406 - Multicellular Animals, Especially Man 



a clearer picture of the physiological role of 

 the parathyroid glands. They exert homeo- 

 static control over the Ca 2 + ion content of 

 the body fluids, both blood and lymph. The 

 parathyroid glands, in fact, have been called 

 the calciostats of the body. This is a very im- 

 portant role, since cells in general — but nerve 

 and muscle tissues in particular — are very 

 sensitive to the Ca-+ ion level of the internal 

 environment. Also a proper Ca-+ ion level 

 is essential to the maintenance of the mineral 

 content of bone, which is mainly a cal- 

 cium-phosphate complex, called hydroxy- 

 apatite. 



The primary target organs of the parathy- 

 roid hormone are the intestines, the kidneys, 

 and certain ol the cells in bone tissue. More- 

 over, a release of hormone by the glands is 

 triggered when a chop occurs in the level of 

 Ca 2 + ion in the circulating fluids. Stimu- 

 lated by the parathyroid secretion, the in- 

 testinal mucosa absorbs more calcium, the 

 kidney tubules reabsorb more calcium but 

 less phosphate, and the bone-eroding cells, 

 or osteoclasts, become more numerous and 

 more active in decomposing the mineral de- 

 posits of the bone, thus liberating calcium 

 and phosphate into the < irculation. The main 

 effect, accordingly, is to raise the circulating 

 Ca 2 + ion level, although simultaneously 

 the phosphate level tends to fall because of 

 the extra phosphate excretion by the kidney. 

 This, in fact, more than compensates for the 

 influx of phosphate from the bone. 



A direct feed-back control (p. 412) of ac- 

 tivity in the parathyroid glands appears to 

 be operative. An excess of Ca 2 + ion in the 

 body fluids tends to suppress the mobilization 

 of parathyroid hormone by the gland. More- 

 over, it has been found that the parathyroid 

 hormone fails to function effectively in cases 

 of vitamin D deficiency. 



THE ADRENAL GLANDS 



In man, each adrenal gland perches like a 

 small cap on the upper end ol the corre- 

 sponding kidney (Fig. 22-6). Both glands 



ADRENAL GLAND 



CORTEX 

 MEDULLA 



KIDNEY 



URETER 



Fig. 22-6. The adrenal gland, perched atop the 

 kidney, produces an assortment of important hormones 

 in its cortex and adrenalin in its medulla. 



together weigh less than one ounce, but the 

 adrenal blood supply is very copious. 



Each adrenal is a duplex gland that in 

 section displays a dark-brown central core, 

 the medulla, surrounded by a paler shell of 

 tissue, the cortex (Fig. 22-6). These two dis- 

 tinct parts of the adrenal arise separately in 

 the embryo: the cortex from the mesodermal 

 lining ol the coelom; and the medulla from 

 an ectodermal outgrowth of the neural tube. 

 Likewise the cortex and medulla have dis- 

 tinctly different functions in the adult body, 

 and, in fact, an animal survives quite well 

 without any adrenal medulla, but cannot 

 live in the absence of all cortical tissue. 



The Adrenal Medulla. Adrenalin," or epi- 

 nephrin (C<jH 13 3 N), was isolated quite early 

 (1904). Adrenalin has been synthesized arti- 

 ficial!)', and chemists have likewise produced 

 several other drugs, such as ephedrin, that 

 exert a comparable action in the body. 



The physiological effects of an adrenalin 

 injection are widespread and potent. The 



5 In addition to adrenalin, the adrenal medulla 

 produces smaller quantities of a related compound, 

 noradrenalin, which lacks a terminal CH 3 radical and 

 has slightly dilfercnt physiological effects. 



