THE ENDOCRINE SYSTEM 535 



259. Endocrine Interrelationships 



In the course of our discussion some of the effects of one hormone 

 on the production or action of another have been described. It is now 

 becoming clear that eacli gland affects the functioning of almost every 

 other one, and tliat they together constitute an interrelated and inter- 

 dependent system which coordinates body activities. AVhen the role of 

 the pituitary in regulating the activity of the thyroid, adrenal and gonads 

 was first discovered, the pituitary was described as a "master controlling 

 gland." But in view of the reciprocal effects of the hormones of these 

 glands on the pituitary, and of the further control of the pituitary im- 

 posed by the hypothalamus, it is probably unwarranted to regard the 

 pituitary as a special master gland. 



The interplay of estradiol, progesterone, FSH and LH in regulating 

 the menstrual cycle, and of estrogen, progesterone and prolactin in pro- 

 ducing the development and functioning of the breasts, is now well 

 established. The rate of cell metabolism and the relative rates of utiliza- 

 tion of carbohydrates, fats and proteins are under the complex control 

 of thyroxin, insulin, epinephrine, glucagon, growth hormone, hydro- 

 cortisone, estradiol and testosterone. Normal growth requires not only 

 growth hormone and thyroxin but also insulin, androgens and others. 



Hans Selye, of the University of Montreal, has done much in recent 

 years to investigate the role of hormones in adapting the body to en- 

 vironmental stresses. Stresses such as trauma, burns, cold, starvation, 

 hemorrhage, intense soimd or light and anoxia provoke a pattern of 

 adaptation which tends to resist damage from the stress. The stress 

 stimulates the release of epinephrine from the adrenal medulla, which in 

 turn leads to the release of ACTH by the anterior lobe of the pituitary. 

 The adrenal cortical hormones released by the action of the ACTH 

 produce changes in mineral and carbohydrate metabolism and in tissue 

 reactivity which adapt the animal to resist the effects of the stress. Long 

 continued stresses eventually overcome the body's adaptive ability and 

 produce exhaustion and shock. In the absence of either the hypophysis 

 or the adrenal cortex, the body's ability to tolerate stress is gieatly de- 

 creased. 



Questions 



1. Contrast the integrative effects of the nervous and endocrine systems. 



2. Define a hormone. Distinguish between a hormone and a vitamin; a hormone and an 

 enzyme. 



3. ^Vhat kinds of experiments might be used to determine whether a newly discovered 

 gland in a vertebrate secretes a hormone? 



4. Name and gi\e the functions of the hormones secreted by the mammalian thyroid, 

 parathyroid and adrenal medulla. 



5. What radioactive substance is particularly useful in studying thyroid physiolog)? 

 Why? 



6. What hormone dysfunctions result in (a) myxedema, (b) .Addison's disease, (c) diabetes 

 insipidus, (d) diabetes mellitus. (e) Cushing's syndrome and (f) tetanv? 



7. Why can thyroxin be effective when administered orally whereas insulin must be 

 injected subcutaneously? 



