WATER 



183 



evaporating surface. Generally speaking, the 

 greater the intensity of turbulent mixing, the 

 drier wiU the surface of the air become, the 

 larger will be the vapor pressure gradient, and 

 the greater the evaporation." 



RELATION OF ANIMALS TO MOISTURE 



Water makes up a large proportion of 

 the bodies of plants and animals, whether 

 they live on land or in the water. Active 

 protoplasm holds about 70 to 90 per cent 

 of water. Measured values for active ani- 

 mals lie between 50 per cent for meal 

 worms (tenebrionid beetle larvae) (Hall, 

 1922) and 98 per cent for medusae from 

 brackish water (Hyman, 1940). Animal 

 tissues tend to lose water with age; man, 

 for example, has about 72.5 per cent at 

 birth and 66 per cent when adult. The 

 water content of various insects is given in 

 Table 13. 



The difficulties encountered in maintain- 

 ing the proper water balance by aquatic 

 animals (p. 169) become much greater for 

 terrestrial forms. This was one of the obsta- 

 cles to the invasion of land habitats by 

 plants and animals. The internal supply of 

 water must be maintained; loss of one-third 

 of the water present produces death in 

 taxonomically widely separated forms such 

 as land isopods and the house mouse, 

 though other animals can lose much more 

 and live. Despite the danger of desiccation, 

 an intimate contact must be maintained be- 

 tween actively respiring cells with a high 

 water content and the surrounding atmos- 

 phere, which may be relatively dry. Plants 

 face these difficulties as well as animals. In 

 relatively dry continental climates, 99.9 per 

 cent of the water taken from the soil passes 

 through the plant and is dispersed into the 

 atmosphere in transpiration; in many 

 moister climates, the percentage falls only 

 to 99.7 (Klages, 1942). Evaporation of this 

 water helps control the temperature, and 

 its passage though the plant performs other 

 valuable functions. 



The evaporation of water is the one im- 

 portant means of dissipation of body heat in 

 hot climates or on hot days in any climate. 

 The humiditv of the air has a large effect in 

 determining how high a temperature can be 

 endured. In desert heat, a decrease of 1 per 

 cent in relative humidity is almost as effec- 

 tive as lowering the temperature 1° F. 

 (Adolph, 1943). Water has high heat ca- 

 pacity and high heat of vaporization. The 



animal or plant body consists so largely of 

 water that it approaches the heat capacity 

 of the latter and so is well buffered against 

 temperature fluctuations. Dill (1938) re- 

 ports that a resting man kept in thermal 

 equilibrium with his environment would in- 

 crease in body temperature at the rate of 

 2° C. an hour as a result of his own metab- 

 olism. If he were made of steel and could 

 still carry on the same metabolism as at 

 present, his rise in temperature in an hour 

 would be about eight times as fast as it is 

 at present. 



The blood is largely water and so has 

 high heat capacity that enables it to trans- 

 fer heat from the deeper parts of the body 

 to the skin and respiratory tracts, where it 

 is dissipated by evaporation of water or by 

 heat radiation. The vaporization of 1 liter 

 of water at 33° C. requires 580 kilogram- 

 calories (Dill, 1938). Much water is lost in 

 respiration. Air is taken into the lungs of 

 man in quantities of from 5 to 100 liters 

 per minute. The inspired air may be warm 

 or cold, saturated or almost entirely dry. On 

 expiration it is practically saturated with 

 water vapor at 33° C. About 400 cc. of 

 water are lost daily in this way by the aver- 

 age man. So-called insensible perspiration 

 increases water loss by 300 per cent, and 

 man in the desert can sweat at the rate of 

 a liter or more an hour or even roughly 

 three times that amount. Vaporization of 

 this amount of water would remove over 

 ten times the heat produced in basal metab- 

 olism of man and over four times that of 

 hard physical labor (Brody, 1945). Human 

 sweating varies greatly with different indi- 

 viduals and is much affected by acclimatiza- 

 tion. It also varies still more widely between 

 species. The loss of water in heat regula- 

 tion is closely associated with salt losses, 

 and both are correlated with the quahty 

 and quantity of urinary secretion.* 



Brody (1945) states that up to about 

 29° C, water vaporization in man shows an 

 irregular increase and dissipates as much as 

 35 per cent of the heat. There is a steep 

 and orderly rise above this point, and a 

 balanced heat budget for the body is at- 



• Dill (1938) gives a readable, accurate 

 summary of heat and water regulation of man 

 and several other animals under a variety of 

 climatic conditions, especially those of hot 

 climates, both arid and humid. He is interested 

 in the physiological and ecological aspects of 

 the subject; see also Brody (1945). 



