(b) By dilating the renal arteries, so that the quantity of blood within them is increased, whilst the pressure in the arterial system generally, and the resistance in the renal veins, remain unchanged. This method of increasing the amount of the renal water may be carried out by acting on the vaso-motor system of the kidney either locally or centrally. Local depressants of the renal nerves include Digitalis and Scilla in the second stage; Spirit of Nitrous Ether; all Volatile Oils and Resins, such as Turpentine, Juniper, Copaiba, Hops, Savin, Cantharides, Camphor, etc.; Alcohol, Belladonna, Aconite, Nitrates, and Nitrites. Central renal vascular depressants are chiefly or solely emotional impressions which are not available as pharmacodynamical means. A powerful reflex dilator of the renal vessels is cold to the surface. Such measures are local vascular diuretics.

(c) By combining the two previous means, when still more profuse diuresis will be the result. This occurs in the second stage of the action of Digitalis and Scilla, and in the application of cold to the surface.

2. Measures For Diminishing The Volume Of Urine

Measures For Diminishing The Volume Of Urine. The volume of urine might be diminished by employing the opposite set of influences to those just described. These are obscure, however, and of less therapeutical interest; and the student may be left to work out the different systems for himself.

3. Measures Affecting The Secretion Of Urinary Solids

Measures Affecting The Secretion Of Urinary Solids. The activity of the renal epithelium, i.e. the excretion of solids and of a certain amount of the water, may be modified by influences of two classes :

(a) By measures and conditions which affect the renal cells through the composition of the blood in general. - Of these, the state of digestion, including the selection of food, is the most important. The quantity of food; its richness in proteids, carbohydrates, and salts of different kinds; the relative amount of work thrown upon gastric or acid, and duodenal or alkaline digestion; and the vigour of hepatic metabolism, as determined by so many causes, including exercise, oxygenation, and the use of drugs-may all be made use of by the pharmacologist in altering the composition of the urinary solids.

One of the most easy and important of these alterations is in the chemical reaction of the urine. The natural acidity of the urine can be increased by excess of proteids, sugar, and starch, by deficiency of water, by certain wines and spirits, by Salicylic and Benzoic Acids, and by an excess of Tartaric and Citric Acids. The mineral acids have an insignificant or even negative power on the acidity of the urine, a fact which is to be carefully noted. Sulphuric Acid is excreted by the kidneys (in part), but as neutral sulphates; Hydrochloric Acid as neutral chlorides, Phosphoric Acid as phosphates; Nitric Acid is believed to increase the ammonia in the urine by decomposition in the blood, so that it may have an alkaline influence; and Tartaric, Citric, and Acetic Acids in combination with Alkaline bases, escape as Alkaline Carbonates.

On the other hand, we possess abundant and powerful means of rendering the urine alkaline. Amongst foods, the most effective in this direction are fruits, milk, and fish, as they throw into the blood a quantity of Alkaline Citrates,Tartrates, Acetates, Carbonates, and Phosphates, which are directly or indirectly excreted by the kidneys. Amongst drugs, the whole group of Alkalies and Alkaline Earths have an alkalinising effect on the urine, excepting Ammonia, which is completely broken up in the system. Thus the alkalies are entirely unlike the mineral acids in exercising a powerful and available influence on the reaction of the urine.

(b) By measures which affect the renal epithelium specifically. Whatever may be their alkalinising value in the blood, certain substances have a special influence on the urine by specifically acting upon the renal cells. Thus Potash and Soda possess equal values as alkalinisers of the blood, but potash will much more powerfully and quickly neutralise the acidity of the urine, because whilst Soda is excreted partly by the bile and bronchial mucus, or locked up in the system as the neutral chloride of sodium, Potash stimulates the renal epithelium, which excretes it as the carbonate. Soda does, however, possess a degree of specific action on the kidney, especially its Phosphate and Acetate. Lithia closely resembles Potash in this respect; Ammonia, although not an alkaliniser, has a similar influence; and Magnesia and Lime are distinctly stimulants of the renal epithelium, as is well seen in some natural mineral waters. Now, in passing through the cells, these salts necessarily carry with them a certain amount of water from the venous plexus around the tubules, and if abundant, actually produce diuresis. They thus furnish us with another group of diuretic measures, which we call the saline diuretics, chiefly alkaline in their influence on the blood and urine, but at the same time independently active as specific renal stimulants. Let it be carefully noted that the saline diuretics do not, as far as we know, directly affect the renal circulation; but that we possess in them an indirect means of influencing the venous plexus around the tubules, and thus the whole renal circulation and the general blood pressure, especially the pressure in the veins.

Another great group of natural substances in the materia medica have a specific effect on the renal epithelium, namely, the Aromatic Oils, Oleo-resins, and Balsams. The chief of these are Turpentine, Juniper, Copaiba, Cubebs, Cantharides, and Hops; whilst Jaborandi, Alcohol, Aconite, and many more act partly in the same way. All these substances, either as such or after decomposition, are excreted (in part) by the renal cells, and carry with them, like salines, so much water, besides dilating the renal vessels, as we have already seen. The degree in which the different members of this great class act upon the renal cells varies widely, however: thus, Juniper and Copaiba are powerful diuretics, greatly increasing the urinary flow, whilst most of the others have but little effect on the volume of urine, possibly because their action on the renal vessels, which accompanies their action on the cells, does not favour the escape of fluid. Thus Turpentine and Cantharides, two most powerful renal stimulants, sometimes diminish, sometimes increase, the urinary water, and may even cause haemorrhage from the glomerulus.