Control of blood glucose concentration - AL only (3.6.4.2)

The liver plays a central role in regulating blood glucose concentration:

• Glycogenesis: the conversion of glucose to glycogen when blood glucose concentration is high.
• Glycogenolysis: breakdown of glycogen to glucose when blood glucose concentration falls.
• Gluconeogenesis: synthesis of glucose from non-carbohydrate sources, such as amino acids and glycerol, when glycogen stores are depleted.

Insulin is a hormone produced by cells in the pancreas.

It lowers blood glucose concentration by binding to specific receptors on the membranes of target cells such as liver and muscle cells. This causes more glucose channel proteins to be inserted into the cell membrane, increasing the uptake of glucose from the blood into the cells.

Insulin also stimulates glycogenesis by activating enzymes that convert glucose into glycogen for storage in the liver and muscles, helping return blood glucose concentration to normal.

Glucagon is a protein hormone produced by the cells in the pancreas.

It increases blood glucose concentration when levels are too low.

Glucagon binds to receptors on target cells, mainly in the liver, activating enzymes that convert glycogen into glucose (glycogenolysis).

Glucagon also stimulates enzymes that convert glycerol and amino acids into glucose (gluconeogenesis), thereby releasing more glucose into the blood.

Adrenaline is a hormone released by the adrenal glands during a “fight or flight” response. It helps the body respond quickly by increasing blood glucose levels.

Adrenaline acts as a first messenger, meaning it binds to a receptor in the surface membrane of a target cell. The receptor changes shape, causing the activation of adenyl cyclase (an enzyme) within the membrane which converts ATP into cyclic AMP, a second messenger.

Cyclic AMP activates protein kinase enzymes which trigger an enzyme cascade that stimulates glycogenolysis (conversion of glycogen to glucose).

Glucose is a sugar that circulates in the blood and serves as the main respiratory substrate for ATP production. A constant supply is needed, particularly by the brain and muscles.

If blood glucose concentration is too low (hypoglycaemia), insufficient glucose is available for respiration, leading to symptoms such as fatigue, confusion, and, in severe cases, unconsciousness.

If blood glucose concentration is too high (hyperglycaemia), the blood water potential decreases, causing water to leave body cells and enter the blood by osmosis, leading to cellular dehydration.

Blood glucose concentration is affected by the availability and use of glucose.

After a carbohydrate-rich meal, digestion and absorption increase blood glucose levels. During exercise and respiration, glucose is removed from the blood, and levels fall.

Hormones such as insulin and glucagon regulate blood glucose concentration by controlling glucose uptake, storage and release. Changes in blood water potential can also affect glucose concentration by making the blood more dilute or more concentrated.

Diabetes mellitus is a condition in which the homeostatic control of blood glucose concentration is impaired.

Type I diabetes typically develops in childhood and is caused by an autoimmune response that destroys the cells of the islets of Langerhans in the pancreas. As a result, little or no insulin is produced. Without sufficient insulin, cells take up less glucose from the blood, causing blood glucose concentration to remain abnormally high.

Type II diabetes occurs when body cells become less responsive to insulin due to the reduced sensitivity of insulin receptors – this is called insulin resistance.

As the condition progresses, the pancreas may also produce insufficient insulin to maintain normal blood glucose levels. It usually develops in adults.

Risk factors include obesity, poor diet and lack of exercise.

Without the homeostatic control of blood glucose concentration, blood glucose levels can rise to dangerous levels, potentially causing damage to blood vessels, nerves, kidneys and other organs.

A lack of insulin reduces glucose uptake by muscle and adipose (fat) cells and decreases glycogen synthesis. As a result, cells have less access to glucose for respiration, which can lead to fatigue and weakness.

Type I diabetes is treated with insulin injections or an insulin pump. Blood glucose levels can be monitored using biosensors, helping patients adjust insulin doses. Insulin is typically administered several times a day.

Type II diabetes can often be controlled through lifestyle changes, including a healthy diet, regular exercise and weight loss. Medications may be used to improve insulin sensitivity or reduce glucose absorption, and insulin therapy may be required in later stages of the disease.

The increase in people suffering from Type II diabetes is linked to obesity, poor diet, and lack of exercise.

Health advisers believe governments and food companies should help to reduce diabetes through education, food labelling, and limiting the sale of unhealthy foods.

Conversely the food industry argues that individuals are responsible for their own lifestyle choices and that foods that they produce are safe in moderation.