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 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.