Human Physiology, 14th edition (2016)

346 Chapter 11

Pineal Gland

The small, cone-shaped pineal gland is located in the roof of

the third ventricle of the diencephalon (chapter 8), where it is

storage. Insulin thereby causes glucose to leave the plasma and

enter the target cells, where it is converted into the energy stor-

age molecules of glycogen (in skeletal muscles and liver) and

fat (in adipose tissue). Through these effects, insulin lowers the

blood glucose concentration ( fig. 11.31 a ) as it promotes anabo-

lism (chapter 19, section 19.3). The ability of the beta cells to

secrete insulin, and the action of insulin to lower the plasma

glucose concentration, are tested in an oral glucose tolerance

test for diabetes mellitus (chapter 19, section 19.4).

Glucagon, secreted by the alpha cells of the pancreatic

islets, acts antagonistically to insulin—it promotes effects that

raise the plasma glucose concentration. Glucagon secretion is

stimulated by a fall in the plasma glucose concentration and

insulin secretion that occurs when a person is fasting. Under

these conditions, glucagon stimulates the liver to hydrolyze gly-

cogen into glucose (a process called glycogenolysis ), allowing

the liver to secrete glucose into the blood ( fig. 11.31 b ). Gluca-

gon, together with the glucocorticoid hormones, also stimulates

gluconeogenesis —the conversion of noncarbohydrate mol-

ecules into glucose—to help raise the plasma glucose level dur-

ing times of fasting. In addition, glucagon and other hormones

promote other catabolic effects, including lipolysis (the hydro-

lysis of stored fat) and ketogenesis (the formation of ketone

bodies from free fatty acids by the liver). These free fatty acids

and ketone bodies serve as energy sources for cell respiration

during times of fasting.

Aorta

Tail of pancreas

Body of pancreas

Pancreatic duct

Duodenum

Common bile duct

Gallbladder

Celiac

artery

Pancreatic islet

(of Langerhans)

Beta cell

Alpha cell

Figure 11.29 The pancreas and

associated pancreatic islets (islets of

Langerhans). Alpha cells secrete glucagon and

beta cells secrete insulin. The pancreas is also

exocrine, producing pancreatic juice for transport

via the pancreatic duct to the duodenum of the

small intestine.

CLINICAL APPLICATION

Diabetes mellitus is characterized by fasting hyperglyce-

mia and the presence of glucose in the urine. There are two

major forms of this disease. Type 1 diabetes, once called

insulin-dependent diabetes mellitus, is caused by destruc-

tion of the beta cells and the resulting lack of insulin secre-

tion. Type 2 diabetes, once called non-insulin-dependent

diabetes mellitus, is the more common form. It is caused

largely by insulin resistance, or decreased tissue sensitivity

to the effects of insulin, so that larger than normal amounts

of insulin are required to produce a normal effect. The

causes and symptoms of diabetes mellitus are described in

more detail in chapter 19, section 19.4.

Gestational diabetes occurs in about 4% of pregnan-

cies due to insulin secretion that is inadequate to meet the

increased demand imposed by the fetus. Pregnant women

who do not develop gestational diabetes have a sufficiently

increased insulin secretion, probably due to the proliferation

of beta cells in the islets.