The science of life : fully illustrated in tone and line and including many diagrams

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

It is curious to note in passing a system of air spaces running into the bones of the face and communicating with the nasal cavity by means of small openings. Two such spaces may be seen in a_ skull. Apparently the purpose of these spaces is to lighten the architecture of the skull. Sometimes they may be invaded by bacteria through the openings into the nose, or, in the case of the large sinus in the upper jaw, from the root of a bad tooth, and inflammatory conditions may set in. Owing to their situation the treatment of such inflammations involves very difficult and painful operations.

We have said that the oxygen from the air reaches the blood by diffusion. It is dissolved in the plasma. But the amount the plasma could carry in a state of solution would not be enough for the energy-production of a mammal’s body. And here it is that the importance of the red blood corpuscle comes in. They are specialized oxygen-carriers. The haemoglobin, the red substance colouring the red_blood-cells, combines chemically with oxygen and so increases enormously the amount of oxygen that any given volume of blood can carry. The oxygen diffuses through into the plasma and the red corpuscles pick it up. The haemoglobin-oxygen compound is an unstable one ; it can exist only where there is abundant oxygen. If there is much oxygen about it will be formed, and if there is little it will dissociate. Hence haemoglobin combines very readily with oxygen in the lungs and gives it up very easily in the other tissues where there is a want of it.

As has already been said, it is to haemoglobin that blood owes its colour. The colour of oxygen-free haemoglobin differs from that of the haemoglobin-oxygen compound; the former is purple, the latter scarlet. This is why arterial blood is red while venous blood is purplish-blue. If you cut an artery the blood jets quickly because of the pulse of the heart and is scarlet from the outset. If you cut a vein the blood comes in a steady slow stream ; it is darker but brightens to red in the oxygen of the air. Anaemic people are people who suffer from a shortage of red blood corpuscles ; they are pallid and lack energy because of the insufficient supply of oxygen to the cell engines. “Red-blooded” is not a bad phrase for an energetic person whose tissues are all vividly alive.

In passing, let us note the structure of the cells involved in this respiratory exchange. As we have seen, the processes are very simple. The cells of the alveolar walls

and the red cells of the blood do not perform complex and varied functions ; they do not contract and relax, or secrete digestive juices, or transmit nervous impulses. Their duties involve no energy-consuming processes at all; the cells are merely water-containing structures through which dissolved gases diffuse. We find corresponding to this simplicity of function an extreme simplicity of structure. A human red-cell is little more than a minute bag containing haemoglobin, and so shaped that it has a very large available surface. A lining-cell of our lungs is little more than a thin, flat plate. They are the extreme of cell simplicity in our bodies.

A few cells with cilia, from the windpipe (greatly magnified).

§ 6 Kidneys and Other Exhaust Organs

The next round in our itinerary of the immensely complicated aggregation of living cell-mechanisms which constitutes a human being must be the organs that get rid of the exhaust products of billions of these internal combustion engines, and which also get rid of the waste matters resulting from their wear and tear. The substances constituting the former class are two—water and carbon dioxide. The members of the second class are more numerous. The most important is ammonia, but there are also other substances, such as uric acid, creatinine, and sulphuric acid, containing nitrogen, phosphorus, sulphur, and other elements in various forms. These substances are shed into the blood in small amounts only, and (with the exception of ammonia) their presence does not constitute any immediate danger

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Fig. 21.