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

THE GOMPLEX BODY-MACHINE AND HOW IT WORKS

because of the high pressure and swift return. Like the liver, the kidneys also censor the blood that passes them, but in

addition they remove undesirable molecules -

altogether from the blood-stream, instead of merely neutralizing them. The liver rebukes and neutralizes, but the kidneys expel. In addition to the blood-vessels, there is another tube leading out of each kidneythe ureter—down which there runs a continual trickle of water, carrying dissolved in it those substances which have been separated from the blood. This trickle of fuid—the urineaccumulates in the bladder, whence from time to time it is expelled.

It should not be thought that the only function of the kidneys is to weed out poisonous substances from the blood. ‘Their importance is very much more general than that. Any substance, even the most salutary, can be harmful if it is present in too great an amount, and the kidneys exercise a general standardizing influence on all the ingredients (except gases) of the blood-plasma which passes through them.

They regulate the proportions in which the various blood-constituents are present. As an example of this influence, we may consider the excretion of salt—for there is always a little salt in urine. We saw in a previous section that a certain amount of salt is a necessary ingredient of the fluid that bathes living tissue ; moreover, this amount must be accurately maintained, for either excess or lack injures the cells. The kidneys assist in regulating the saltconcentration of blood by varying the proportion of salt to water in the urine it produces. Suppose, for example, that after violent sweating (which involves loss of salt) we drink a lot of water, the blood becomes watery and contains too little salt. ‘To compensate for this deviation, the kidneys produce a large amount of urine which is practically pure water. On the other hand, during dry weather, as a result of water-evaporation from the lungs and skin, there may be too little water (and therefore too much salt) in the blood; under these circumstances, the kidneys produce a urine containing an unusually high proportion of salt. In this way the kidneys control many of the con-

stituents of circulating blood; if for any reason the amount of potassium, or sugar, or sulphate in the blood is abnormally high, the offending substance appears at once in theurine. The kidney is a bloodregulator, and removal of actual poisons is only one aspect of its regulating function.

Evidently the amount and composition of urine produced by the kidneys will vary

Vena cava Aorta Kidney

Ureter

Ly, wide

Fig. 22. The organs of excretion.

from hour to hour according to the everfluctuating condition of the body, depending upon the amount of water drunk, the amount of exercise taken, the size and nature of the last meal, the stage which its digestion has reached, and so on. On the average a healthy person voids about a litre and a half of urine per day, containing about thirty grams of urea (converted ammonia),