Homeostasis: Blood Sugar

Question 1

Regulation of Blood Sugar

Answer 1

• After a meal, blood glucose concentration can rise sharply.
• Homeostatic mechanisms then begin to operate to reduce the blood glucose concentration and maintain it at the normal level.
• Any excess glucose in the blood must be removed and stored ready for use in cellular activities between meals.
• Glucose is stored in the liver and muscle cells as glycogen, a molecule made of long chains of glucose molecules.
• The pancreas and adrenal glands secrete hormones that affect the level of glucose in the blood.

Question 2

Role of the Liver

Answer 2

• The liver converts:
• glucose to glycogen for storage,
• glycogen to glucose for release into the blood.
• Most of the liver’s blood supply comes through the hepatic portal vein that brings blood directly from the stomach, spleen, pancreas and small and large intestines.
• Thus, the liver has the first chance to absorb any of the nutrients from digested food.
• Any number of things may occur to the glucose carried to the liver:
• Glucose may be removed from the blood by the liver to provide energy for liver functioning.
• It may be removed by the liver and/or muscles and converted into glycogen for storage.
• It may continue to circulate in the blood where it is available for body cells to absorb and use as a source of energy.
• Glucose in excess of that required to maintain both the normal blood sugar level and the tissue glycogen level is converted to fat for long-term storage.
• The body is able to store about 500 grams of glycogen:
• about 100 grams is stored in the liver
• the remainder in skeletal muscle cells.
• Glucose molecules are chemically combined in long chains to form glycogen molecules.
• This process, known as glycogenesis, is stimulated by the pancreatic hormone: insulin.
• Glycogen itself cannot be used by cells—it must be converted back to glucose or to other simple sugars. (glycogenolysis)
• Glycogen stored in the liver is available for conversion to glucose to maintain blood sugar levels and to supply energy for liver activity.
• Glycogen in muscle cells provides the glucose requirement for muscle activity.
• If the level of glucose in the blood drops below normal, the glycogen stored in the liver and muscle cells can be broken down into glucose.
• This process of converting glycogen back to glucose is called glycogenolysis.
• Most frequently, it occurs between meals and is stimulated by another pancreatic hormone, glucagon.

Question 3

Role of the Liver Processes

Answer 3

Glycogenesis: Formation of glycogen from other carbohydrates, especially glucose. (hormone: insulin)

Glycogenolysis: Breakdown of glycogen to glucose. (hormone: glucagon)

Gluconeogenesis: Conversion of fats or proteins into glucose.
(hormone: glucagon)

Question 4

Decreasing Blood Sugar Levels - Role of the Pancreas

Answer 4

• The pancreas is a pale grey gland, 12–15 cm long, lying partly in the curve of the duodenum.
• Within the pancreas are clusters of hormone-secreting cells called the islets of Langerhans.
• The cells in the islets are of two types.
• One type, the alpha cells, secrete glucagon, and the other type, the beta cells, secrete insulin.
• Both these hormones are secreted into the bloodstream and both are concerned with the control of blood sugar levels.

Question 5

Increasing Blood Sugar Levels - Role of the Pancreas

Answer 5

• Glucagon from the alpha cells causes an increase in blood sugar levels.
• Glucagon does this by stimulating glycogenolysis— the conversion of glycogen to glucose—in the liver.
• Glucagon also stimulates the liver to produce new sugar molecules from fats and amino acids, a process called gluconeogenesis.
• In addition, it may have a mild stimulating effect on protein breakdown.

Question 6

Role of the Adrenal Glands

Answer 6

• The adrenal cortex is stimulated to secrete its hormones by adrenocorticotrophic hormone (ACTH) from the anterior lobe of the pituitary.
• The hormones secreted are glucocorticoids, the best known of which is cortisol.
• They regulate carbohydrate metabolism by making sure enough energy is provided to the cells.
• In doing so, they stimulate the conversion of glycogen to glucose.
• Cortisol also increases the rate at which amino acids are removed from cells (mainly muscle cells) and transported to the liver.
• Some of these amino acids may be converted to glucose by the liver if glycogen and fat levels are low.
• This process of converting a substance other than carbohydrate into glucose is called gluconeogenesis.
• Glucocorticoids also promote the mobilisation of fatty acids from adipose tissue, allowing muscle cells to shift from glucose to fatty acids for much of their metabolic energy.