---
title: "Blood — what it does and the numbers"
description: "About 5 litres in an adult, roughly 7% of body weight. What it does, where it is, the figures people look up, and the widely repeated claims that turn out to be wron"
url: https://atoneplace.net/body/blood/
site: "At One Place"
---

# Blood

**Blood: About 5 litres in an adult, roughly 7% of body weight.**

Carries oxygen and nutrients, removes carbon dioxide and waste, distributes heat and hormones, and delivers immune cells. Red cells make up about 45% of volume, with plasma most of the rest.

## The numbers

- **Volume:** 4.5–5.5 litres in an adult
- **Red cells:** about 25 trillion
- **Red cell lifespan:** roughly 120 days
- **Circuit time:** about 60 seconds

## Worth knowing

A red blood cell completes a full circuit of the body in about a minute and survives roughly 120 days, in which time it makes around 170,000 circuits. Mature red cells have no nucleus, which is what leaves room for more haemoglobin and is why they cannot repair themselves.

## A common claim that is wrong

Deoxygenated blood is not blue. It is dark red. Veins look blue because of how skin scatters light, not because of the blood inside them.

> Structural reference only — this is not medical advice, and individual variation in every figure here is substantial. Anything concerning your own health belongs with a clinician rather than a reference page.

## Common questions

### What does the blood do?

Carries oxygen and nutrients, removes carbon dioxide and waste, distributes heat and hormones, and delivers immune cells.

### Blood — key numbers

Volume: 4.5–5.5 litres in an adult. Red cells: about 25 trillion. Red cell lifespan: roughly 120 days. Circuit time: about 60 seconds.

### Is it true that…

Deoxygenated blood is not blue. It is dark red. Veins look blue because of how skin scatters light, not because of the blood inside them.

## Related pages

- [Blood types](https://atoneplace.net/body/blood-types/)
- [Heart](https://atoneplace.net/body/heart/)
- [Brain](https://atoneplace.net/body/brain/)
- [Bones](https://atoneplace.net/body/bones/)
- [Teeth](https://atoneplace.net/body/teeth/)
- [Liver](https://atoneplace.net/body/liver/)
- [Lungs](https://atoneplace.net/body/lungs/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)