Water 59 



glomerular filtration was 1.22 ml./kg./min. and total urine output was 25-70 

 ml./kg./day. The urine concentration was higher during water deprivation 

 and after injection of pitressin from the posterior pituitary, hence water reab- 

 sorption occurs, and antidiuretic hormone increases water reabsorption. Water 

 is also reabsorbed in the cloaca, ^'^*' and the urine may be a semisolid mass of 

 uric acid crystals. Chick mesonephric tubules form cysts in tissue culture 

 which accumulate water so that the cyst contents are 10 per cent more dilute 

 than the medium, suggesting a water-secreting capacity. '■'" 



The physical properties of yolk make osmotic measurements difficult. Freez- 

 ing-point measurements^^' '-^ indicate a difference between yolk and white; 

 vapor pressure measurements show a higher osmotic concentration in the 

 center of the yolk than just beneath the vitelline membrane, the mean for the 

 yolk corresponding to a Af.p. of 0.55, and for the white to a Af.p. of 0.44. ^^ 

 The difference is not maintained by metabolism but by slow diffusion,--'*" 

 and some authors fail to find any osmotic difference between white and 

 yolk. ^0-' 



Water regulation in mammals has been studied extensively and reviewed 

 frequently. The daily water loss of a man weighing 70 kg. is 600-2000 ml. by 

 the kidneys, 50-200 ml. by feces, 350-700 ml. by insensible evaporation from 

 the skin, 50-4000 ml. by sweat, and 350-400 ml. from the lungs. A lactating 

 mother may lose an additional 900 ml. as milk. Thus the total normal daily 

 loss may range from I to more than 8 liters.^ The total loss \'aries with 

 temperature and humidity, and with various physiological conditions, and 

 must be made up by water drunk, water in food, and metabolic water. Most 

 common mammals have a blood concentration which is about 0.30 osmolar 

 (equivalent to 0.95 per cent NaCl). Dog plasma freezes at —0.576° (0.31 

 osmolar). "'^ In man, urine concentration varies but is usually about 0.65 

 osmolar; the maximum concentration of urine in thirsting man is 1.4 osmolar 

 (Au=2.6). *^^' Much water is reabsorbed along with salts in the proximal 

 tubule of the kidney (guinea pig), although the fluid distal to the loop of 

 Henle may still be isotonic with the plasma and the site of urine concentration 

 is not definitely known. -"'- The permeability of the skin of mammals to 

 water is extremely low --^^ as measured both by transfer of heavy water and 

 by evaporative loss. Low permeability is particularly important in aquatic 

 mammals. In man loss of 10 per cent of the body water causes serious illness, 

 and 20 per cent loss results in death; much greater proportions of protein, fat, 

 and glycogen can be lost with little ill effect. ""* A water loss of 24 to 30 per 

 cent of the bodv weight is fatal to a mouse. '"' 



The water load of the blood is kept delicately balanced by nervous and 

 hormonal factors. When it rises by excess water intake a diuresis appears after 

 a certain time. This diuresis can be inhibited by emotional disturbance, by 

 epinephrine or tvramme. -•" Injections of hypertonic solutions of salt or 

 sugar which cause onlv a 2 per cent or lower rise in osmotic concentration 

 reduce the diuresis bv 90 per cent. Osmoreceptors, apparently located in the 

 brain, stimulate the liberation of antidiuretic hormone from the posterior lobe 

 of the pituitary which reduces urine production (see Ch. 22). 



Water balances were compared in three rodents, Microtiis, which inhabits 

 drv prairie, Peromvscus, which lives on the forest floor, and Blarina, which is 

 sukerranean. ^' 'Blarina shows the highest water turnover; Peromysciis 



