2. Endocrines and Populations 205 



In summary, three mechanisms are involved in regulating the secretion 

 of aldosterone. The first and most important regulating factor is the volume 

 of the extracellular, probably intravascular, fluid, changes in which act 

 through atrial stretch receptors and other as yet unknown pathways to 

 effect reciprocal changes in the rate of secretion of aldosterone. Decreased 

 pulse pressure also stimulates increased aldosterone secretion and may be 

 one way in which changes in blood volume act. Depression of the secretion 

 of aldosterone by increases in blood volume requires an intact vagus nerve. 

 The second mechanism responds to changes in body potassium; a rise in 

 potassium resulting in elevation of the rate of secretion of aldosterone and a 

 fall in potassium permit the secretion of aldosterone to fall back to normal. 

 Finally, adrenocorticotropin, or at least a fraction thereof, is capable of 

 stimulating the secretion of aldosterone in the intact animal, but only to a 

 moderate degree and for a relatively short period of time, although the 

 secretion of aldosterone or its regulation and the functional integrity of 

 the adrenal zona glomerulosa apparently do not depend upon adrenocorti- 

 cotropin. Glomerulotropin, a recently described hormone from the pineal 

 complex region of the brain which stimulates the secretion of aldosterone, 

 may be an important link in the regulating system depending on the volume 

 of the extracellular fluid or body potassium or both, but this work requires 

 confirmation. 



The actions of aldosterone are essential in combatting incipient shock 

 mammals, and this hormone apparently plays a vital role in the daily 

 maintenance of fluid and electrolyte homeostasis. Aldosterone also may be 

 more directly responsible for maintaining blood pressure and counteracting 

 hemoconcentration through its activity in correcting alterations in blood 

 volume. 



h. The zona fasciculata. (1) The hormones. This zone of the adrenal 

 cortex normally secretes hydrocortisone (Kendall's compound F), corti- 

 costerone (Kendall's compound B), small amounts of cortisone (Kendall's 

 compound E), 11-deoxycorticosterone (Kendall's compound A), 11-de- 

 oxycorticosterone (DOC, DCA, or DOCA), ll-deoxy-17-hydrocorti- 

 costerone (Reichstein's compound S), and C19 ketosteroids, usually andro- 

 genic, the amounts and proportions depending on the species and the circum- 

 stances. Although modification of this concept is required in the light of the 

 work of Symington and his co-Avorkers (1958) (c/. above). Their experi- 

 ments indicate that the reticularis is the part of the cortex that normally 

 produces corticoids and 17-ketosteroids at rest, and that the fasciculata be- 

 comes functional with increased stimulation. In other words, the reticularis 

 is the active part of the cortex and the fasciculata is a resting portion. 

 Actually this work indicates that the morphologic separation of the cortex 

 into fasciculata and reticularis is unjustified. In addition to aldosterone, the 



