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Glucose Metabolism Glucose Absorption

Physiology of Glucose Metabolism

Glucose metabolism is critical to normal physiological functioning. Glucose acts both as a source of energy and as a source of starting material for nearly all types of biosynthetic reactions.

The brain uses about 120 grams of glucose daily: 60-70% of the total body glucose metabolism. The brain has little stored glucose, and no other energy stores. Brain function begins to become seriously affected when glucose levels fall below ~40 mg/dL; levels of glucose significantly below this can lead to permanent damage and death. The brain cannot use fatty acids for energy (fatty acids do not cross the bloodbrain barrier); ketone bodies can enter the brain and can be used for energy in emergencies. The brain can only use glucose, or, under conditions of starvation, ketone bodies (acetoacetate and hydroxybutyrate) for energy.

Only 1% of pancreatic tissue is endocrine. This tissue is found in the Islets of langerhans. Surrounding the islets are adipose tissue deposits.

There are four types of cell in the islets of langerhans, alpha, beta, delta and F. Alpha and beta secreted substances involved with control of glucose, delta and F cells control the level of action of the gastrointestinal tract.

Glucose enters the blood from three sources

  • Carbohydrate containing food (grains, fruits, milk and sweets) in the gut.
  • Breakdown of complex glucose stored in the liver.
  • New synthesis of glucose in the liver

In order that the glucose entering the blood be utilised, a hormone called as insulin is required. Rise in blood glucose is sensed by specialised cells called ‘beta cells’ of pancreas, which is turn secret insulin. Insulin allows glucose to enter body cells, where it can be converted into energy.

Insulin and Pancreas

Pancreas is situated in the abdomen, behind the stomach and duodenum(first part of small intestine).

It can be broadly divided into 2 parts

  • The acini
  • The islets of Langerhans

The acini, which form major part of pancreas, secrete digestive juices into the duodenum. The islets are islands of cells interspersed in the acini are portion of the pancreas. These islands are composed of three different groups of cell: alpha(25%), beta (60%), and delta (10%) cells. Alpha cells secrete the harmone glucagon; beta cells secrete insulin, and the delta cells secrete somatostatin. Insulin and glucagon are the important hormones that regulate glucose metabolism.

The close interrelations among these cell types in the islets of Langerhans allow cell-to-cell communication and direct control of secretion of some of the hormones by the other hormones. For instance, Insulin inhibits glucagon secretion, and somatostatin inhibits the secretion of both insulin and glucagon.