G

gastric inhibitive polypeptide (GPI) See DIGES-
TIVE HORMONES.

gastrin See DIGESTIVE HORMONES.

glucagon A peptide HORMONEthe alpha cells of
the ISLETS OFLANGERHANSproduce in response to
low blood GLUCOSE levels (humoral regulation).
Intense physical exercise also causes the release of
glucagon into the bloodstream. Glucagon stimu-
lates the LIVERto convert glycogen (a storage form
of glucose) into glucose to raise the level of glu-
cose in the BLOOD. Glucagon opposes the action of
INSULIN(which stimulates glucose use or conver-
sion to glycogen to decrease blood glucose levels).
High blood glucose levels inhibit glucagon produc-
tion as does release of SOMATOSTATIN, another pep-
tide hormone synthesized by the HYPOTHALAMUS,
gastrointestinal tract, and delta cells of the islets of
Langerhans. Glucagon is also available as a phar-
maceutical DRUGused to treat HYPOGLYCEMIA(low
blood glucose) and insulin shock.
For further discussion of glucagon within the
context of the endocrine system’s structure and
function please see the overview section “The
Endocrine System.”
See also DIABETES; DIGESTIVE HORMONES; LIVER;
METABOLISM.

glucose A simple sugar molecule (monosaccha-
ride) that is the primary energy source for most
cells in the body. The body metabolizes (breaks
down) carbohydrates, which enter the body from
the gastrointestinal tract, into glucose. The body
requires a fairly narrow but constant level of
available glucose circulating in the BLOODat all
times (70 to 110 milligrams of glucose per deciliter
of blood). Glucose and two hormones the ISLETS OF

LANGERHANSin the PANCREASproduce, INSULINand

GLUCAGON, function in synchronization and oppo-

sition to maintain this level.

Rising glucose levels in the blood signal the

islets of Langerhans to secrete insulin. The insulin

binds with receptors on cell membranes, enabling

glucose to enter the cells. Within the cell the glu-

cose causes a series of biochemical actions that

result in the formation of adenosine triphosphate

(ATP), which fuels cellular METABOLISM. Insulin (in

coordination with CORTISOL) also stimulates the

LIVERto extract excess glucose from the blood and

convert it to glycogen, a storage form of glucose

the liver then deposits within its tissues as well as

within MUSCLEtissue throughout the body. When

the blood glucose level drops, the islets of Langer-

hans release glucagon, a hormone that causes the

liver to convert glycogen back to glucose and

release it into the blood circulation. Glucagon also

stimulates DIGESTIVE HORMONES and enzymes that

result in the sensation of hunger, encouraging

food intake that can more rapidly replenish the

body’s glucose supply.

FASTING BLOOD GLUCOSE LEVELS

Health Condition Milligrams of Glucose per

Deciliter of Blood

hypoglycemia < 50

normal 70–110

hyperglycemia (prediabetes) 111–125

DIABETES > 126

See also CELL STRUCTURE AND FUNCTION.

goiter Swelling and enlargement of the THYROID

GLAND. Most often goiter occurs as a symptom of

thyroid dysfunction though can develop when

thyroid function is normal, such as sometimes

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