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.
Nervous Control of Urinary Secretion.
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.