Chloride and water secretion and absorption by the gills of the eel

Summary Perfusion experiments with the heart-gill preparation of the common eel, Anguilla vulgaris, are presented. Various concentrations of perfusion medium were used with eels from both sea water and fresh water. The external medium was sea water in the case of the sea water eels and fresh water in the other case.Perfusate was collected over a period of hours in each experiment and the exchange of chloride and water was studied.In the early experiments it was shown that normal permeability of the gills to these substances is small. The development of an extremely precise chloride method made it possible to evaluate the magnitudes of the exchanges in accurate quantitative terms.Experiments with fresh water as external medium showed that there is a small but perfectly significant change in chloride concentration in the perfusion medium as it passes through the gills so that it becomes more dilute. Reasons are adduced from which it is concluded that the dilution is due to the diffusion of water into the internal medium through the surface of the gills. This branchial water intake is calculated to be something like 40 cc. per kilogram of eel per day. This figure is in substantial agreement with Smith's measurements of the rate of urine formation in fresh water fishes.It was found that when the external medium is sea water, the chloride concentration of the internal medium decreases at a considerably faster rate than in the case where fresh water is present outside the gills. Experiments in which the external medium was analysed showed that chloride was being secreted into the concentrated sea water. It was found possible to measure volume changes in the external medium as well as the changes of chloride concentration in both mediums. These experiments demonstrated beyond doubt that a concentrated chloride solution is secreted by the gills in opposition to a large concentration gradient.Calculations made from several different bases showed that the measurements of volume and of chloride concentration consistently agree in yielding similar values for the volume and concentration of the chloride solution secreted by the gills of the sea water eel.It is shown that the magnitude of the concentration changes effected are related to the concentration of the internal medium used so that a small increase in the concentration of the perfusion medium results in a large increase in the amount of chloride concentration change occurring in the perfusion. Below a certain concentration of the internal medium the gills no longer give evidence of doing concentration work, and may even become permeable as evidenced by the internal medium increasing in concentration as it passes through the gills.Similarities between the activities of the gill of the sea water eel and of the normal mammalian kidney are pointed out. A rough calculation indicates that the work performed by the two organs is of the same order when computed in terms of grams of tissue involved.Experiments in which urea analyses were made showed that the gills of the eel are permeable to urea and that the amount of urea excreted through the gills as determined by direct measurement agrees quite well with Smith's figures for extfa-renal urea excretion.Possible objections to some of the conclusions are brought forth and their untenability demonstrated.Fellow of the National Research Council of America.