The results of the present study confirm previous observations and provide further evidence that water and solute are primarily transported from the intestine by the lymphatic system of rat jejunum in vitro, regardless of the nature of the serosal bathing medium (moist air, mineral oil or Ringer solution), or degree of intestinal motility. The transport of fluid by the venous system was negligible under all circumstances so far observed. Venous transport occurred only over a brief duration but lymphatic transport continued with little diminution in rate for at least 40 min. Lymph flow decreased with the increase of mucosal fluid osmolarity but was not affected by changing serosal fluid osmolarity. Lymph was always isosmotic with the mucosal fluid whether the latter was hypo-, iso-, or hypertonic. The composition of lymph with respect to concentrations of glucose; Na, K and Cl was the same as that of the absorbed fluid. The glucose concentration in the lymph was closely related to that in the bathing fluid on the mucosal side, rather than to that on the serosal side. It is concluded that the formation of lymph in the intestine is closely associated with water and solute absorption.
A test has been carried out in rats of the possibility of measuring with the aid of doubly labeled water (D2O18) the following components of the material balance of an animal: output of CO2 and water; intake of oxygen, food and water. The items of information used for the measurement were a) isotopic analyses of initial and final blood samples, b) composition of the diet with respect to percentage protein, carbohydrate and fat, c) initial and final body weight, d) final percentage body water. Initial percentage body water obtained from a by the volume of dilution principle could substitute for d. CO2 and water output were estimated isotopically; O2 consumption, from the CO2 output and dietary R. Q.; food intake, from CO2 output and dietary composition; water intake, from the difference between water output and dietary metabolic water. A rough correction for storage of materials was made from the change in body weight. The average difference between observed values for each of the above components of the material balance and values calculated by the isotope procedure was less than 10%. The fact that dry air was supplied to the animal in the metabolism chamber used to obtain the observed values probably favored better agreement between calculated and observed values for water intake and output than would prevail in ordinary moist air.
With an in vitro rat jejunal preparation it was found that at low distention pressures the absorption rate of segments without mesentery are about 40% lower than the rate of those with intact mesentery. The decrease of absorption rate in segments with lacteal ducts sectioned near the gut wall was the same as the rate for those completely devoid of mesentery. Section of blood vessels showed no decrease but rather a slight increase in rate. The mesenteric lymphatic ducts during water absorption in vitro and in vivo showed rhythmical contractions with an average frequency of 10/min. The lymphatic ducts of an isolated mesentery may continue to contract and transport water for a few minutes. The lymphatic pressure of the isolated segment is assumed to be an approximate measure of absorbing force. Epinephrine may augment lymphatic contractility and may also elevate lymphatic pressure. These observations suggest that the mesenteric lymphatics may play a significant role in water transport.
Studies have been made by microscopic observation of blood vessels in skin, skeletal muscle, mesentery and intestinal wall. In cats anesthetized with pentobarbital sodium, the effects of histamine upon the diameters of small blood vessels have been observed and it has been found that, in general, larger microscopic vessels respond to topically and intravenously applied histamine by constriction, whereas smaller venules and arterioles, in general, respond by dilation. After blood vessels have been constricted by continuous infusions of epinephrine or sympathetic stimulation, the administration of histamine induces a dilation. Mesenteric and intestinal small veins are more sensitive to histamine than are small arterioles in the same structures.
Studies have been made of the diameters of microscopic and macroscopic blood vessels in the leg skin of 8 dogs and 13 cats, under pentobarbital anesthesia in parallel with observations on blood pressures in similar vessels, using two techniques, micro-cannulation and a recently described micro-chamber method. During stimulation of the homolateral sympathetic trunk, with or without curarization, there was a rise in venule and small vein pressure, associated with an initial decrease in venous vessel diameter. After cessation of stimulation the venous diameters returned to or exceeded control values while the arteriolar diameters were still markedly reduced. These observations provide direct evidence that even in the absence of mechanical obstruction to venous blood flow the pressures in venules, and therefore necessarily in capillaries when blood is flowing forward, may be elevated by sympathetic nerve stimulation which reduces the blood flow into the vascular bed in question. The possible importance of this mechanism in edema formation and in blood pooling is pointed out.
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