This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
On account of these complex factors we must not forget, in treating patients, that the volume of the urine is made up of water, and that, therefore, the quantity of urine excretion is not necessarily a measure of the excrementitious materials that are being removed from the body. Indeed, von Noorden states that a concentrated urine may carry out just as much deleterious matter as one less concentrated. As the normal powers of healthy kidneys are vastly more than sufficient to maintain a proper blood composition, our endeavor in disease must be to restore the kidney functions or to minimize the amount of kidney activity required. We have no proof that the removal of edema by diuresis benefits the kidneys themselves, however much it may benefit the patient. We cannot confer upon the kidneys any abnormal powers, or functions new to kidney tissue.
Walker and Dawson, Christian, and others have shown that the life of rabbits with severe acute experimental nephritis may be definitely shortened by the repeated administration of diuretics, at least those of the caffeine series, and potassium acetate. Their experiments would suggest that diuretics are contraindicated in acute nephritis, but the success of diuretic methods in mercuric bichloride poisoning points otherwise. Christian says that following an active diuresis there may be for a day or two a decrease in renal function as measured by the index of urea excretion. This is probably due to renal fatigue and not to renal damage.
From these remarks it will be seen that the site of the diuresis may be the glomerulus or the tubule, or both; and that diuresis may be brought about by:
I. Measures which increase the glomerular fluid.
(a) By increasing the blood-flow through the kidney.
(b) By lowering the osmotic pressure of the blood. II. Measures which increase the tubular secretion.
III. Measures which decrease the tubular reabsorption.
(a) By Increasing the Blood-flow Through the Kidney
It is evident that constant replacement of the blood of the kidneys must take place or the urine will cease to flow. It is evident, also, that glomerular filtration is dependent upon the maintenance of a certain capillary pressure, for experiments show that when general arterial pressure falls below about 40 mm. of mercury, the urine ceases to flow. The capillary pressure in the glomerulus is maintained by the general arterial pressure, by the small size of the efferent vessel of the glomerulus as compared with its afferent vessel, and by the friction of the second set of capillaries. About the pressure in the efferent vessel, and about its dilatation and contraction, we know nothing; but it is found by experiment that even a moderate resistance to the venous outflow from the kidney checks the flow of urine. We know at present, therefore, that the flow of urine is readily influenced by changes in the amount of blood passing through the kidneys; and that this amount of blood is regulated by the general arterial pressure, by the caliber of the kidney arterioles, by the back pressure in the kidney veins, and by the viscosity of the blood. Digitalis is one of the best of diuretics in conditions with impaired circulation. (See Digitalis.)
The kidney arterioles are the sluice-gates to the capillaries. If general arterial pressure remains constant, dilatation of the kidney arterioles allows a greater blood-flow through the kidney capillaries, and contraction of the arterioles determines a lesser blood-flow. If the caliber of the arterioles remains constant, a rise in general arterial pressure causes more blood to pass through, and a fall in pressure causes less blood to pass through.
It is a general rule that diuresis is accompanied by dilatation of the kidney arterioles through a local action, and in most instances it is observed that diuresis is dependent upon such dilatation. But there are exceptional instances where diuresis has occurred in the absence of dilatation of the renal arterioles, or where diuresis has failed even though the arterioles were dilated.
In experimental vascular nephritis Pearce reports dilatation of the vessels from caffeine and from 5 per cent. sodium chloride, but diuresis from the caffeine only. Also, if the kidney is prevented from expanding, i. e., the vessels not allowed to dilate, there is diuresis from caffeine, but not from various diuretic salts and dextrose.
If sodium chloride, sodium acetate, urea, or dextrose in hypertonic solution is injected into the blood, the osmotic pressure of the blood is at once raised. Fluid passes to it from the tissues, the blood swells up, and a condition of hydremic plethora with lowered osmotic pressure is brought about, i. e., the quantity of blood is greater than normal, the tissues or tissue spaces having been drawn upon for a diluting fluid. If an isotonic saline solution is injected into a vein, swallowed, or administered by rectum, this hydremic plethora results without the imbibition of fluid from the tissues or tissue spaces.
In hydremic plethora, under the influence of the slightly raised arterial pressure and the lessened viscosity of the blood, this swollen volume of blood tends to promote rapid blood-flow, and, as a consequence, to favor transudation of the excess of fluid through capillaries. The kidney capillaries are the ones by which the body gets rid of excessive fluid; therefore if the kidneys are functionating properly, there is diuresis, and the excess of water with certain dissolved materials is rapidly got rid of.
Hydremic plethora and its resulting diuresis may be the consequence of the absorption of dropsical fluid, as under the administration of digitalis. It may be produced intentionally by the ingestion of water, or of solutions of dialyzable substances.