MAINTENANCE REQUIREMENTS OF MATTER 335 



Some of the more important factors leading to the excretion 

 of ash ingredients from the body and hence to the depletion of 

 its stock are considered in the following paragraphs : - 



426. Maintenance of osmotic pressure. In the discussion 

 of excretion in Chapter IV it was stated. (198) that the essential 

 function of the kidneys is to maintain a constant composition 

 of the blood, those organs acting somewhat like an overflow 

 valve by means of which any excess of a substance above the 

 normal limit begins to be excreted. In this way the osmotic 

 pressure in the body is regulated and an excess of any salt in 

 the feed, sodium chlorid for example, is disposed of. 



The matter is not quite so simple, however, as would appear 

 from the foregoing statement. The action of the kidneys in 

 eliminating surplus salts and so preventing an increase of osmotic 

 pressure is not confined to the particular salt supplied in excess, 

 but extends to others also. This is most strikingly shown in 

 the case of the alkalies. If, for example, unusual amounts 

 of potassium salts are consumed, an increased excretion of 

 this element results in the urine, but the need of keeping 

 the osmotic pressure at its normal level seems to be so great 

 that more or less of the sodium salts are also excreted, even 

 though their concentration in the blood may not be above the 

 normal. 



This relation as regards potassium and sodium has been 

 shown by the well-known investigations of Bunge, 1 who holds 

 that the desire for common salt on the part of herbivora is due 

 to the presence of relatively large amounts of potassium in 

 their feed and the consequent tendency towards impoverish- 

 ment of the body as regards sodium. The occurrence of salt 

 hunger in animals receiving feed with an abnormal ratio of 

 potassium to sodium has been explained in the same way. 



In other words, the effort of the body to maintain the osmotic 

 pressure of its fluids by removing a surplus of some one ingre- 

 dient may bring about an impoverishment as regards other 

 elements and so create a need for an increased supply of the 

 latter in the feed. 



427. Maintenance of neutrality. Attention was called in 

 Chapter V (271) to the fact that practical neutrality of the 

 blood serum and lymph is necessary for the normal functioning 



1 Physiologic des Menschen, 1905. 



