The Relation between Blood Osmotic Pressure, Fluid Distribution and Voluntary Water Intake

The physiological basis of thirst has been a debatable subject for well over a century. Recent investigations have centered around the old controversy as to whether thirst is a general or localized sensation. The historical literature on this subject has been presented by Cannon (1918). Montgomery (1931) has briefly summarized the more recent contributions. Schiff in 1867 (cited by Cannon, 1918) was of the opinion that thirst was a generalized sensation which arose from a lessened water content of the body. The local reference of the sensation of thirst to the pharynx he placed in t*he same category as the local reference of hunger to the stomach, or of sleepiness to the heaviness of the eyelids. Recent investigations, however, have stressed the relationship of water intake to the local conditions of the mouth especially with reference to salivary flow (Montgomery, 1931). The influence of general body hydration and the distribution of body fluids upon thirst has received scant attention. In the present study, observations on voluntary water intake were correlated with changes in blood osmotic pressure and the general distribution of the body fluids. PROCEDURE. Dogs were used as the experimental animals. A normal blood sample was taken from the femoral artery, after which the animals received, intravenously, 2.5 ct./kilo of either 20 per cent NaCl or 40 per cent urea. These concentrations are approximately isosmolar. Thirty minutes were al .lowed to elapse for the complete equilibration of the substances injected during which time the dogs were kept quietly on the table and were not given access to water. At the end of this period a second arterial blood sample was procured and the animals then returned to their cages where water was available ad libitum. Their fluid intake was recorded at fifteen minute intervals. Blood samples were obtained at intervals over six hours. Measurements were made on the serum of total osmotic pressure by the vapor pressure method of Hill (1930) and specific gravity by the falling drop metshod of Barbour and Hamilton (1926).