Glucose in the blood provides a source of fuel for all tissues of the body. Blood glucose levels are highest during the absorptive period after a meal, during which the stomach and small intestine are breaking down food and circulating glucose to the bloodstream. Blood glucose levels are the lowest during the postabsorptive period, when the stomach and small intestines are empty. Despite having food only periodically in the digestive tract, the body works to maintain relatively stable levels of circulatory glucose throughout the day.
The body maintains blood glucose homeostasis mainly through the action of two hormones secreted by the pancreas. These hormones are insulin, which is released when glucose levels are high, and glucagon, which is released when glucose levels are low. The accompanying animation depicts the functions of these hormones in blood glucose regulation.
Throughout the day, the release of insulin and glucagon by the pancreas maintains relatively stable levels of glucose in the blood.
During the absorptive period blood glucose levels tend to increase, and this increase stimulates the pancreas to release insulin into the bloodstream. Insulin promotes the uptake and utilization of glucose by most cells of the body. Thus, as long as the circulating glucose supply is high, cells preferentially use glucose as fuel and also use glucose to build energy storage molecules glycogen and fats.
In the liver, insulin promotes conversion of glucose into glycogen and into fat. In muscle insulin promotes the use of glucose as fuel and its storage as glycogen. In fat cells insulin promotes the uptake of glucose and its conversion into fats. The nervous system does not require insulin to enable its cells to take up and utilize glucose.
If glucose is being absorbed from the small intestine and there is no circulating insulin, most cells of the body cannot make use of the circulating glucose and its levels can rise to dangerous levels. This is what happens in a form of diabetes in which the pancreas does not secrete insulin. This condition is called diabetes mellitus (mellitus is derived from the Latin word for honey) because the excess glucose is lost in the urine and the urine tastes sweet.
During the postabsorptive period when there is not more glucose being absorbed from the intestine, the critical controlling element is the fact that the pancreas stops secreting insulin. As a result, most cells of the body cannot take up insulin and the glucose that is circulating in the blood is reserved for the nervous system. If, however, blood glucose levels drop below normal postabsorptive levels, the pancreas is stimulated to secrete glucagon. Glucagon has many actions that are opposite those of insulin — promotion of breakdown of glycogen and fats in the liver, promotion of breakdown of fats in the adipose tissue, and the stimulation of gluconeogenesis in the liver. The actions of glucagon bring blood glucose levels back to normal postabsorptive levels.
Textbook Reference: Concept 30.5 The Processing of Meals Is Regulated