COUNTERCURRENT HEAT EXCHANGE IN THE RESPIRATORY PASSAGES.

One of the important functions of the upper respiratory tract is to condition the inspired air prior to its arrival at the lungs. This conditioning involves heating the air to body temperature and humidifying the air to full saturation at that temperature. As a consequence of conduction and evaporation, heat is lost from the nasal surfaces, and the temperature of these surfaces falls below body temperature. During expiration, therefore, air leaving the lungs at body temperature and full saturation is cooled as it passes along the respiratory tract. This cooling of the expired air accomplishes a second function of the respiratory tract, that of conserving body heat and body water. MIeasurements of air and surface temperatures have been made in human respiratory passages by Seely in 19401 and later with improved techniques by Cole2 and by Inglestedt.3 These studies have demonstrated that, although air is expired at temperatures below body temperature, the human respiratory tract is not a very efficient heat exchanger. According to Cole2 temperatures in the central portion of the air stream near the nasal opening seldom are within 3VC of the surface temperature of the nasal passage at that location, and the mean temperature of the expired air of subjects breathing at normal room temperatures is about 330C. Previous studies have shown that the loss of water by evaporation from kangaroo rats (Dipodomys merriami), relative to the oxygen consumption, is lower than in man and lower than expected if the expired air were saturated at body temperature.4' 5 This could theoretically be accomplished in two ways: (a) through a higher degree of utilization of alveolar oxygen before the air is expired, or (b) through expiration of air at a temperature considerably lower than that of the body. (A third possibility, that the air is not saturated by passage over the moist surfaces of the lung, seems highly improbable.) Studies of the oxygen loading characteristics of the blood as well as its carbon dioxide content excluded possibility (a).6 The present study was undertaken to make direct measurements pertaining to possibility (b). Materials and Methods.-Temperatures were measured with small bead thermistors, 0.125-mm diameter. Resistance changes in the thermistor due to temperature changes were observed as voltage deflections on an oscilloscope connected across a simpleWheatstone bridge circuit. The bridge balance point was arbitrarily selected at a temperature midway in the anticipated temperature range. Unanesthetized animals were restrained in a prone position on a board by means of loops of soft plastic tubing around the legs, neck, and snout. The thermistor was placed in the nasal passage by first inserting a length of polyethylene tubing (0.61 mm o.d.); the thermistor and leads were then inserted into the polyethylene tubing which subsequently was retracted, leaving the thermistor in place. Before insertion, the polyethylene tubing was lightly coated with topical xylocaine ointment to minimize irritation to the mucous membranes. The animal was then