Cells burn glucose in cellular respiration, so they need a steady supply. But you eat only a few times a day. Two hormones from the pancreas keep the blood's glucose level steady between and after meals, a pair of negative feedback loops.
Key idea
After a meal, blood glucose rises, and the pancreas releases insulin. Insulin helps body cells take up glucose and tells the liver to store it as glycogen, so blood glucose falls. Between meals, blood glucose falls, and the pancreas releases glucagon, which tells the liver to break glycogen down and release glucose, so blood glucose rises.
Why it works
Why two hormones instead of one? With one signal you could only push in one direction. Two opposing signals let the body correct both a rise and a fall, like having both a furnace and an air conditioner. Clusters of cells in the pancreas (islets) do the sensing: beta cells make insulin and alpha cells make glucagon.
Worked example: After a meal
An hour after lunch, which hormone is high, and what does the liver do?
- Blood glucose has risen from digested food.
- The pancreas releases insulin.
- Insulin signals the liver to take up glucose and store it as glycogen.
Answer: Insulin is high; the liver stores glucose as glycogen.
Worked example: During a long hike
Hours after breakfast, a hiker's blood glucose starts to fall. What keeps it from falling too far?
- The pancreas senses the drop and releases glucagon.
- Glucagon tells the liver to break glycogen down into glucose and release it.
Answer: Glucagon and the liver's glycogen store.
The liver is the body's glucose bank. Glycogen, the stored form, is a branched chain of many glucose units (see Carbohydrates), so it can be built up quickly after a meal and broken down quickly when glucose runs low. Muscles store glycogen too, but they keep it for their own use.
Common mistake
Two common confusions: thinking insulin is a kind of sugar, and thinking both hormones push the same way. Insulin is a protein hormone that lowers blood glucose; glucagon is another protein hormone that raises it. Check: "insulin in, sugar in (to cells)"; glucagon makes the liver let glucose "go".