HUMAN BIOLOGY

hoW do endocrine cells in pancreatic

islets regulate blood sugar?

• Alpha cells secrete glucagon when the blood level of glucose
  (sugar) falls below a set point. Beta cells secrete insulin when
  blood levels of glucose rise above the set point.


• Somatostatin from delta cells regulates the functioning of
  alpha and beta cells.

taKe-home message

pancreatic islets Clusters
of endocrine cells in the
pancreas.

a lesser extent, proteins, and inhibits the conversion

of proteins to glucose. In this way insulin lowers the

glucose level in the blood.

3. Delta cells secrete somatostatin. This hormone acts on
   beta cells and alpha cells to inhibit secretion of insulin
   and glucagon, respectively. Somatostatin is part of
   several hormone-based control systems. For example,
   it is released from the hypothalamus to block secretion
   of growth hormone; it is also secreted by cells of the
   GI tract, where it acts to inhibit the secretion of various
   substances involved in digestion.
   Even with all the variations in when and how much we
   eat, pancreatic hormones help keep blood glucose levels
   fairly constant (Figure 15.10).

alpha cells

glucagon

Body cells, especially in muscle and

adipose tissue, take up and use more

glucose.

Cells in skeletal muscle and liver store

glucose in the form of glycogen.

insulin

beta cells alpha cells

glucagon

Stimulus

Increase in blood glucose

Stimulus

Decrease in blood glucose

insulin

beta cells

Cells in liver break

down glycogen faster.

The released glucose

monomers enter blood.

++

Response

Decrease in blood glucose

Response

Increase in blood glucose

Pancreas

X– X–

Liver Muscle Fat cells

2

1

3

4

5

6

7 8

9

10

Figure 15.10 Animated! cells that

secrete insulin and glucagon respond

to a change in the level of glucose

in blood. These two hormones work

antagonistically to maintain normal

blood sugar levels.

1 After a meal, the blood level of

glucose increases. In the pancreas,

the increase 2 stops alpha cells from

secreting glucagon and 3 stimulates

beta cells to secrete insulin. In response

to insulin, 4 adipose and muscle cells

take up and store glucose, and liver

cells make more glycogen. As a result,

insulin lowers blood sugar 5.

6 Between meals, blood sugar falls.

The decrease 7 stimulates alpha cells

to secrete glucagon and 8 slows the

insulin secretion by beta cells. 9 In the

liver, glucagon causes cells to convert

glycogen back to glucose, which enters

the blood. As a result, glucagon raises

blood sugar 10. (© Cengage Learning)

15.9

n The pancreas hormones insulin and glucagon work

antagonistically—the action of one opposes the action of

the other. Controls over the release of these hormones

regulate the glucose level in blood.

n    Link to Accessory organs of digestion 11.5

The pancreas has both exocrine and

endocrine functions. Its exocrine

cells release digestive enzymes into

the small intestine. It also has some

2 million scattered clusters of endo-

crine cells. Each cluster is a pancreatic islet and contains

three types of hormone-secreting cells:

1. Alpha cells secrete glucagon. Between meals, cells use
   the glucose delivered to them by the bloodstream.
   When the blood glucose level decreases below a set
   point, secreted glucagon acts on cells in the liver and
   muscles. It causes glycogen (a storage polysaccharide)
   and amino acids to be converted to glucose. In this way
   glucagon raises the glucose level in the blood.


2. Beta cells secrete the hormone insulin. After meals,
   when a lot of glucose is circulating in the blood, insulin
   stimulates muscle and adipose cells to take up glucose.
   It also promotes synthesis of fats, glycogen, and to

the pancreas: regulating Blood sugar

298 Chapter 15

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