HORMONES 



287 



the organism to adjust its metabolic activities so as to cope with changes 

 in the outside environment and to maintain a stable internal condition. 

 The system operates for the most part automatically. Thus, for example, 

 an increase in the blood glucose level stimulates the pancreas to secrete 

 more insulin, which promotes utilization of the sugar and hence brings 

 the concentration down again. The pituitary produces, among others, 

 hormones which stimulate gro-wth and activity of the thyroid, ovaries, 

 testes, and adrenal cortex. The characteristic hormones of these glands 

 depress pituitary function. In general, the rate of hormone production 

 is controlled either by other hormones, by various other chemical sub- 

 stances in the body, or to a lesser degree by nervous stimulation originat- 

 ing in the external environment. 



Hormone metabolism and function 



The smooth operation of various bodily processes often involves the 

 concerted action of a whole series of hormones. For example, carbo- 

 hydrate metabolism cannot proceed normally without the help of hor- 

 mones from the pancreas, pituitary, thyroid, and adrenal cortex. Sexual 

 reproduction in mammals depends on the hormones of the ovaries, testes, 

 pituitary, adrenal cortex, and, to some extent, the thyroid. 



Just how the observed effects are brought about is, in most cases, not 

 known. Presumably, since they are active in very small amounts, the 

 hormones must act through certain enzyme systems {e.g., effect of 

 epinephrine on phosphorylase, p. 289). Likewise the metabohc fate of 

 the hormones, that is, what becomes of them after being secreted into 

 the blood, is still largely unknown, although some of the steroid hormones 

 have been found recently to be converted into modified products and 

 excreted in the urine. 



With this general introduction, attention will now be directed to the 

 individual endocrine organs and the hormones they produce. 



NONPROTEIN HORMONES 



Hormones of the adrenal medulla 



Epinephrine (adrenalin) and norepinephrine (noradrenalin or arterenol) 

 are the hormones produced by the adrenal medulla, the inner part of a 

 small endocrine gland located just above each kidney. About five to 

 six times as much of the former substance is normally formed by the 

 adrenal as of the latter. These two hormones are almost certainly syn- 

 thesized from phenylalanine in the body. The biosynthetic pathway used 

 is not entirely known, but is probably somewhat as follows: 



