Temperature regulation in the new-born lamb. VI. Heat exchanges in lambs in a hot environment

At ambient temperatures below about 30°C, respiratory and cutaneous evaporation were constant in normal lambs and lambs without sweat glands. Above 30°C, respiratory water loss increased steeply. Cutaneous water loss also increased, but at a slower rate than respiratory loss and only in the lambs with sweat glands. The efficiency of evaporation in cooling the lamb was close to 100%. The contribution of cutaneous blood flow to facilitation of heat loss in lambs lying down appeared to be low. At low environmental humidity, respiratory evaporation at all rates of normal shallow panting was approximately 4 mg per respiration; but in "second phase" breathing this was increased up to 12 mg per respiration, and total respiratory evaporation was not reduced. Lambs showed no evidence of distress when exposed for 6–12 hr to air temperatures of 40°C and water vapour pressures of' less than I5 mm Hg. Cutaneous loss tended to fall and respiratory loss to increase. Maximum rates of cutaneous and respiratory evaporation were estimated by suppressing evaporation from the skin or respiratory tract at 43°C. These values tended to be higher in crossbred lambs then in Merinos, and Merinos tended to reach maximum "sweating" rates under less severe heat stress than the crossbreds. Homeothermic equilibrium was approached when evaporation from neither site was suppressed, but rectal temperatures increased rapidly when cutaneous evaporation was prevented, and more rapidly still when respiratory evaporation was much reduced. The results also illustrate how a high metabolic rate decreases heat tolerance. These experiments provide good evidence that sheep do sweat, but that respiratory evaporation is quantitatively more important than sweating.

CARBON AND OXIDATION–REDUCTION BALANCES AND THE ESTIMATION OF FERMENTATION EFFICIENCIES OF AEROBIC FERMENTATIONS

A method for preparing carbon and oxidation–reduction balances of aerobic fermentations is described. In spite of the complexity of the microbial cell, relatively few and simple analyses of the material are necessary for the purpose of constructing the balances. To test the method, balances were made on citric acid and ustilagic acid fermentations. Excellent results were obtained. Based on the principle of material balance, a number of formulae for calculating the fermentation efficiency and respiratory loss have been established for fermentations involving substrates and products of varied oxidation–reduction status. Limitations of the method and formulae are also discussed. Results obtained by the described methods and equations are of value in correlating the various processes occurring in an aerobic fermentation and in providing an over-all picture of the fermentation.