Blood Glucose Homeostasis is the body requires volumes of glucose in order to create ATP. The amount of ATP demanded will fluctuate, and therefore the body regulates the availability of glucose to maximise its energy making potential.
Two hormones are responsible for controlling the concentration of glucose in the blood. These are insulin and glucagon. The diagram illustrates the principle of negative feedback control in action involving blood/sugar levels.
Both insulin and glucagon function as important control system for maintaining a normal blood glucose concentration, responding to the high and low blood glucose level, respectively. When the blood glucose level to decrease towards normal. Conversely, a decrease in blood glucose level stimulates glucagon secretion; the glucagon then functions in the opposite direction to increase the blood glucose toward normal. Under most normal conditions, the insulin feedback mechanism is much more important than the glucagon mechanism. However, the glucagon mechanism becomes valuable in times of starvation or when extra glucose is required by the body during exercise and other stressful situations.
Insulin and Glucose Disposal in Normal Individuals
Almost all organs in the body, under most conditions are dependent upon glucose as an immediate source of energy. Immediately after a high-carbohydrate meal, the glucose that is absorbed into the blood causes rapid secretion of insulin from the beta cells of islets of Langerhans in the pancreas. The insulin in turn causes rapid uptake, storage and use of glucose by almost all tissues of the body, but especially by the muscles, adipose tissue and liver.
The receptors of the pancreas are responsible for monitoring glucose levels in the blood, since it is important in every cell for respiration.
Two types of cell release two different hormones from the pancreas, insulin and glucagon. These hormones target the liver, one or the other depending on the glucose concentration
- In cases where glucose levels increase, less glucagon and more insulin is released by the pancreas and targets the liver
- In cases where glucose levels decrease, less insulin and more glucagon is released by the pancreas and targets the liver
Effect of Insulin on the Muscles
The muscles are stimulate by the post meal secretion of insulin to rapidly take up glucose. If the muscles are exercising or working, they utilize glucose to produce energy. If the muscles are not exercising after a meal and yet glucose is transported into the muscle cell in abundance, then most of the glucose is stored in the form of muscle glycogen for later use, instead of being used for energy.
Effect of Insulin on the Liver
The liver acts as a storehouse for glycogen, the storage form of glucose. When either of the above hormones target the liver, the following occurs
- Insulin – Insulin is released as a result of an increase in glucose levels, and therefore promotes the conversion of glucose into glycogen, where the excess glucose can be stored for a later date in the liver
- Glucagon – Glucagon is released as a result of an decrease in glucose levels, and therefore promotes the conversion of glycogen into glucose, where the lack glucose can be compensated for by the new supply of glucose brought about from glycogen
An important action of insulin is to promote most of the glucose absorbed after a meal to be stored almost immediately in the liver in the form of glycogen. It also decreases the breakdown of already formed glycogen in the liver to glucose. Then, between meals, when food is not available and the blood glucose concentration begins to fall, insulin secretion decreases rapidly and the liver glycogen is broken down into glucose, which is released back into the blood to keep the glucose concentration from falling too low. This process is known as glycogenolysis. Insulin also inhibits gluconeogenesis i.e., production of glucose by the liver from amino acids and other precursors by decreasing the activity of enzymes required for gluconeogenesis in the liver.
Effect of Insulin on the Other Body Organs(Except Brain)
Insulin increases glucose transport into and glucose usage by most other cells of the body (with the exception of the brain cells) in the same way that it affects glucose transport and usage in muscle cells. The transport of glucose into fat cells promotes synthesis of fats. Therefore, in this indirect way, insulin promotes deposition of fat in these cells.
Insulin and Brain
Insulin has little effect on uptake of glucose in brain, as the brain cells can take up and use glucose without the inter-mediation of insulin. Brain cells cannot use fats, and are totally dependent on glucose for energy. This is one of the reasons why blood glucose has to be maintained above a certain level always.
Glucagon and Glucose Disposal in Normal Individuals
Glucagon is a hormone secreted by the alpha cells of the islets of Langerhans when the blood glucose concentration falls. Glucagon increases the blood glucose concentration, an effect that is exactly the opposite to that of insulin. It does this by means of:
- Promoting the breakdown of liver glycogen (glycogenolysis)
- Increasing gluconeogenesis in the liver.
Both of these effects greatly enhance the availability of glucose to the other organs of the body.