§5.311 BALANCE OF MONOVALENT ELECTROLYTES 215 



may be concerned with these processes than with osmotic regula- 

 tion. Yet osmotic regulation also requires an increase in metabolic 

 energy that might be stimulated by thyroxine (§ 5.111); it is 

 noteworthy that the thyroid is more active in conditions of 

 changing salinity than in stable conditions, even if these are 

 abnormal (D. C. W. Smith, 1956). Experimental investigation of 

 fish is clearly fraught with many difficulties; recently Jorgensen 

 and Rosenkilde (1956) have issued a timely warning, in the form 

 of observations on the very wide range of spontaneous variations 

 that occur in the chloride content of relatively undisturbed and 

 undamaged starving goldfish. 



Reptilia. Little is known of the electrolyte control in this class 

 of vertebrates, but it is attractive to speculate that secretions of the 

 adrenal cortex may act at the renal tubular level in reptiles to 

 increase potassium excretion and sodium retention (Chester Jones. 

 \9S7a). 



Mammalia. Hormones from the adrenal cortex (Fig. 5-15fl). 

 facilitate active transport of ions from the lumen of each convoluted 

 kidney tubule into its cells, the surfaces of which are increased by a 

 brush border. Reabsorption of sodium (Na + ) and potassium ions 

 (K+) from the glomerular filtrate, which is isosmotic with the 

 plasma, occurs in the proximal tubules. Chloride ions (C1-) also 

 move back into the cells in the same region, either by independent 

 active transport, or in company with the cations, because of the 

 difference in electric potential set up by the cation transport 

 across the cell surface. 



Cortical insufficiency, or removal of the adrenal cortex, is 

 followed by loss of Na+ from the plasma and tissues and its 

 excretion in the urine (Fig. 5-16 and Table 26). The sodium 



OXYTOCIN, increasing salt excretion so that endosmosis can keep 

 pace with the reduced rate of salt reabsorption ; and antidiuretic 

 HORMONE (vasopressin or ADH), increasing water reabsorption and 

 reducing urine output (antidiuresis). The reactions in the tubules 

 are similar to those in the Amphibia; but the mammal is peculiar 

 in having a urine concentrating mechanism, probably in the 

 collecting ducts (large arrows). More concentrated urine is indi- 

 cated by closer striations. 



