Human Physiology, 14th edition (2016)

Endocrine Glands 345

11.6 Pancreas and Other Endocrine Glands

The pancreatic islets secrete two hormones, insulin and glu-

cagon. Insulin promotes the lowering of blood glucose and

the storage of energy in the form of glycogen and fat. Glu-

cagon has antagonistic effects that raise the blood glucose

concentration. Additionally, many other organs secrete hor-

mones that help to regulate digestion, metabolism, growth,

immune function, and reproduction.

Figure 11.27 A posterior view of the parathyroid
glands. The parathyroids are embedded in the tissue of the
thyroid gland.

Pharynx

Thyroid gland

Parathyroid

glands

Esophagus

Trachea

LEARNING OUTCOMES

After studying this section, you should be able to:

12. Identify the endocrine cells and hormones of
    the pancreas, and describe the actions of these
    hormones.


13. Identify the pineal gland and its hormone, and
    describe its effect.


14. Describe the endocrine functions of the gonads and
    placenta.

Figure 11.28 The actions of parathyroid hormone
and the control of its secretion. An increased level of
parathyroid hormone causes the bones to release calcium and
the kidneys to conserve calcium that would otherwise be lost
through the urine. A rise in blood Ca^1 can then exert negative
feedback inhibition on parathyroid hormone secretion.

Decreasing blood

Ca2+

Parathyroids

Parathyroid

hormone

Kidneys Bone

Reabsorption

of Ca2+

Dissolution of

CaPO 4 crystals

Decreased urinary

excretion of Ca2+

Increased blood

Ca2+

Negative

feedback

Sensor

Integrating center

Effector

The pancreas is both an endocrine and an exocrine gland. The

gross structure of this gland and its exocrine functions in diges-

tion are described in chapter 18, section 18.5. The endocrine

portion of the pancreas consists of scattered clusters of cells

called the pancreatic islets or islets of Langerhans. The human

pancreas contains approximately one million islets, which are

most common in the body and tail of the pancreas ( fig. 11.29 ).

Pancreatic Islets (Islets of Langerhans)

On a microscopic level, the most conspicuous cells in the islets are

the alpha and beta cells ( fig.  11.29 ). The alpha cells secrete the

hormone glucagon, and the beta cells secrete insulin. A human

islet contains about 50% beta cells, 35% to 40% alpha cells, and

10% to 15% delta cells, which secrete the hormone somatostatin.

Insulin is the only hormone that acts to lower the blood

glucose concentration. After a carbohydrate meal or sugary

drink, the plasma glucose level rises. This rise in plasma glu-

cose stimulates the beta cells of the islets to secrete increased

amounts of insulin. Insulin then binds to its receptors in the

plasma membrane of its target cells, and, through the action

of signaling molecules, causes intracellular vesicles con-

taining GLUT4 carrier proteins to translocate to the plasma

membrane ( fig.  11.30 ). These carrier proteins promote the

facilitated diffusion of glucose into the cells of insulin’s tar-

get organs—primarily the skeletal muscles, liver, and adi-

pose tissue.

Also, insulin indirectly stimulates the activity of the enzyme

glycogen synthetase in skeletal muscles and liver, which pro-

motes the conversion of intracellular glucose into glycogen for