The big idea: All the blood leaving your gut passes through the liver before reaching the rest of the body. This makes the liver the body's chemical hub.
What the liver does
- It adjusts the nutrients in the blood, stores some, removes others, and breaks down toxins.
- The cells that do this work are called hepatocytes (liver cells).
- A key job is keeping the level of glucose in the blood steady — neither too high nor too low.
- Liver
- A large organ that processes nutrients and toxins in the blood coming from the gut; the body's main 'chemical factory'.
- Hepatocyte
- A liver cell — the cell type that carries out the liver's many chemical jobs, including regulating blood nutrients.
- Glycogen
- A storage carbohydrate (a polymer of glucose) that the liver makes when blood glucose is high and breaks down when it is low.
- Blood glucose regulation
- Keeping the concentration of glucose in the blood close to a steady set point, mainly by storing or releasing glucose in the liver.
- Bile
- A fluid made by the liver; it carries excess cholesterol and the waste pigment bilirubin out of the body in the faeces.
Why the liver sits where it does: Blood from the gut, loaded with newly absorbed nutrients, reaches the liver first.
That lets the liver adjust the nutrient levels before the blood reaches every other organ — so the rest of the body receives blood with a steady, safe composition.
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Your blood glucose changes throughout the day — it rises after a meal and falls between meals. The liver smooths these swings out.
The liver is the effector in a negative-feedback loop: whichever way glucose drifts, the liver's response pushes it back toward normal.
- Negative feedback
- A control system in which a change in a level triggers a response that OPPOSES the change, returning the level toward its set point.
- Insulin
- A hormone (from the pancreas) released when blood glucose is HIGH; it makes the liver take up glucose and store it as glycogen, lowering blood glucose.
- Glucagon
- A hormone (from the pancreas) released when blood glucose is LOW; it makes the liver break glycogen back into glucose, raising blood glucose.
When blood glucose is HIGH (after a meal): After eating, glucose floods into the blood and the level rises.
The pancreas releases insulin. In response, hepatocytes take up glucose and join the molecules together into glycogen for storage.
Removing glucose from the blood makes the level fall back toward normal — the response opposes the rise.
When blood glucose is LOW (between meals): Between meals, the blood glucose level falls.
The pancreas releases glucagon. In response, hepatocytes break glycogen back down into glucose and release it into the blood.
Adding glucose makes the level rise back toward normal — again the response opposes the change.
Blood glucose is held near a set point by negative feedback: a rise after a meal triggers a response (insulin → store glucose as glycogen) that lowers it again; a fall triggers the opposite (glucagon → release glucose) — the response always opposes the change.
Interactive diagram
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| Situation | Blood glucose is HIGH (e.g. after a meal) | Blood glucose is LOW (e.g. between meals) |
|---|---|---|
| Hormone released | Insulin (from the pancreas) | Glucagon (from the pancreas) |
| What the liver does | Takes up glucose and stores it as glycogen | Breaks glycogen down and releases glucose |
| Effect on blood glucose | Falls back toward normal | Rises back toward normal |
| Why | The response OPPOSES the rise | The response OPPOSES the fall |
Glucose too HIGH
- Happens after a meal
- Insulin is released
- Liver stores glucose as glycogen
- Blood glucose falls back to normal
Glucose too LOW
- Happens between meals
- Glucagon is released
- Liver breaks glycogen back into glucose
- Blood glucose rises back to normal
A memory hook: Insulin → IN (glucose goes into store as glycogen, lowering blood glucose).
Glucagon → Glucose comes back (glycogen is broken down, raising blood glucose).
Both are negative feedback: the liver's response always opposes the change.
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How this is tested: Liver questions are short 2-mark Outline items on Paper 3, plus data-handling questions on blood glucose.
The question types
- Paper 3, 2-mark Outline: how hepatocytes regulate nutrient levels in the blood. The expected answer is the store-as-glycogen / release-from-glycogen glucose loop.
- 2-mark Outline: how the body removes excess cholesterol. The liver puts it into bile, which is lost in the faeces.
- Data handling: a blood-glucose-versus-time trend after a meal. Reason about insulin, glycogen storage and negative feedback from the shape of the curve.
IB-style question — how hepatocytes regulate blood glucose
Outline how hepatocytes help to regulate the level of glucose in the blood. [2]
How to score both marks
- Deal with high glucose. When blood glucose is high (after a meal), hepatocytes take up glucose and store it as glycogen, which lowers the blood glucose level.
- Deal with low glucose. When blood glucose is low (between meals), hepatocytes break glycogen back down into glucose and release it, which raises the blood glucose level. (Award 1 mark for the high-glucose response, 1 mark for the low-glucose response.)
Final answer
When blood glucose is high, hepatocytes store glucose as glycogen (lowering it); when blood glucose is low, they break glycogen back into glucose (raising it) — keeping the level steady.
Why this scores full marks: The two marks come from the two opposite responses — store glucose as glycogen, and break glycogen back to glucose.
Writing only one direction (e.g. just 'stores glucose') caps you at 1 mark. A good answer always shows both halves of the negative-feedback loop.
| Liver role | What the hepatocytes do | Why it matters |
|---|---|---|
| Regulating blood glucose | Store glucose as glycogen when blood glucose is high; break glycogen back to glucose when it is low | Keeps blood glucose steady between meals and after eating |
| Handling cholesterol | Remove excess cholesterol from the blood and release it into bile | Stops cholesterol building up in the blood |
| Storing nutrients | Store glycogen, iron and some vitamins | Acts as a reserve the body can draw on later |
| Detoxification | Break down toxins such as alcohol into safer products | Protects the rest of the body from harmful substances |
| Breaking down red blood cells | Break down old red blood cells; the pigment bilirubin is passed into bile | Recycles components and removes waste pigment |