PROBLEM U What Substances Help Regulate Cell 



Activities? 



Chemical regulators. It is curious that 

 although our bodies contain dozens of 

 organs, hundreds of tissues, and billions 

 of cells, all parts normally work together. 

 A complex organism remains alive be- 

 cause all its parts work together and all 

 its cell activities go on at a proper rate, 

 sometimes more slowly, sometimes more 

 rapidly. They go on in a balanced way. 

 Why is this so? The nervous system is 

 partly responsible. But much regulating 

 is also done by chemical substances, 

 called hormones. 



Hormones, or chemical regulators, af- 

 fect the rate at which substances are 

 made and broken down in the cells. 

 Through their activities they affect the 

 activities of organs. The rate of heart 

 beat, the width of small arteries or small 

 air tubes, and the secretions of certain 

 glands are all, partially at least, regulated 

 by hormones. You have already read of 

 a hormone, secretin, which regulates the 

 activity of the secreting cells in the pan- 

 creas. The effects of some hormones may 

 be very far reaching indeed; they may 

 determine how large or small a person 

 will be when full grown or whether he 

 will be of normal intelligence. 



Ductless glands. To have such impor- 

 tant effects these hormones must reach 

 the cells in all parts of the body. If this 

 seems difficult, remember that there is 



one substance, blood, that does get close 

 to all cells. Hormones simply travel in 

 the blood. They diffuse directly into the 

 blood from the glands that produce them. 

 For this reason hormones may be defined 

 as chemical "messengers" or regulators 

 made in one part of the body and pro- 

 ducing their effects in another part. The 

 glands which secrete hormones are called 

 ductless glands or glands of i?iternal se- 

 cretion as distinguished from glands with 

 ducts such as sweat glands or gastric 

 glands. Another name is endocrine (en'- 

 doh-creen) glands. The hormones may 

 be called internal secretions because the 

 secretions go directly from the gland 

 into the blood. 



The thyroid gland and its secretion. 

 One of the best known of the ductless 

 glands is the thyroid gland, which lies in 

 the neck just below the voice box (lar- 

 ynx). The gland is about two inches 

 long. See Figure 239. The hormone pro- 

 duced by the thyroid gland is called 

 thyroxifi. Thyroxin was first extracted 

 from the gland; later its chemical compo- 

 sition was discovered and finally it was 

 made in the laboratory. Thyroxin is 

 a compound containing comparatively 

 large amounts of the element iodine. 

 Iodine gets into the body in the form of 

 chemical compounds that are found both 

 in food and in drinking water. 



