Under normal conditions, without sweating, about two thirds of the insensible water loss occurs by diffusion through the skin. The rate of water loss from the respiratory tract depends on the volume of respiratory exchange and the content of water of the inhaled and exhaled air. These are in turn dependent on the temperature and humidity of the environmental air, since the exhaled air is about 88 per cent saturated with water at 33 C. When there is hyperpnea, the magnitude of the water losses through the lungs is difficult to measure but probably reaches values five times as great as the normal rate.55In estimating the importance of the losses from the lungs, it should be remembered that an increase in the loss of water from the lungs may be partially compensated by a decrease in the activity of the sweat glands. Water loss from the respiratory tract