262 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



being about 5% of the initial value obtained at a normal body temperature. 

 Since the decrement in total body oxygen consumption is of a similar 

 magnitude, it appears that although the diffusing capacity at low tempera- 

 tures is considerably decreased, it is still adequate for existing needs. 



If the lung were a physical system having constant geometry we should 

 expect the diffusing capacity to decrease by only about 1.2% per degree 

 drop in temperature over the range involved in our experiments. Our ob- 

 served decrease being about four times larger suggests that some physio- 

 logical factor is involved in addition to purely physical ones. We have 

 suggested that a decrease in the size and/or number of functioning pul- 

 monary capillaries with a consequent decrease in area available for dif- 

 fusion may be responsible. 



CIRCULATION 



If pulmonary ventilation and diffusing capacity are adequate the hemo- 

 globin in the systemic arterial blood will be nearly fully saturated. This 

 should be especially true at low temperatures because the affinity of hemo- 

 globin for oxygen is increased; the dissociation curve is shifted to the left 

 (8). Furthermore the amount of oxygen that can be carried in physical 

 solution is increased at low temperatures. There is therefore a more than 

 ample content of oxygen in the blood as it enters the tissue capillaries. 

 Whether the tissues actually receive an adequate supply of oxygen de- 

 pends now on whether the flow of this oxygen-rich blood is fast enough, 

 i.e. whether the ratio of blood flow to oxygen consumption is maintained at 

 a sufficiently high value. 



The experimental evidence indicates that during cooling the frequency 

 of the heart beat decreases in an approximately linear fashion while the 

 stroke volume remains nearly constant (see fig. 2). The net result is a 

 drop in cardiac output that is just about in proportion to the droj) in 

 oxygen consumption. Thus the venous blood oxygen content and the ar- 

 terial-venous oxygen difference remain approximately constant during 

 cooling (9, 10). 



At body temperatures below 25°C ventricular fibrillation is a fairly 

 common occurrence. The heart seldom regains its normal rhythm spon- 

 taneously and death from circulatory failure results. There are numerous 

 theories which attempt to explain why the hypothermic heart is so prone 

 to fibrillate, but none of them seems to be convincingly proven. 



As long as fibrillation can be avoided the over-all total body blood flow 

 appears to be quantitatively adequate for the transport of oxygen and 

 carbon dioxide at temperatures down to 20°C or below. The over-all blood 

 flows to the brain (11) and to the heart (12-14) are likewise sufficient as 



