# Hormone

A chemical messenger made by an endocrine gland, carried in the blood, and acting on target cells with the right receptors. Hormone effects are slower but last longer than nerve signals.

Biology (university) · Part 09: Animal physiology · Card 4 of 9 · Last updated 2026-10-19

## Analogy

A letter in the post rather than a phone call.

## Example

**Blood sugar loop**

Meal: glucose rises.
Insulin released.
Glucose falls.
Fasting: glucagon released,
glucose rises.

## In practice

Insulin from the pancreas tells cells to take up glucose and lowers blood sugar. Glucagon does the opposite.

## Practice

### Flashcards

- **Which hormones control blood sugar?** Insulin lowers it and glucagon raises it.
- **What does ADH do in the kidney?** It makes the tubule take back more water, so urine is more concentrated.

### Interview questions

**Explain how negative feedback regulates blood glucose, naming the hormones and organs involved.**

- Pancreas detects rise
- Insulin released, cells and liver take up glucose
- Glucose falls
- Glucagon released when low, liver releases glucose

## Questions people ask

### Why do hormones only affect certain cells?

Hormones travel through the blood to every cell, but only target cells have the matching receptor. Cells without it ignore the message. Water-soluble hormones such as insulin bind receptors on the cell surface, while steroid hormones such as oestrogen pass through the membrane and bind receptors inside.

### What is diabetes?

Diabetes is a condition in which blood glucose stays too high. In type 1 the immune system destroys the insulin-making cells of the pancreas, so little insulin is made. In type 2, body cells respond poorly to insulin, and it is strongly linked to lifestyle and genes. Both need careful management.

## Related concepts

- [Homeostasis](https://studyvisually.com/learn/science/biology-university/biology-university-part-9/biology-university-part-9-concept-1)
- [Nephron](https://studyvisually.com/learn/science/biology-university/biology-university-part-9/biology-university-part-9-concept-7)

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