511
CHAPTER THIRTEEN
Endocrine System
Neurons, including those of the brain, obtain glucose by
a facilitated diffusion mechanism not dependent on insulin,
but rather only on blood glucose concentration. For this rea-
son, neurons are particularly sensitive to changes in blood
glucose concentration. Conditions that cause such changes—
excess insulin secretion, for example—are likely to affect
brain functions.
At the same time that insulin concentration is decreas-
ing, glucagon secretion is increasing. Therefore, these hor-
mones function together to maintain a relatively constant
blood glucose concentration, despite great variations in the
amounts of ingested carbohydrates.
Somatostatin
(similar to the hypothalamic hormone),
which the delta cells release, helps regulate glucose metabo-
lism by inhibiting secretion of glucagon and insulin.
Table
13.12
summarizes the hormones of the pancreatic islets, and
Clinical Application 13.4 and From Science to Technology
13.1 discuss diabetes mellitus, a derangement of the control
of glucose metabolism that affects millions of people.
PRACTICE
39
Name the endocrine portion of the pancreas.
40
What is the function of glucagon?
41
What is the function of insulin?
42
How are the secretions of glucagon and insulin controlled?
43
Why are nerve cells particularly sensitive to changes in blood
glucose concentration?
13.10
OTHER ENDOCRINE GLANDS
Additional organs produce hormones. These are part of the
endocrine system too. They include the pineal gland; the
thymus; reproductive glands; and certain cells of the diges-
tive tract, the heart, and the kidneys.
Pineal Gland
The
pineal gland
(pin
e-al gland) is a small, oval structure
deep between the cerebral hemispheres, where it attaches to
the upper portion of the thalamus near the roof of the third
ventricle. It largely consists of specialized
pineal cells
and
supportive neuroglia (see ±
g. 11.20
b
).
FIGURE 13.36
Insulin and glucagon function together to stabilize
blood glucose concentration. Negative feedback responding to blood
glucose concentration controls the levels of both hormones.
too high
too low
Normal
blood glucose
concentration
Control center
Beta cells secrete
insulin
Control center
Alpha cells secrete
glucagon
Stimulus
Rise in blood glucose
Effectors
Insulin
• Promotes movement
of
glucose into certain cells
• Stimulates formation of
glycogen from glucose
Response
Blood glucose drops toward
normal (and inhibits insulin
secretion)
Receptors
Beta cells detect a rise
in blood glucose
Receptors
Alpha cells detect a drop
in blood glucose
Effectors
Glucagon
• Stimulates cells to break down
glycogen into glucose
• Stimulates cells to convert
noncarbohydrates into glucose
Stimulus
Drop in blood glucose
Response
Blood glucose rises toward
normal (and inhibits glucagon
secretion)
TABLE
13.12
|
Hormones of the Pancreatic Islets
Hormone
Action
Source of Control
Glucagon
Stimulates the liver to break down glycogen and convert noncarbohydrates into glucose; stimulates
breakdown of fats
Blood glucose concentration
Insulin
Promotes formation of glycogen from glucose, inhibits conversion of noncarbohydrates into glucose, and
enhances movement of glucose through adipose and muscle cell membranes, decreasing blood glucose
concentration; promotes transport of amino acids into cells; enhances synthesis of proteins and fats
Blood glucose concentration
Somatostatin
Helps regulate carbohydrates
Not determined
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