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

BOOK 1

to the living body. Nevertheless, if through any derangement of the usual devices for their removal they accumulate in the system, quietly and imnsidiously they work evil, stiffening the connective tissues, hardening the arteries, and producing subtle disorders of nutrition. If, for example, there is difficulty in getting rid of uric acid, it collects in the joints, giving rise in course of time to the stabbing pains of gout. Mr. Everyman as he gets on in life is apt to consume a considerable amount of trustworthy and untrustworthy solvents for his uric acid. If the removal of other waste products is inefficient, there may be such symptoms as headaches, nausea, and vomiting. In general, the results of defective elimination are chronic and cumulative ; they represent, not an immediate disaster, but a slow fouling of the living machine.

Excretion is the word used to express the removal of these substances. Carbon dioxide can exist in gaseous form and is therefore excreted by the lungs. It diffuses into the alveoli as oxygen diffuses into the blood. The other substances, however, have to be expelled in watery solutions. The principal agent in this process are the kidneys and liver, although other parts of the body take minor shares. The wise physician watches for clogging of the kidneys and liver as the wise chauffeur watches for carbonisation in his cylinders. The kidneys are chemical separators ; blood reaches them with various undesirable matters dissolved in it and it flows away cleansed, while the impurities leave along a different channel, which guides them safely out of the body. The liver is a chemical accomplice of the kidneys; it performs certain operations on the waste products in the blood, and converts them into other substances with which the kidney can more readily deal.

The duties of the liver are many ; it is an organ to which we shall constantly refer in this and subsequent sections. From our present point of view, its most important function is concerned with the ammonia produced by active tissues. Ammonia is a definitely harmful substance, and has a convulsant action on nerve-centres ;_ there is, therefore, danger in the ammonia. which living cells are continually shedding into the blood. It is interesting to note that every living cell in us is doing its best to poison us as a whole and has to be specially restrained. It so happens that as blood flows through the liver the ammonia is converted into urea, a comparatively harmless substance, and in this way its sting is re-

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THE SCIENCE OF LIFE

CHAPTER 2

moved. This process becomes particularly important just after a meal, for, as we shall see, the digestion of the nitrogenous substances known as proteins involves the entry of large quantities of ammonia into the blood. Since the whole of the blood from the intestines flows directly along a special system of veins (portal veins) to the liver, the ammonia is all dealt with before it can reach and injure any other part of the body. If it were not for this protection, the most serious disturbances would occur, for during the digestion of a heavy meal of meat the amount of ammonia produced would be quite enough to derange the nervous system and throw us into convulsions. Besides ammonia, there are other dangerous products which are made harmless by the liver. For example, bacteria in the intestines produce two foul-smelling poisons called indol and skatol, which find their way in small quantities into the portal blood ; but here again the liver intervenes and converts them, before they can reach the general circulation, into harmless substances that the kidneys can remove. ‘The liver may be imagined as a chemical censor of the blood that leaves the intestines, detecting such poisons as result from the digestive process. It does not actually expel them : it modifies them and then returns the disarmed products into the blood stream for the kidneys to expel at their convenience. It has been called the Ellis Island of the body. It arrests and it marks the undesirable immigrant for potential deportation, it manacles him, but it does not actually remove him.

Our kidneys are much the same shape as those of a sheep that figure on the breakfasttable ; they are about four and a half inches long and lie just in front of the veriebral column in the small of the back. Their blood-supply is copious ; they receive blood by a short branch from the aorta; the great main artery of the body, and return it to the main venous trunk, the posterior vena cava. Since, as we have already pointed out, the pressure in the aorta is higher than that in any other blood vessel in the body, and that in the vena cava lower, the head of pressure that drives blood through the kidneys is enormous, and the current through them is full and swift. Nevertheless, what they are doing is a work by no means so urgent as the oxygenation of the blood by the lungs, and it suffices that at each heartbeat only a part of the arterial blood goes through them. But it is a large part