The actual mechanism of the secretion of fluid into the intestine is difficult to determine. It seems improbable that a change in blood pressure plays any very important rôle, if indeed it has an influence at all. There is much evidence to show that many glands consisting of cells resembling those of the intestine roughly, secrete their characteristic fluids quite independently of blood pressure. In Sida crystallina, a small fresh-water crustacean, it was found[89] that if a small quantity of one of the saline purgatives or of BaCl2 or pilocarpine be added to the water in which these crustaceans are lying, there is not only a rapid increase of intestinal movement and a rapid evacuation of faeces, but there is also an increased secretion of fluid into the intestine, so that the whole lumen becomes filled with a pale greenish fluid. It was pointed out further that in this organism there is no closed blood vascular system, the blood simply running in wide channels in more or less definite directions. There can therefore exist nothing here comparable with the blood pressure of higher animals, and yet secretion normally takes place without changes in blood pressure. Further, it can be greatly increased by chemicals without an increase in blood pressure being possible. A similar secretion without blood pressure as a causative factor is seen in the skin of the common slug (Ariolimax). Here the secretion of the skin may be markedly increased by the injection or local application of a solution of any of the saline purgatives. This takes place equally well when the heart of the animal is removed, and also in an isolated portion of the animal, or in a piece of the skin cut off with the scissors. In these latter cases there can be no possibility of blood pressure taking a part in the secretion.
It has been further shown[90] that loops of intestine entirely removed from the body may be caused to secrete a measurable quantity of fluid by immersing them in certain purgative solutions, especially those containing BaCl2. Other solutions such as pure m/6 NaCl do not cause this secretion, although active peristaltic movements go on in NaCl. In this case the secretion must be entirely independent of the blood pressure. Pilocarpine also in the salivary gland causes an enormous increase in the secretion, without raising the blood pressure in the carotid.
It is certain from these facts that in many glands secretion is quite independent of any change in blood pressure; and it seems probable that such changes must play a very subordinate part in the secretion of fluid from the intestine.
On the other hand, it is to be noted that in many instances muscular and secretory activities are controlled by the same conditions. There seems to be a common factor in the production of the two functions. Saline purgatives produce not only muscular activity, but also increased secretion; and calcium and magnesium are capable of inhibiting both. Atropin also quiets the movements of the intestine, and at the same time is conspicuous in suppressing the secretion. Section of the splanchnic nerves causes not only increased muscular movements, but an increased secretion of fluid in the intestine. These instances could be greatly increased in number. From them it seems that something exists in common in muscular movements and in glandular activity. What first suggests itself is that the gland cells themselves are made to contract rhythmically by the various conditions which cause rhythmical contractions in muscle. That the stimulus for this must be greater in the case of secretion is shown by the fact that in the intestine peristaltic movements may be maintained in a solution (m/6 NaCl) in which no secretion takes place. It seems not at all improbable that one factor in the production of secretory activity is dependent on a property of the gland cell closely related to muscular contractility.
A further factor is suggested by the action of certain diuretics. In the kidney the changes in the quantity of blood flowing through the organ and to some extent changes in blood pressure influence the flow of urine. The diuresis produced by such substances, however, as saponin, digitalin, potassium chlorate, etc., probably depends on an increase in permeability of the capsule of Bowman. As shown[91] recently, these substances produce haemolysis, and are also strong diuretics. Calcium chloride, which inhibits the flow of urine produced by them, inhibits also the haemolysis. Haemoglobinuria, which readily appears with small doses of saponin or digitalin, is inhibited by simultaneously injecting calcium chloride. There thus seems to be something in common between haemolysis and diuresis; and what suggests itself as most probable is that the permeability of the red blood corpuscle, as well as that of the kidney cell is increased, so that on the one hand haemoglobin escapes into the blood (is secreted into the blood), and the amount of urine on the other hand passing through the kidney cell is increased. Calcium, according to the same idea, would decrease the permeability in both cases.
In secretion we have therefore among other things two factors which probably play a rôle, namely, a property of the gland cell resembling that of muscular contractility and controlled in many cases by the same conditions, and a change in permeability of the cells which are secreting. In the kidney there is a third factor dependent on the flow of blood through the organ. A continuous supply of blood is of course necessary in all glands for a continued secretion.
FOOTNOTES:
[76] Recherch. expériment. sur les mouvements des liquides dans les tubes de petits diamètres, Paris, 1828. Quoted from Hay.
[77] Über die Saftbewegung, 1848.
[78] L’Union médicale, 1871, 50, 51. Gaz méd. de Paris, 1879.