Guards proved readily available. At the dawn of life all the cells of the organism were capable of capturing and digesting particles of food. At about the same time organisms provided themselves with mobile cells, very similar to modern amoebae. They did not sit idly by and wait for the flow of fluid to supply them with something palatable, but continuously sought their “daily bread”. These wandering hunters, which at the very outset waged war against microbes invading the organism, became known as leucocytes.

Leucocytes are the largest cells in man’s blood, varying in size from 8 to 20 microns. These white-smocked sanitarians in our organism continued for a long time to take an active part in digestion and they still perform that function in modern amphibians. It is not surprising that there are large numbers of leucocytes in lower animals. In one cubic millimetre of fish blood there are as many as 80 thousand leucocytes, ten times as many as in a healthy human.

It takes very many leucocytes to combat pathogenic microbes successfully, and the organism produces large quantities of them. However, it has proved very difficult to determine their life-span. The leucocytes are of course ‘warriors’ and as such they probably never live to old age but are killed off in battle, that is in the fight for health. This may explain why under different experimental conditions in different animals leucocytes possessed life-spans ranging from twenty-three minutes to fifteen days.

More accurate data have only been obtained for lympho­cytes, one of the types of white blood corpuscles in our body. Their life is ten to twelve hours, which means that the organism completely renews its stock of lymphocytes at least twice a day.

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.

At any rate, the highest nobility, the favourites of the King of Castile, an ancient kingdom in Central Spain which had overthrown the yoke of the Moors, claimed that the blood flowing in their veins was ‘blue’. This was meant to prove that they had never been related to the Moors, whose blood was considered to be darker in color. In fact, there are only some Crustacea which really have blue blood.

The waters of our internal ocean have all that the cells of the organism require. The tissue fluids of the lowest animals are, in composition, very much like common sea water. The higher the animal, the more complex the composition of its haemolymph and blood. The blood contains, besides salts, physiologically active substances, vitamins, hormones, proteins, fats and even sugars. Nowadays, birds’ blood is the sweetest, while that of fish contains the smallest amount of sugar.