OSMOSIS 199 



water engulfed with food is held for a time in food vacuoles, later 

 diffusing into the protoplasm and ultimately finding its way into 

 the contractile vacuole. This vacuole can, therefore, be com- 

 pared with the kidney of higher organisms. Other types of 

 smaller vacuoles may serve a similar purpose. 



The contractile vacuole may fill by osmosis, but the mechanism 

 of contraction (emptying) is not known. Furthermore, for 

 continued operation, the concentration within must remain 

 higher than that without, yet, as the vacuole enlarges, during 

 diastole, its contents become diluted. If a contractile vacuole 

 fills by osmosis, then why at a definite point does it suddenly 

 empty itself? We do not know. Syneresis (page 146) or 

 electroendosmosis (page 358) or membrane equilibria (page 203) 

 may be involved. 



The role of osmosis is prominent within the animal body and 

 constitutes an important study in medicine, as is evident from 

 the work of R. Hober on the kidney and other adenoid (gland- 

 like) tissues. The osmotic pressure of the blood is 7 atmospheres. 

 The extent to which water will enter or leave the blood stream is 

 in great measure determined by this value. The pH (acidity) 

 of the blood is 7.38. The pH of the gastric juice is from 1 to 2. 

 Osmotic work must be done in concentrating H+ ions in the 

 gastric juice to the extent of a million times that in the blood 

 stream. 



The kidney is an osmotic machine, as are also the sweat glands 

 of animals and the water-secreting glands (hydathodes) of 

 plants. The functioning of the kidney, like that of most glands 

 {e.g., the nectaries of flowers), may be primarily osmotic, but 

 other forces are certainly involved, for a diffusion takes place 

 against a concentration gradient. The concentration of urea in 

 urine is sixty times that in the blood. The mechanism responsi- 

 ble for this cannot be solely osmotic; certain other forces (again 

 possibly electroendosmosis) must be active. 



