160 E. Kerpel-Fronius 



(b) Water deprivation quickly exhausts the water reserves 

 which are low in relation to metabolism and, consequently, to 

 obligatory urine volume and dermal loss of w^ater. 



(c) Because of the high cardiac output required for meta- 

 bolic processes, and the low reserves of water to guarantee its 

 maintenance, circulation is endangered by even smaller water 

 deficits, the more so since water losses occur rapidly. It will 

 be remembered that the small plasma volume of the infant 

 relative to the cardiac output is compensated for by a high 

 pulse rate to ensure adequate circulation. 



(d) The vulnerability of the circulation facilitates a rapid 

 decrease in renal clearances, which even in the healthy infant 

 are low in relation to his high metabolic rate. Obviously, the 

 infant's rather poor renal blood flow is adjusted to, and only 

 maintained by a relatively high cardiac output. The renal 

 fraction has been calculated to be 10 per cent of the total 

 output of the heart in infants whereas it is 20 per cent in 

 adults. 



As pointed out by McCance and Widdowson (1957) stagna- 

 tion of growth plays a role in the easily disturbed equilibrium. 

 In a growing animal a certain amount of the food goes to the 

 building of its tissues. If growth is arrested, an additional 

 solute load formed by this fraction of the intake presents 

 itself for excretion by the kidneys. This will result either in a 

 higher urine volume, or, if the kidneys are incompetent, in 

 hyperelectrolytaemia and azotaemia. McCance and Widdow- 

 son (1957) have shown that these effects are striking in fast- 

 growing animals and may under certain circumstances be of 

 importance to the human infant. On the basis of some of the 

 data compiled by the American Academy of Pediatrics (1957) 

 an estimate has been made of the effect of arrested growth on 

 solute load and renal water expenditure. Solute load may be 

 expected to rise 13 per cent in the infant who is fed on cow's 

 milk, and 57 per cent in the breastfed child, causing a con- 

 siderable increase in urine volume. When at the same time 

 extrarenal water expenditure is increased by high environ- 

 mental temperature, or diarrhoeal losses, the water balance 



