CHAPTER 17 Functional Organization of the Endocrine System
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Episodic hormone secretion
occurs when hormones are se-creted at fairly predictable intervals and concentrations. Thispattern is often observed in steroid reproductive hormones,which fluctuate over a month in cyclic fashion during thehuman reproductive years. Additionally, because steroid hor-mones also often have binding proteins, they have longerhalf-lives than other hormones, which contributes to their rela-tive stablity in the circulation.
13.
What kinds of activities are regulated by hormones with ashort half-life? With a long half-life?
14.
Describe chronic, acute, and episodic patterns of hormonesecretion.
17.3
Control of Hormone Secretion
LEARNING OUTCOMES
In general, lipid-soluble hormones exhibit the two regular secre-tion patterns (chronic and episodic), whereas because of their shorthalf-life, water-soluble hormones tend to exhibit the irregular (acute)secretion pattern, but there are a few exceptions. For instance, someprotein reproductive hormones exhibit episodic secretion.
After reading this section, you should be able to
A.
List and describe the three stimulatory influences onhormone secretion and give examples of each.
B.
List and describe the three inhibitory influences onhormone secretion and give examples of each.
C.
Describe the major mechanisms that maintain bloodhormone levels.
ASSESS
YOUR PROGRESS
5.
What are the three general characteristics of hormones?
6.
Explain how the half-life of a hormone relates to its stability.
7.
Why do some hormones require a binding protein duringtransport in the blood?
8.
What effect does a bound hormone have on theconcentration of a free hormone in the blood?
9.
What are the two chemical categories of hormones? Giveexamples of both types.
10.
Describe how the chemical nature of a hormone affects itstransport in the blood, its removal from circulation, and itshalf-life.
11.
What happens to the half-life when a hormone binds to abinding protein? What kinds of hormones bind to bindingproteins?
12.
Why do organs regulated by protein hormones havefenestrated capillaries?
Three types of stimuli regulate hormone release: humoral, neural,and hormonal. No matter what stimulus releases the hormone,however, the blood level of most hormones fluctuates within ahomeostatic range through negative-feedback mechanisms (seechapter 1). In a few instances, positive-feedback systems alsoregulate blood hormone levels.
Stimulation of Hormone Release
Control by Humoral Stimuli
Metabolites and other molecules in the bloodstream can directlystimulate the release of some hormones. These molecules arereferred to as
humoral stimuli
because they circulate in the blood,and the word
humoral
refers to body fluids, including blood. Thecells that secrete these hormones have receptors for certain sub-
+
stances in the blood. For example, glucose, Ca
2
, and Na
+
can stimu-late hormone secretion. When the blood level of the particularsubstance changes, the hormone is released in response to the mol-ecule’s concentration (figure 17.5). Forinstance, if a runner has just finished a longrace during hot weather, he may not pro-duce urine for up to 12 hours after the racebecause his elevated concentration of bloodsolutes stimulates the release of a water-conservation hormone called antidiuretichormone (ADH). Similarly, elevated bloodglucose levels directly stimulate insulinsecretion by the pancreas, and elevatedblood potassium levels directly stimulatealdosterone release by the adrenal cortex.
when bloodCa
2+
is too low
Osteoclast
No PTH
when bloodCa
2+
is too high
Control by Neural Stimuli
The second type of hormone regulationinvolves
neural stimuli
of endocrineglands. Following action potentials, neu-rons release a neurotransmitter into asynapse with hormone-producing cells. Inthese cases, the neurotransmitter stimu-lates the cells to secrete their hormone.
FIGURE 17.5
Humoral Regulation of Hormone Secretion