MILLIONS of years ago, when the first living creatures made their appearance in this world, they were composed only of one single cell which extracted its nutriment and life-giving oxygen from the water in which it lived. In those far-off days, nothing could live outside the water which prevented it from drying up and dying and which provided a surrounding always constant in composition. As time went on and more living things developed, certain cells began to form a skin which prevented drying and enabled the organism to live in the air and to be independent of the water which previously nourished it.
Before this could happen, however, a substitute for water had to be found, and it was necessary that this substitute should bathe every cell in the body with its life-giving properties, so that each cell could continue to live. The substitute which the animals developed was blood, and the functions it serves are those which we have mentioned.
Blood is a fluid which, between the narrowest of limits, is always of the same composition; it has the peculiar property of being able to carry oxygen from the lungs and pass it on to all the living cells, no matter where they are situated, and it carries food substances for the cells, which enable them to carry on their various activities and to repair the wear and tear which work imposes on them. In addition, it carries the chemical messengers which we have discussed already, and
also certain substances which the body produces for its defence against various germs.
THE RED MILLIONS WHICH BRING LIFE TO THE TISSUES IF we place a drop of blood under the microscope, we shall find that floating in a straw-coloured fluid are millions upon millions of tiny cells. These cells can be divided into two great groups which serve quite different functions. The first group are the red cells, whose role is to carry oxygen from one part of the body to another. Each red-cell is a tiny circular disc, so small that its diameter is only seven-thousandths of a millimetre and its thickness only one-thousandth. The red-cells are so numerous that in one cubic millimetre of blood, which is a quantity considerably less than the size of a pin’s head, are no less than five million red-cells. If the number falls to three millions, we complain that we are suffering from anaemia.
The cell is composed of a thin membrane outside, with a sponge-work of the same material inside, holding in its meshes a solution of haemoglobin. This chemical substance is responsible for the oxygen-carrying properties of the red-cells. Haemoglobin has the property of combining with several substances if these are presented to it in sufficient quantity. At a given moment, it will take up large quantities of certain substances, if present in excess, and give them up when they are present only in small quantities in its surroundings.
Let us suppose that we have a single red-cell in one of the small arteries leading to the tissues. This red-cell is saturated with oxygen. When it reaches the tissues, its surroundings are nearly devoid of oxygen but highly impregnated with carbon dioxide, a gas which is produced by the tissues when they burn up food substances. The red-cell is forced to give up its oxygen, which is greedily seized and used by the tissues, and at the same time it takes up the carbon dioxide which the tissues have produced and must get rid of.
The red-cell then continues on its journey through the veins and heart and eventually reaches the lungs. Here, because the lungs are filled with air, the concentration of oxygen is high and that of the carbon dioxide is very low, so the red-cell exchanges its load of carbon dioxide for a further load of oxygen and goes upon its beneficent round once more. The function of the red-cell, therefore, is to provide the tissues with oxygen and to carry away their waste products.
TINY CELLS THAT ARE SCAVENGERS OF THE BLOOD IN each cubic millimetre of blood are about nine thousand white cells of several different kinds. The functions of some of these are quite unknown, but others have been seen to do their work under the microscope, and we know a great deal about them. One of the best-known, and one which is fairly typical of the others which do the same work, is known rather grandiosely as the Polymorphonuclear Neutrophil Leucocyte. This name describes it, for it is a white cell (Leucocyte) which takes up neutral dyes when stained (neutrophil) and its nucleus is usually divided up into several lobes and may take on many different shapes. It is known for short as a polymorph.
These cells behave in the blood and tissues exactly as does the single-celled amceba which lives in the water of our country ponds and is taken as the prototype of the commencement of animal life. If an amceba or a polymorph finds itself in close proximity to a small particle of matter, it will throw out long processes of its substance, surround the particle, engulf it and digest it. This procedure cannot be described as a voluntary one on the part of the cell; it is merely the reaction which is forced upon the cell by chemical substances exuding from the particle in question.
The amceba and the polymorph are not free agents in the matter, for they have only two courses open to them. Faced with a body which produces any reaction upon them, they will either move away from it if the substance repels them, or will engulf it, if it attracts them. It is the chemical composition of the body which determines their reaction and not any voluntary process on their own part.
The amceba engulfs a particle in its surroundings only for the purpose of supplying itself with food, but, while the polymorph may do this, its main function is to eat up and digest and destroy foreign bodies, germs and the like which might prove dangerous to the body in general. Very often the germs are so virulent that they may multiply inside the body of the polymorph instead of being digested, and in this case the cell will die in the perfojmance of its duty.
If polymorphs from the blood are placed on a microscope together with disease-producing germs, this process can be watched directly and a graphic record may be taken by the cinematograph which will show it up to perfection. The
polymorphs will engulf the bacteria and, under favourable circumstances, they will be seen to disappear under the influence of digestion. The polymorphs are thus the scavengers of the blood and represent the first line of defence of the body against disease-producing germs.
