Medical Physiology, 3rd Edition

CHAPTER 47. Organization of Endocrine Control

Eugene J. Barrett

With the development of multicellular organisms that have specialized tissues and organs, two major systems evolved to communicate and coordinate body functions:

1. The nervous system integrates tissue functions by a network of cells and cell processes that constitute the nervous system and all subdivisions, as discussed in Chapters 10 through 16.

2. The endocrine system integrates organ function via chemicals that are secreted from endocrine tissues or “glands” into the extracellular fluid. These chemicals, called hormones, are then carried through the blood to distant target tissues where they are recognized by specific high-affinity receptors. As discussed in Chapter 3, these receptors may be located either on the surface of the target tissue, within the cytosol, or in the target cell's nucleus. These receptor molecules allow the target cell to recognize a unique hormonal signal from among the numerous chemicals that are carried through the blood and bathe the body's tissues. The accuracy and sensitivity of this recognition are remarkable in view of the very low concentration (10−9 to 10−12 M) at which many hormones circulate.

Once a hormone is recognized by its target tissue or tissues, it can exert its biological action by a process known as signal transduction (see Chapter 3). Here in Chapter 47, we discuss how the signal-transduction cascades couple the hormone to its appropriate end responses. Some hormones elicit responses within seconds (e.g., the increased heart rate provoked by epinephrine or the stimulation of hepatic glycogen breakdown caused by glucagon), whereas others may require many hours or days (e.g., the changes in salt retention elicited by aldosterone or the increases in protein synthesis caused by growth hormone [GH]). We also examine the principles underlying the feedback mechanisms that control endocrine function.

In Chapters 48 through 52, we see how the principles introduced in this chapter apply to some specific endocrine systems.

 

SMALL INTESTINE

LARGE INTESTINE

Length (m)

6

2.4

Area of apical plasma membrane (m2)

~200

~25

Folds

Yes

Yes

Villi

Yes

No

Crypts or glands

Yes

Yes

Microvilli

Yes

Yes

Nutrient absorption

Yes

No

Active Na+ absorption

Yes

Yes

Active K+ secretion

No

Yes

Principles of Endocrine Function

Peptide Hormones

Amine Hormones

Steroid and Thyroid Hormones

References