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Powerpoint Presentation: Regulating Blood Sugar

 

Homeostasis Index

Homeostasis : Introduction
The hormone pathway
Thermoregulation
Homeostatic control of water balance

Topic Chapters Index

 

The Pancreas

Anatomy

The pancreas is an exocrine gland secreting alkaline digestive juices. into the duodenum via the pancreatic duct. It also contains endocrine cells in the Islets of Langerhans.

 

Islets of Langerhans

An oval collection of cells (75 x l75μm), scattered through the pancreas. There are about 1 to 2 million islets in a human pancreas comprising approximately 1 to 2% of its mass. They are darker red than the surrounding tissue having a rich blood supply. The blood from the islets drains into the Hepatic Portal Vein going directly to the liver.

They contain three types of cells

  • alpha cells. (20%) secrete glucagon

  • beta cells (75%) secrete insulin

  • delta cells (1 to 8%)

 

 

HOMEOSTASIS

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The Pancreas, Insulin and Glucagon Fact Sheet

Insulin and Glucagon

INSULIN

GLUCAGON

A protein made of two short polypeptide chains linked by disulphide bridges. It is synthesised at the ribosome as a single polypeptide (proinsulin) and is later activated by enzymatic cleavage into the two chains.

A small protein made up of 29 amino acids. It is made by the alpha cells in the Islets. It is also made in an inactive form proglucagon before it is released.

Action of Insulin

Action of Glucagon

Increases the entry of glucose into the body cells. Notable exceptions being brain tissue, red blood cells, kidney tubule. and intestinal lining.

 

Inhibits glycogen breakdown in liver and muscle.

Glucagon binds to liver cells, stimulating the breakdown of glycogen into glucose. It does not affect glycogen in muscle tissue.

Inhibits lipid breakdown in liver and adipose tissue.

Increases lipid breakdown though its influence is small.

Increases the uptake of amino acids by cells and increases the rate of protein synthesis (here it is acting as a growth hormone)

Stimulates the formation of glucose from amino acids in the liver.

The action of insulin is very rapid, so is its breakdown (t½ = 10 to 25 min). Once released into the blood stream insulin binds with the receptor sites on its target cells' plasma membrane. This stimulates vesicles carrying glucose pores which lie in the cytoplasm of these cells, to fuse with the plasma membrane. Insulin is broken down by enzyme action in many tissues.

Glucagon is also very short lived (t½ = 5 to 10 min). Glucagon is broken down particularly by the liver tissue. As the hormone is secreted into the blood flowing into the liver little glucagon is seen circulating in the rest of the body.

Control of Insulin Secretion

Control of Glucagon Secretion

Because it has a rapid half-life the islet cells tend to respond more often to stimulating rather than inhibiting chemicals in the plasma. These include: Glucose, amino acids, glucagon,

Glucagon levels rise between meals as the glucose and fatty acid levels in the blood fall.

Secretion is also stimulated by amino acids and hormones secreted by the digestive system.

In many ways it is antagonistic to insulin.

insulin glyogen relationship

 

The Homeostatic Control of Blood Glucose Levels

HOMEOSTATIC CONTROL OF BLOOD GLUCOSE LEVELS

 

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