Relative prevalence in man and animals. Causes of difference. Kidneys as eliminating organs for nitrogenous material, toxins, bacteria, mineral, vegetable and animal poisons, diuretic drinking water, condition powders, cantharides, urea, etc. Suppression of urine, precipitation of urine. Filtration through kidney. Secretion. Urinary solids. Nervous control of secretion. Excess.
Diseases of the urinary organs are less prevalent in the lower animals than in man, owing largely no doubt to the greater simplicity of their habits of life and to the comparative shortness of the lives of those that are kept for meat producing. It is a mistake, however, to suppose that they are so infrequent as would appear, since the absence of subjective symptoms in the animal allows a number of the milder forms of renal disease to be passed over without recognition.
In man the excessive consumption of animal food, the lack of exercise, the abuse of alcohol, the prevalence of venereal diseases, conduce largely to renal troubles, while animals in general escape. Yet animals suffer much more extensively than is generally supposed. The kidneys are, as in man, the eliminating organs for superfluous and waste nitrogenous matter, and in overfed animals may be overcharged with this work. They are the general emunctories for the soluble poisonous products of bacteria and plants, which may stimulate the urinary secretion, and from these irritation may result. It is through the kidneys that the bacteria themselves largely leave the animal body, and trouble is liable to come during their passage. Further, exposure to cold tends to increase the urinary secretion, over-stimulating the kidneys, and the same may come from diuretic drinking waters and condition powders, also from cantharides and other diuretic agents applied to the skin. Urea and many toxins are diuretic, hence the occurrence of polyuria at and after the crisis of fevers.
On the other hand suppression of the urinary secretion may occur in connection with profuse perspirations in hot weather, with prolonged diarrhœa, or with privation of water, and in such cases the liquid becomes concentrated and irritating and there is a disposition to precipitate its solids under slight disturbing causes. As conducive to such precipitation may be named foreign solid bodies, bacterial ferments and probably the goitre poison since gravel and calculus are common in goitrous regions.
There are two forms of elimination through the kidneys. 1, filtration; 2, secretion.
1. Filtration is referred to the glomeruli, and is determined by the relative blood pressure. Increase of pressure causes increase of watery transudation. Digitalis increases heart action and arterial pressure, and accidently urination. Excessive consumption of water and watery liquids increases intravascular tension, and the amount of urine.
2. Secretion is referred to the columnar epithelium of the convoluted tubes. It is by the elective affinity or selective power of this epithelium that the solids of the urine are abstracted from the blood and passed into the urine. Crystals of uric acid have been found in these cells and it is supposed that the abundance of water furnished by the glomeruli, irrigating these convoluted tubes, dissolves and washes on the various solids and other products with which the epithelial cells are charged. The protoplasm of the cells becomes saturated with the urea, uric acid, hippuric acid coloring matter (indican, urochrome, etc.), and this is washed out, passing by exosmosis to the liquid of lesser density with which the tubes are filled.
An electric current through the renal plexus of the sympathetic (vaso-motor) lessens, or suppresses urinary secretion (inhibition).
Cutting the nerves of this plexus causes excessive vaso-dilation, renal pulsations synchronous with heart beats and arterial pulse, and great increase of urine. A similar increase comes from the application of cold to the surface, from fatigue, from heat exhaustion, from irritation of the floor of the fourth ventricle just in front of the origin of the vagus and from section of the splanchnic nerve. This last is, however, much less marked and more transient than from section of the renal nerve noted above; the latter causing dilation of the renal vessels only, and increased pressure, whereas the former causes dilation of the abdominal organs generally, diverting the blood largely to other parts than the kidney and preventing the same increase of pressure in the vessels of the latter. For the same reason transverse section of the medulla oblongata, or of the spinal cord as far back as the seventh cervical vertebra, lessens or interrupts the urinary secretion, the pressure in the kidney being reduced by the diversion of much of the blood elsewhere. This influence of the nervous system on the urinary secretion seems to be mainly or entirely one of increase or decrease of blood pressure in the kidney. For this reason a weak heart tends to lessen urinary secretion.
Excessive increase of urine is only important when continuous and in the absence of visible cause, such as diuretics.