Insulin and glucagon are secreted to maintain blood glucose levels within a normal range. Performing this activity will illustrate the changes in blood glucose levels before and after ingesting carbohydrates and the role of insulin and glucagon in maintaining blood glucose homeostasis.
Play the animation to review your knowledge of the roles of hormones in fuel molecule utilization.
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Hormones secreted by endocrine glands travel in the blood and bind to target cells, changing the activity of the target cells. These changes in target cell activity help maintain body homeostasis. Secretion of most hormones is controlled by negative feedback.
Body cells break down glucose, fatty acids, and amino acids to make ATP. Between meals when blood glucose levels are dropping, the nervous system and red blood cells use glucose as the preferred fuel source to make ATP. During this time, other body cells use noncarbohydrate fuel sources to conserve glucose. Therefore, blood glucose homeostasis is important for brain function as well as other body functions. The pancreatic hormones insulin and glucagon maintain blood glucose levels between 70 and 100 mg/dL Low blood glucose levels (hypoglycemia) can cause dizziness, confusion, and loss of consciousness.
After a meal containing carbohydrates, the ingested carbohydrates are digested and absorbed into blood as glucose. This causes blood glucose levels to rise.
High blood glucose [hyperglycemia] stimulates the secretion of insulin by the pancreas and inhibits the secretion of glucagon. Insulin increases the transport of glucose into body cells causing blood glucose levels to decrease. Insulin also stimulates storage of excess glucose (glucose not needed for ATP production) by increasing glycogenesis and lipogenesis. In liver and muscle cells the excess glucose is stored as glycogen and in adipose cells it is stored as triglycerides (fat).
Low blood glucose [hypoglycemia] stimulates the secretion of glucagon, another pancreatic hormone, which increases the transport of glucose into the blood causing blood glucose to rise and decreases the secretion of insulin.
The insulin/glucagon ratio is a measure of the balance between the storage of glucose and increasing blood glucose levels. Higher ratios indicate that glucose storage (via glycogenesis or lipogenesis) prevails over glucose availability in the blood. Lower ratios indicate that increasing blood glucose levels (via gluconeogenesis and lipolysis) prevails over glucose storage.
Fatty acids released by lipolysis and amino acids released by protein catabolism may be broken down directly to make ATP. In the liver, the breakdown of fatty acids also forms ketones which are released into the blood. Ketones can be transported into cells and used to make ATP. The use of fatty acids, ketones, and amino acids to make ATP conserves blood glucose, thereby helping to maintain normal blood glucose levels.
When blood samples are sent to clinical laboratories for measurement of glucose, ketones, insulin, and glucagon, the blood is separated into plasma and formed elements (red blood cells, white blood cells, and platelets) by centrifugation. The plasma is used to measure glucose and ketones by enzymatic analysis and insulin and glucagon by radioimmunoassay.
