The position which the kidney occupies in the circle of the great physiological systems gives a special character to its diseases, and to the actions and uses of remedies in connection with it. The series of vital processes which commences with the admission of food, air, and medicines, ends chiefly with the excretion of urine. Digestion, assimilation, sanguification, metabolism, circulation, and respiration, all, therefore, affect the activity of the kidney. This is chiefly due to the fact that the kidney does not itself form the urea, uric acid, pigments, salts, and water which form the hulk of the urine -that these bodies reach it by the blood, and it has but to sweep them from the circulation. This dependent position of the kidney is of great interest to the practical therapeutist. Clinically, the condition of the urine is a key to the manner in which the various viscera are discharging their functions; pathologically, we often find in other organs the cause of renal disease; and pharmacologically, we discover that if we wish to affect the composition of the urine and the activity of the kidney, we must, in many cases, direct our measures to the digestive organs, the heart and the vessels.

Conversely, the kidney makes its influence felt backwards upon the other organs. Disturbance of the renal function quickly tells upon the blood and viscera. We saw this under the heads of the liver and metabolism, and noted how quickly the retention of waste products checked functional activity, like ashes choking out a fire. As striking a relation exists between the kidney and the organs of circulation. Thus the practitioner, adopting the inverse order of investigation, estimates the condition of the kidney by the pulse, bowels, and appetite; the pathologist finds in the enlarged heart and ruptured vessels of the brain the outcome of disease of the renal glomeruli; and the pharmacologist relieves the blood pressure or the liver by measures directed to the kidneys. These preliminary considerations will prepare us for the systematic discussion of this complex subject.

I. Physiological Relations

The source of the urine is believed to be certainly double. The bulk of the water is excreted in the Malpighian bodies, being squeezed from the glomerulus into the capsule by the blood pressure within the former. The excreting force is determined (1) by the pressure of the blood entering the glomerulus by the afferent vessel, and (2) by the resistance to its flow through the efferent vessel; whilst the freedom of filtration depends upon the fact that the uriniferous tubules have a free outlet, and thus present but little obstruction to the entry of water into their channel.

The size of the renal vessels is regulated by vaso-motor nerves, coming chiefly from the splanchnics, which derive their renal fibres from the medulla oblongata, in part at least through the first thoracic ganglion. The spot in the fourth ventricle which thus presides over the vessels of the kidney is a centre, i.e. it receives impressions through afferent nerves, and sends impulses through efferent nerves to the kidneys. Thus powerful emotions will disturb the flow of urine, and the temperature of the surface of the body affects the amount of urine secreted, partly at least reflexly.

The solid constituents of the urine-urea, uric acid, and their allies, and many of the salts, dissolved of course in a small quantity of water-are probably separated from the blood by the cells of the convoluted tubules. The activity of the renal epithelium no doubt depends, like that of the salivary glands, upon an inherent secreting force of its own, probably controlled by trophic nerves; upon the activity of the circulation; and especially upon the quality of the blood. "We have already seen that the materials which the blood conveys to the kidney for excretion will depend upon the activity of all the bodily functions, and we will not return to this subject except with respect to the influence of digestion and assimilation on the urine. During gastric digestion a quantity of acid is withdrawn from the blood to furnish the gastric juice, and this loss of acidity in a fluid already alkaline makes itself felt in the urine, which soon becomes less acid, or even alkaline. This reaction increases when absorption begins. "Water and salts enter the blood; augment still further the alkalinity of the urine, the salts being chiefly alkaline, and the total volume of the blood, and thus of the renal secretion, is increased; the arterial pressure rises. Finally, the products of the action of the liver, lungs, and other metabolic organs, upon the peptones and carbohydrates (urea and its allies) also enter the blood and appear in the urine, in comparative excess. This condition of the urinary function and urine, consequent on a full meal, gradually declines. The excess of water escapes; the alkaline salts are voided; the excess of urea and uric acid disappears; and therewith the general characters of the urine change. By the end of three or four hours from the admission of food, the urine is again moderate in amount, more acid, and clear, an increase of acidity following the previous reduction.

II. Pharmacodynamics

The preceding considerations prepare us for the conclusion that what power we may possess over the excretion of urine will be exercised, as far as its water is concerned, chiefly through the circulation; as far as the solids are concerned, chiefly through the blood. These points must be separately studied.

1. Measures For Increasing The Volume Of Urine

Measures For Increasing The Volume Of Urine. The amount of water, that is, the volume of urine which is excreted from the glomerulus, may be increased by diuretics, the effect being called diuresis (II Pharmacodynamics 16 through and II Pharmacodynamics 17 the urine). This may be accomplished in various ways:

(a) By raising the pressure in the arteries generally, including the renal, whilst the pressure in the veins is constant. This is most easily effected by temporarily increasing the amount of water in the system by drinking; by raising the force or the frequency of the heart, or both, by Alcohol, Digitalis, Scilla, Ammonia, and Scoparium; or by constricting the peripheral vessels through the vasomotor system, e.g. by cold to the surface, Digitalis, Scilla, or other vascular stimulants. These measures are called cardio-vascular diuretics.