Blood glucose is the main sugar carried in the blood, and it is a major fuel source for body cells. The brain, muscles, liver, and many other tissues depend on a steady supply of glucose to function well. If blood glucose rises too high or falls too low, cells and organs can be harmed.
The pancreas helps keep blood sugar near a healthy range by releasing the hormones insulin and glucagon.
Insulin and glucagon work as an opposing pair in a negative feedback system. After a meal, rising blood glucose causes the pancreas to release insulin, which helps cells take in glucose and helps the liver store glucose as glycogen. Between meals or during exercise, falling blood glucose causes the pancreas to release glucagon, which signals the liver to break down glycogen and release glucose.
In diabetes, this control system does not work properly because insulin is missing, too low, or not used effectively by body cells.
Understanding Biology: Insulin, Glucagon, and Blood Sugar
The pancreas contains tiny groups of cells called islets. Beta cells make insulin, while alpha cells make glucagon. These hormones travel in the bloodstream, but they affect different tissues in different ways.
Insulin attaches to receptor proteins on cell surfaces. In many muscle and fat cells, this causes glucose transport proteins to move to the cell membrane. More glucose can then enter the cell.
This is not the main method used by the brain. Brain cells can take up glucose without needing insulin in the same direct way. Insulin still matters greatly because it helps control the amount of glucose available in the blood.
The liver has a special role because it can store fuel after eating, then supply fuel later. It joins many glucose units into glycogen, a compact storage molecule. Liver glycogen can be broken apart when the body needs glucose between meals.
The liver can make new glucose from substances such as lactate, glycerol, and some amino acids. This process becomes more important during a long fast, after glycogen stores become low.
Muscle cells store glycogen too, but their glycogen mainly fuels muscle contraction. Muscles cannot release their stored glucose directly into the blood in the same useful way as the liver.
Blood sugar control changes during exercise, illness, and stress. Active muscles use glucose quickly, but they can use some of their own glycogen and fat as fuel. During hard exercise, hormones such as adrenaline help signal the liver to release more glucose.
This prevents blood glucose from dropping too far while muscles are working. Low blood sugar can cause shaking, sweating, hunger, dizziness, confusion, or weakness.
Severe low blood sugar is especially dangerous because the brain needs a continuous energy supply. High blood sugar over long periods can damage blood vessels, nerves, kidneys, and eyes.
Diabetes shows why hormone signals and cell responses both matter. In type 1 diabetes, the immune system destroys most insulin producing beta cells. A person must replace insulin with medicine.
In type 2 diabetes, body cells often respond poorly to insulin at first. The pancreas may produce extra insulin for a time, but it may not keep up later. Food, physical activity, sleep, illness, and medicines can all affect glucose readings.
When learning this topic, track the direction of each change. Insulin supports storage and use after food enters the body.
Glucagon supports fuel release when food has not recently been absorbed. Remember that negative feedback reduces the original change rather than making it larger.
Key Facts
- Normal fasting blood glucose is often about 70 to 100 mg/dL.
- Insulin lowers blood glucose by increasing glucose uptake into cells and promoting glycogen storage in the liver and muscles.
- Glucagon raises blood glucose by stimulating glycogen breakdown and glucose release from the liver.
- Glucose homeostasis depends on negative feedback: a change in blood glucose triggers responses that reverse the change.
- Glycogenesis is glucose storage: glucose molecules are joined to form glycogen.
- Glycogenolysis is glycogen breakdown: glycogen is split into glucose molecules that can enter the blood.
Vocabulary
- Insulin
- Insulin is a hormone made by pancreatic beta cells that lowers blood glucose by helping cells absorb glucose and store it as glycogen.
- Glucagon
- Glucagon is a hormone made by pancreatic alpha cells that raises blood glucose by signaling the liver to release glucose.
- Homeostasis
- Homeostasis is the process of keeping internal body conditions, such as blood glucose level, within a stable range.
- Glycogen
- Glycogen is a storage form of glucose found mainly in the liver and muscles.
- Diabetes
- Diabetes is a group of conditions in which blood glucose remains too high because insulin production or insulin response is impaired.
Common Mistakes to Avoid
- Saying insulin breaks down glucose directly is wrong because insulin is a signaling hormone, not an enzyme that chemically destroys glucose.
- Thinking glucagon and glycogen are the same is wrong because glucagon is a hormone, while glycogen is a stored carbohydrate.
- Assuming only the pancreas controls blood sugar is wrong because the liver, muscles, fat tissue, intestines, and brain also play important roles.
- Forgetting the direction of negative feedback is wrong because high blood glucose should trigger insulin to lower it, while low blood glucose should trigger glucagon to raise it.
Practice Questions
- 1 A student's fasting blood glucose is 92 mg/dL. After eating, it rises to 138 mg/dL. Which hormone should increase, and what should happen to glucose uptake by cells?
- 2 A person's blood glucose drops from 85 mg/dL to 62 mg/dL during a long run. Which hormone should increase, and what liver process helps restore blood glucose?
- 3 Explain why insulin and glucagon are described as antagonistic hormones in a negative feedback loop.