WHEN THE BLOOD CLOTS FOR OUR PROTECTION INCORPORATED in the blood are a third group of tiny bodies, smaller even than the red-cells, which are known as platelets. Their job is to assist in the formation of a clot, when this proves necessary. When small blood vessels are cut by accident, blood will pour out and, unless something is done to stop it, the bleeding will continue indefinitely until no more blood is left in the body. Clotting of the blood is the first defence which the body puts up to prevent severe bleeding after an accident, and it is also one of the ways in which the spread of infection is stopped in severe cases because, through it, the circulation is stopped in the infected area, with the result that the infecting organism dies and is cast off without the infection becoming generalised.
Clotting, therefore, is essentially a reaction to damage, and only this may start the processes which lead to clotting. How disastrous it would be if all the blood in our body were liable to clot for no apparent reason! Nature has arranged therefore that severe damage, and only this, shall lead to a clot forming in the vessels of the affected part. The whole chemical mechanism is a very complicated one and cannot be gone into fully here, but we may briefly say that three substances, including calcium, are always present in the circulating blood. When tissues are damaged and bleeding takes place, the platelets also become damaged and from the tissues and platelets is produced a substance known as thrombokinase which works upon the three chemicals we have mentioned until a clot is produced.
When a vessel is cut, clotting is the first defence against severe bleeding. Soon, however, cells begin to wander into the clot which becomes organised later into solid, immovable living fibrous tissue which ensures that no further bleeding can take place. What disastrous consequences can follow inability of the blood to clot can well be seen in the condition known as Haemophilia. This disease is a rarity and no one yet knows the exact cause of it. The essential trouble is that the blood will not clot in the normal time, with the
result that the unfortunate victim will bleed to death after the most trivial of injuries—for example, if a tooth is removed.
HOW THE TISSUES DEFEND AND REPAIR THEMSELVES IF the tissues are damaged in any way, either from violence or as the result of infection by genus, a definite train of events follows. First, as there is much work to be done by the cells in defending themselves, the blood supply must bt increased. This is accomplished by dilating the blood vessels which supply the infected part, the dilation itself being the result of a reflex which is set in motion at the site of damage.
After a time the circulation slows and large quantities of fluid from the blood escape into the tissues through the damaged vessel walls in order to dilute the poisons which the infection is producing and to carry substances which will destroy these poisons. At the same time, polymorphs actively make their way through the walls of the vessels and creep into the tissues, attracted by the chemical substances produced by germs and damaged tissues. Here they eat up and remove the germs and any tissues that may be dead. Later fibrous tissue grows into the wound which is thus replaced by a fibrous scar.
Everyone knows the signs of inflammation. These are heat, redness, swelling and pain. All these are explainable on the basis of the facts we have just mentioned; because more blood is arriving at the part, it becomes hot and red. Fluid exuding into the tissues makes it swell and stretches the delicate nerves which produce pain. Because the part is damaged, it ceases to work, and loss of function results. Thus an inflamed joint is not moved and an inflamed muscle will not contract. Rest is essential for repair, and this is automatically secured.
THE FORCES THAT KEEP US BREATHING
WE have seen how the respiration of each cell of the body is effected by the red-cells carried in the blood, and we have discussed in the section on Anatomy how the lungs expand and relax with the chest when breathing takes place. What we have not yet explained are the forces which keep breathing going on—why in fact we breathe at all. Situated in the lower part of the brain, just where the spinal cord joins it, are a number of nerve cells which control respira-
tion. These cells are connected by nerve fibres directly to the muscles which raise the ribs and to the diaphragm.
These cells, just like the heart, have the property of sending out at regular intervals impulses which travel down the nerves and activate the muscles. They exhibit, in fact, rhythm. Moreover, they are exquisitely sensitive to the composition of the blood which bathes them. We have seen that the blood takes up the waste products of combustion, namely carbon dioxide, from the tissues, and we have seen also that it carries a large quantity of oxygen picked up in the lungs. These two substances are the factors which control respiration, and they do it quite automatically. They control, increase and decrease, and even abolish the inherent rhythm of these cells which have been called the respiratory centre.
Primarily, the centre depends on the amount of carbon dioxide in the blood for the impulses it sends out to start breathing. Increase in the carbon dioxide will increase the number of impulses sent out in a given time and will increase the frequency of breathing, so that more carbon dioxide is washed out of the lungs and more will leave the blood. A decrease in the carbon dioxide of blood will result in depression of respiration, until the carbon dioxide increases once more sufficiently to stimulate the cells again.
A simple experiment that anyone who is sufficiently interested can do for himself will prove this. Breathe very deeply and at a rapid rate for, say, thirty breaths. You will then find that you will stop breathing and not recommence for quite a long while. The explanation is that by voluntary forced breathing, you have washed out so much carbon dioxide from your lungs and blood that your respiratory centre is no longer stimulated and ceases to function until the carbon dioxide content of the blood rises again to a sufficiently high level.
