§ 5.322 WATER BALANCE 239 



hypertonic on the treated side, while remaining iso- or even 

 hypotonic on the other side. 



This concentration may or may not be hmited to the collecting 

 ducts rather than occurring in the renal tubules (Fig. 5-15). It is 

 certainly limited to a maximal rate of water uptake, no matter what 

 volume of fluid is delivered to it from the tubules ; it is also limited to 

 a maximal osmotic concentration of the urine, relative to, but con- 

 siderably above, that of the plasma. "The degree of urinary concen- 

 tration in the mammal is determined . . . largely by the volume of 

 water delivered to this concentrating mechanism " (Sawyer, 1957). 



The natural secretion of the antidiuretic hormone can be brought 

 about by stimulation of osmo-receptors in the anterior hypo- 

 thalamus of the brain, a region supplied by branches from the 

 internal carotid arteries. It has been shown in dogs that infusing 

 these branches of the carotids for about half an hour with suffi- 

 cient saline to increase the osmotic pressure of the blood by some 

 2 per cent can reduce the urine flow by 90 per cent. This treatment 

 is much more eff"ective than giving similar saline infusions else- 

 where in the circulation (Jewell and Verney, 1957). The presence 

 of the saline presumably has the same eff'ect as tissue dehydration ; 

 yet it can be shown that although ADH injection into an isolated 

 heart-lung preparation causes antidiuresis it does not reduce the 

 G.F.R., whereas natural dehydration does both. The factor 

 controlling G.F.R. in mammals is not known, since recent work 

 all points to the vascular constrictor, "vasopressin", being one 

 and the same hormone as ADH. 



Haemorrhage also stimulates secretion of ADH, possibly by 

 affecting vagal nerve endings sensitive to hydrostatic pressure in 

 the heart or the great veins (Heller, 1956). 



Sawyer (1957) wrote: "Diuresis and antidiuresis can ... be ex- 

 plained in the mammal simply in terms of an increase in perme- 

 ability of the distal segment to water under the influence of the 

 antidiuretic hormone. This increase in permeability, since it 

 involves reabsorption of water only up to the isosmotic state, 

 could be interpreted in terms of a change in pore size, as the 

 evidence indicates to be the case in the [frog] skin, and as we have 

 inferred to be the case in the frog bladder" and kidney. But this 

 statement was made prior to the publication of Chester Jones's 



