult erythrocytes of mammals of their nuclei. The main function of the erythrocytes is to act as containers for haemoglobin. This function is a passive one and could not be disturbed, whereas a decrease in metabolism is very conveniently followed by a sharp reduction in oxygen consumption.

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.

In lower animals, oxygen and all the other essential substances are merely dissolved in the fluid which circulates throughout their bodies. Higher animals evolved a special substance which not only readily combines with oxygen when it is plentiful, but parts with oxygen equally readily when it is scarce. Such remarkable properties have also been found in certain complex proteins whose molecule contains iron and copper. Hemocyanin, a protein containing copper, is blue; haemoglobin and similar proteins whose molecules contain iron are red.

A molecule of haemoglobin may be said to consist of protein proper and an iron-containing part. The latter is identical in all animals, but the protein-containing part has certain special features which enable even very closely related animals to be distinguished.

The blood contains everything that the cells of our body require. They simply remove what they need as the blood passes through the blood vessels. Only the oxygen-containing substance has to remain intact. If it is left in the tissues, broken down there and used for the body’s needs, difficulties arise in the transportation of oxygen.