Oxygen has rather a different effect upon the centre, for when its blood content is low, it makes the centre more sensitive to the existing content of carbon dioxide, so that a deeper and more rapid breathing results. Lack of oxygen has only an indirect effect; carbon dioxide has a direct effect. When we climb a high mountain or go up in an aeroplane to a great height, we breathe automatically faster, for there is less oxygen in the air and therefore less in the blood, with the result that the respiratory centre is stimulated. It will thus be seen that the respiratory centre tends to keep the amount of oxygen and carbon dioxide in the blood always within the limits which
are best suited to the body. An increase or decrease in either will result in immediate and automatic compensation. Thus is the blood always kept properly aerated, and thus is the removal of some of its waste products automatically ensured. We have spoken so far only of the automatic control of breathing. We can, of course, regulate our breathing at will. The rib muscles, like any other muscles of their kind, are under the voluntary control of the brain. This mechanism, however, ensures that we shall always breathe, and enables us to breathe without having to devote our attention to it.
CLEARING OUT THE WASTE: THE KIDNEYS AT WORK THE purpose of the kidneys is to remove from the blood those waste products which have resulted from cell activity in all parts of the body; the kidneys remove and secrete the ‘ash ‘left over from the fires of life, which, if it accumulated, would clog the delicate mechanisms and eventually kill us owing to its poisonous effects. Urea and uric acid are the main substances which the kidney removes from the blood.
The tiny blood vessels which result from the division of the kidney arteries pass to an enormous number of structures which are known as glomeruli and which are situated only in the outer layers of the kidney. A glomerulus may be likened to a ball made of a very thin filtering membrane to which is attached a tube which opens into the inside of the ball. One side of the ball is deeply indented by a mass of capillaries which are coiled upon themselves and continually branch, forming a densely-packed network. Under the influence of the blood pressure a large volume of the fluid part of the blood containing the waste products is forced through the glomerular wall and filters through into the interior of the ball.
This fluid is, of course, identical with the blood fluid, and not only contains the waste products but also many substances which are valuable to the body and must be reabsorbed. The only substances which cannot get through the glomerular wall are the proteins contained in the blood fluid, for these are formed, as we have seen, of very large molecules and it is by virtue of their size that they are retained. The glomerular filtrate, therefore, is identical with the blood fluid except that it contains no proteins. This has been actually proved, for fluid has been drawn off under the microscope from a living glomerulus and has been analysed.
From the glomerulus the filtered fluid passes into the tube which leads out of it and here it starts on a long journey. The glomerulus is situated in the outer layers of the kidney and the first part of the tube remains too in this layer coiling and twisting upon itself amongst the glomeruli. Then it starts to move straight towards the centre of the kidney and having gone about half-way it turns back upon itself, forming a loop, and returns again to the outer layers where it becomes coiled and twisted for the second time. After this it turns again towards the kidney centre and joins up with many similar tubes which are each coming from a similar glomerulus and passes its contents eventually into the ureter.
During this long journey the glomerular filtrate becomes greatly changed. It is changed, in fact, from blood fluid into urine. All the substances which the body requires and must not waste are absorbed into the blood by the cells of the tubules through which it passes. At the same time an enormous volume of water is also reabsorbed by these cells so that the urine which leaves the kidney is greatly concentrated and contains only waste products and nothing which could be of value to the body. A great deal of work is therefore done by the cells lining these tubules, even if we consider only the water which has been reabsorbed. It has been estimated that, in the cat, if the glomeruli filter off twenty-four pints of blood fluid, twenty-three and four-fifths pints of water is reabsorbed and that only one-fifth of a pint of urine is produced.
Many people say that this simple account is not the whole story of how the kidney produces urine. They say that in addition to absorbing water and some solids the kidney tubules also actively secrete waste products into the fluid passing down them. Some even go so far as to state that the tubules only secrete and do not absorb water, and that it is this secretion only which concentrates the urine.
The problem of how the kidney works is one of the most vexed questions in physiology and perhaps more ink and paper have been wasted and more wordy battles have occurred on it than upon any other subject in the whole of physiology. We will leave the matter here and say only that there is good experimental evidence that both processes occur, and therefore that both parties are probably in a measure right. Anyhow it is not a matter of the greatest importance for we have made for ourselves a theory which, if not absolutely correct, is sufficient to explain the main workings of the kidney both in health and in disease, and from it many excellent methods of treating kidney diseases have been worked out.
SOME books TO READ NEXT
books about these two scientific subjects, Anatomy and Physiology, are of necessity inclined to be technical, but there are some works which may be tackled with interest and profit by the layman as well as by the medical student. Far and away the best work to read on Anatomy as a reference book is Gray’s Anatomy. It is, of course, technical, but is
profusely illustrated. Three books of great interest to the student of human anatomy are : Human Embryology and Morphology by Sir Arthur Keith, History of the Human Body by Wilder and F. Wood, Jones’s Man’s Place Among the Mammals. These three books, although containing a number of technicalities, are much more interesting than those dealing only with pure Human Anatomy.
Physiology, like so many sciences to-day, has many different divisions, and as many different books have been written about them. An interesting, simple and well-written book on the chemistry of the body is T. R. Parsons’ Elements of Biochemistry. Bainbridge and Metroes’ Textbook of Physiology is an elementary book which covers the whole field of Physiology, but being condensed makes somewhat difficult reading. For those who want to learn more about advanced Physiology Starling’s Physiology is the standard work.