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Sanitarians of Leucocytes

Blood is not merely a means of transportation, it per­forms other important functions. Flowing through the body’s vessels, the blood in the lungs and intestines comes into close contact with the environment. Both the lungs and especially the intestines are the ‘dirtiest’ places in the organism. It is no wonder that in these sites bacteria can easily penetrate the blood. Why should this not happen? Blood is an excellent nutrient medium rich in oxygen. Were it not for the watchful and merciless guards at the gate, the course of the organism’s life would turn into the path of its death.

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.

Containers for Haemoglobin

It is very convenient to transport haemoglobin in a spe­cial container inside the erythrocytes, but, as the saying goes, every cloud has a silver lining. The erythrocyte, being a living cell, does itself consume a great deal of oxygen. Nature hates wastefulness and had to think hard of a way of cutting down this unnecessary expenditure.

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.

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.

Blood’s Main Function

The main function of the blood is transportation. It carries warmth all over the body, takes nutrients from the intestine and oxygen from the lungs and delivers them where necessary.

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.

Color and Taste of Blood

The shores of our own ‘personal ocean’ are washed by waves, which are not blue but scarlet. However, the venous blood saturated with carbon dioxide and other products of metabolism has a bluish tint, a fact which seems to have been known as early as the 11th century.

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.