The pressure of blood in the glomeruli may be raised:

(1) By increase of the arterial tension generally.

(2) By increased tension locally.

Such a general increase may be brought about by greater action of the heart, or by contraction of the blood-vessels in other vascular areas, such as the intestines, muscles or skin, by nervous stimulation, exposure to cold, or the action of drugs.

The pressure may be increased locally by dilatation of the renal arteries, e.g. from section of the vaso-motor nerves, or possibly stimulation of vaso-dilating nerves.

In addition to such increase of pressure in the glomeruli by increase of blood-supply to them, we must not, however, forget the possibility of increased pressure in them by contraction of the efferent vessels leading from them, as well as of those arterial twigs (arteriae rectae) which pass directly to the venous plexus surrounding the tubules, and which form no inconsiderable part of the vascular supply of the kidney.

Alterations in the size of the renal vessels were formerly ascertained simply by exposing the kidney and observing its colour, contraction of the arteries being associated with paleness, and dilatation with redness of the organ. A much more exact method has been introduced by Roy, who encloses the kidney in a capsule filled with oil and connected with a registering apparatus. When the vessels dilate, the kidney increases in size, and diminishes when they contract, so that the alterations can be readily recorded on the same revolving cylinder on which the general blood-pressure is registered by the manometer.

The pressure of blood in the glomeruli may be diminished generally:

(1) By failure of the heart's action; or

(2) By dilatation of the vessels of large areas, as the intestines, muscles, or skin.

The pressure of blood in the glomeruli may be diminished locally by contraction of the renal arteries, or of the afferent branches to the glomeruli.

The heart's action may fail from many causes, which have already been discussed more particularly.

Dilatation of the vessels in the skin, intestines, etc, may be caused by exposure to warmth, by the action of drugs, or by paralysis due to nervous injury.

Section of the splanchnics or of the spinal cord causes paralysis of the renal arteries, and ought, therefore, to increase the secretion of urine. This does occur, though not invariably, when the splanchnics are divided; but section of the spinal cord, by paralysing the intestinal and other vessels, lowers the blood-pressure so much that the supply of blood to the kidney is not only much below the normal, but is so small that the secretion of urine is generally almost completely arrested.

The nerves of the kidney consist of a number of small branches running along the renal artery and containing a number of ganglia. When these nerves are cut the vessels of the kidney dilate; when they are stimulated the vessels contract. A number of those fibres pass to the kidney from the spinal cord through the splanchnics, so that when the splanchnics are cut the vessels of the kidney usually dilate, and when they are irritated they contract.

The whole of the nerves, however, do not pass through the splanchnics, for stimulation of a sensory nerve, of the medulla oblongata, or of the spinal cord in the neck, will cause contraction of the renal vessels after both splanchnics have been cut, and section of the splanchnics does not always cause the renal vessels to dilate.

The nervous centre for the renal arteries is probably, like the chief vaso-motor centre for the body generally, in the medulla oblongata; but in all probability there are also subsidiary centres in the spinal cord and in the solar and mesenteric plexuses.

The reason for supposing these latter centres to exist is, that stimulation of the peripheral end of the splanchnic, divided at its passage through the diaphragm, causes contraction of both kidneys, and the vessels of the kidney of the side opposite to the stimulated nerve commence to contract later than those on the same side. A delay like this in the action of the stimulus indicates that it has not acted directly, but through the medium of ganglia.

When the splanchnics are divided the vessels of the kidney sometimes dilate and the kidney increases in size; a profuse secretion of urine may take place, which quickly increases to a maximum and remains for a considerable time. This, however, is not a constant effect, and not unfrequently the vessels do not dilate, and the kidney, instead of increasing, diminishes in size. This is what to a certain extent might be expected, inasmuch as a section of the splanchnics causes dilatation of the intestinal vessels and lowers the blood-pressure, and thus diminishes the supply of blood to the kidney.

When a puncture is made in the medulla oblongata in the floor of the fourth ventricle, profuse secretion also occurs, but this differs from that caused by section of the splanchnics, in being preceded by a slight diminution, by rising rapidly to a maximum and then rapidly falling. These characters seem to show that it is due to irritation of some vaso-dilating mechanism,1 rather than to any paralysis.

Stimulation of the vaso-motor centre in the medulla oblongata by venous blood, or by drugs such as strychnine or digitalis, has a twofold action on the kidney, for it tends to cause contraction not only in the vessels of the kidney, but in those of other parts of the body. The effect upon the kidney is thus a complicated one, for the contraction of the intestinal and other vessels by raising the blood-pressure tends to drive blood into the kidneys, at the same time that the contraction of the renal arteries tends to keep it out. When the renal nerves are cut, the renal vessels no longer oppose the entrance of blood, and therefore the renal vessels dilate very greatly when the vaso-motor centre is stimulated; but when the renal nerves are intact the result is a varying one, for sometimes contraction of the renal vessels may be so great as to prevent the entrance of blood into the kidney, however high the general blood-pressure may rise; at other times the general high blood-pressure may be able to dilate the renal arteries in spite of any resistance they may offer. These different conditions may occur subsequently to one another; and this stimulation of the vaso-motor centre may cause contrac1 Heidenhain, Hermann's Handbuch der Physiologie, vol. v. Th. 1, p. 366.