Chapter VIII _137_ Active Relations 



group. When mechanically or electrically stimulated its density increases, 

 and this is accompanied by an excretion of acid. Thus there seemed to be 

 some relation between the distribution of acid in the cell (around pH 3) 

 and its ability to float or sink (Gross, 1934). Harvey (1917) found that 

 cyanide treatment or absence of oxygen did not alter the density of the 

 organisms, indicating that respiratory energy is not a factor. 



The contractile vacuole of many fresh water protozoa is believed to 

 function as an osmoregulatory device (review by Kitching, 1938). Ap- 

 parently it is able to control body volume by excreting water (or a very 

 dilute solution) as fast as it enters by osmosis through the surface mem- 

 brane much as water may be pumped from a leaky vessel. That cyanide 

 slows the vacuolar rhythm and results in swelling may be taken to mean 

 that osmotic work is involved. Transfer to hypertonic solution causes 

 slowing or cessation of vacuolar activity. 



Beadle (1934) studied the water relations of Giinda ulvae, a small 

 worm living in estuaries and adapted to existence both in fresh and sea 

 water. He concluded that on change from sea water to fresh, water enters 

 through the skin and the animal swells. The additional water is absorbed 

 by cells of the gut and secreted into intercellular vacuoles by expenditure 

 of energy. Following this original restoration of balance, the ectoderm is 

 believed to inhibit further entry of water because of a reduced permeability. 

 The latter is visualized as an "osmotic resistance" of the ectoderm cells 

 similarly dependent on energy. The secretion of water into vacuoles seems 

 to resemble that of plant cells postulated by Bennet-Clark, et al. (1936). 



Krogh states that in higher aquatic animals, most of the known osmo- 

 regulatory functions are confined to a more or less advanced type of kidney. 

 In some organisms extra-renal excretion apparently occurs. One form of 

 kidney characteristic of fresh water organisms produces a urine markedly 

 hypotonic to the blood, sometimes nearly as dilute as the surrounding water ; 

 another type, as in mammals, is able to produce a concentrated urine hyper- 

 tonic to the blood. 



Other organisms adapted to changing environment are certain teleost 

 fishes. Study of the eel Anguilla vulgaris (Keys, 1932) revealed mechan- 

 isms by which the animal is able to maintain a fairly constant internal con- 

 centration when changed from fresh to sea water, and vice versa. Fresh 

 water is hypotonic, sea water hypertonic. Keys states that in the latter, 

 water is swallowed and the excess sodium chloride is subsequently excreted 

 by the gills. In fresh water the kidney is capable of filtration, and salt 

 conservation. 



According to Adolph (1930), a skinless frog may act more or less as 

 a perfect osmometer. To the skin is assigned a role of active water trans- 

 port, water passing inward more readily than outward. 



In man the kidney is believed to perform a highly specialized type of 

 osmotic work. According to common understanding the process involves 

 filtration by the glomeruli and subsequent absorption of water by the 

 tubules. The latter operate against an osmotic gradient. Other examples 

 of active water transfer may be recognized in the function of the salivary 

 glands and possibly the intestine. 



The part played by hydrophilic colloids in the water balance of man 

 has been stressed by some investigators (Fischer, 1921 ; Gortner, 1938). 

 For example, in many instances transfer of water from one part of the 

 body to another is explained by imbibitional phenomena. Since the ability 



