Page 60 - 86 human physiology part-2

The Endocrine System

No organ except the pancreas Insulin acts to lower blood sugar levels by allowing the
makes significant amounts of insulin sugar to flow into cells. Glucagon acts to raise blood sugar
or glucagon. levels by causing glucose to be released into the circulation
from its storage sites. Insulin and glucagon act in an opposite but balanced fashion to keep blood sugar
levels stable.

A healthy working pancreas in the human body is important for maintaining good health by
preventing malnutrition, and maintaining normal levels of blood sugar. The digestive tract needs the
help of the enzymes produced by the pancreas to reduce food particles to their simplest elements, or the
nutrients cannot be absorbed. Carbohydrates must be broken down into individual sugar molecules.
Proteins must be reduced to simple amino acids. Fats must be broken down into fatty acids. The
pancreatic enzymes are important in all these transformations. The basic particles can then easily be
transported into the cells that line the intestine, and from there they can be further altered and
transported to different tissues in the body as fuel sources and construction materials. Similarly, the
body cannot maintain normal blood sugar levels without the balanced action of insulin and glucagon.

The pancreas contains exocrine and endocrine cells. Groups of endocrine cells, the islets of
Langerhans, secrete two hormones. The beta cells secrete insulin; the alpha cells secrete glucagon.
The level of sugar in the blood depends on the opposing action of these two hormones.

Insulin decreases the concentration of glucose in the blood. Most of the glucose enters the cells of
the liver and skeletal muscles. In these cells, this monosaccharide is converted to the polysaccharide
glycogen. Therefore, insulin promotes glycogenesis (glycogen formation), stimulating the breakdown
of glycogen into glucose for release into the blood..

Insulin deficiency leads to the development of diabetes mellitus, specifically type I, juvenile
diabetes. As the pancreas does not produce sufficient insulin, it is treated by insulin injections. In type
II or maturity onset diabetes, the pancreas does produce enough insulin, but the target cells do not
respond to it.

As already stated, the pancreas is a mixed gland having both endocrine and exocrine functions.
The exocrine portion secretes digestive enzymes into the duodenum via the pancreatic duct. The
endocrine portion secretes two hormones, insulin and glucagon, into the blood.

Insulin is a hormone that acts directly or indirectly on most tissues of the body, with the exception
of the brain. The most important action of insulin is the stimulation of the uptake of glucose by many
tissues, particularly the liver, muscle and fat. The uptake of glucose by the cells decreases blood
glucose and increases the availability of glucose for the cellular reactions in which glucose participates.
Thus, glucose oxidation, fat synthesis, and glycogen synthesis are all accentuated by an uptake of
glucose. It is important to note that insulin does not alter glucose uptake by the brain, nor does it
influence the active transport of glucose across the renal tubules and gastrointestinal epithelium.

As stated, insulin stimulates glycogen synthesis. In addition, it also increases the activity of the
enzyme that catalyzes the rate-limiting step in glycogen synthesis. Insulin also increases triglyceride
levels by inhibiting triglyceride breakdown, and by stimulating production of triglyceride through fatty
acid and glycerophosphate synthesis. The net protein synthesis is also increased by insulin, which
stimulates the active membrane transport of amino acids, particularly into muscle cells. Insulin also has
effects on other liver enzymes, but the precise mechanisms by which insulin induces these changes are
not well understood.

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