---
title: "Breathing"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 26
exercises: 11
source: https://one-course.com/books/biology/1/en/chapter/26-breathing
---

# Chapter 26 — Breathing

Hold your breath and start counting. Long before one minute, your chest demands, more and more urgently, to move again. You can skip a meal, [sleep](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#prop-g1-healthy-habits-needs) can wait until tonight — but air is needed *now*, around the clock, about fifteen times a minute. What is so urgent in a breath of air?

## 26.1 The breathing movements

**Definition 26.1 (Breathing in and out).**

*Breathing* is the endless alternation of two movements:

- *breathing in* (inhaling): the chest expands and air flows in through nose or mouth;
- *breathing out* (exhaling): the chest relaxes and air flows back out.

At rest, the pair repeats about $15$ times a minute — faster in babies, faster in effort — without your ever needing to think about it.

**Example 26.2 (Watching the movements).**

Hands flat on your [ribs](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton): [breathing](#def-g4-breathing-movements) in, the [rib](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) cage rises and widens under your palms; [breathing](#def-g4-breathing-movements) out, it sinks back. A sleeping cat’s flank shows the same rise and fall. The movements never stop — awake, asleep, from first cry to last day.

## 26.2 Where the air goes

**Definition 26.3 (The breathing organs).**

Inhaled air travels through the nose (or mouth), down the *windpipe* — the firm ringed tube you can feel at the front of your neck — which divides into two [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree), one to each *lung*. The lungs, two spongy organs filling the [rib](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) cage (behind the [ribs](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) of [Definition 17.1](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton)), end in millions of tiny air pockets, each wrapped in fine [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) vessels.

![The path of a breath: windpipe, two branches, two lungs — and, at the branches’ finest tips, millions of tiny air pockets wrapped in fine blood vessels, far too small to see at this scale.](https://one-course.com/images/onecourse/chapters/biology-1/g4-breathing/fig-56018970cee1.svg)

*The path of a breath: [windpipe](#def-g4-breathing-organs), two [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree), two [lungs](#def-g4-breathing-organs) — and, at the [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree)’ finest tips, millions of tiny air pockets wrapped in fine [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) vessels, far too small to see at this scale.*

**Remark 26.4 (Spongy on purpose).**

Millions of pockets instead of two simple bags: it is the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s trick again ([Remark 25.6](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#rem-g4-journey-of-food-surface)). All those pocket walls add up to an enormous meeting surface between air and [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) — the size of half a tennis court, folded into your chest.

## 26.3 What breathing is for

**Proposition 26.5 (Taking oxygen, returning carbon dioxide).**

Air is a mixture of gases, and the body wants one of them: *oxygen*. In the [lungs](#def-g4-breathing-organs)’ pockets, oxygen from fresh air passes into the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), which delivers it to every part of the body; there it is used, together with [nutrients](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion), to release the [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) of [Proposition 18.1](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs). The body’s waste gas, *carbon dioxide*, makes the return trip and is breathed out. Exhaled air has less oxygen and more carbon dioxide than inhaled air — the difference is what [breathing](#def-g4-breathing-movements) is *for*.

**Proof.** *Admitted at this level.* ∎

**Example 26.6 (Why running takes your breath).**

Sprinting, your [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) burn [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) fast — so they need oxygen fast. [Breathing](#def-g4-breathing-movements) obliges: from about $15$ calm breaths a minute to $40$ or more, deep ones, while the heart ([Chapter 27](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#ch-g4-heart-and-blood) tells its side) races to speed the deliveries. The panting after the race is the body settling its oxygen accounts.

**Example 26.7 (The misted mirror).**

Breathe on a cold mirror: mist. Exhaled air comes back changed — warmed, moistened, and, invisibly, poorer in oxygen and richer in carbon dioxide. The mirror shows the moisture; the gas swap it cannot show is the important part.

![Exhaled air made visible: warm, moist — and invisibly poorer in oxygen, richer in carbon dioxide.](https://one-course.com/images/onecourse/chapters/biology-1/g4-breathing/img-5763472b4d4e.jpg)

*Exhaled air made visible: warm, moist — and invisibly poorer in oxygen, richer in carbon dioxide.*

**Method 26.8 (Caring for the breath).**

1. Air rooms daily — many breathers in a closed room means stale, carbon-dioxide-rich air;
2. breathe through the nose in cold or dusty air: it warms, moistens and filters;
3. run, swim, play — exercised [lungs](#def-g4-breathing-organs) and [breathing](#def-g4-breathing-movements) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) grow stronger and deeper;
4. keep smoke away from your [lungs](#def-g4-breathing-organs) : it dirties the delicate pockets, and they clean themselves only slowly.

## 26.4 Exercises

**Exercise 26.1 ★.**

Name the two [breathing](#def-g4-breathing-movements) movements and what the chest does in each.

**Solution of Exercise 26.1.**

[Breathing](#def-g4-breathing-movements) in: the chest expands and air flows in. [Breathing](#def-g4-breathing-movements) out: the chest relaxes and air flows out.

**Exercise 26.2 ★.**

Trace a breath’s path from the nose to the tiny air pockets.

**Solution of Exercise 26.2.**

Nose (or mouth), [windpipe](#def-g4-breathing-organs), the two [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree), the [lungs](#def-g4-breathing-organs), and finally the millions of tiny air pockets wrapped in [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) vessels.

**Exercise 26.3 ★.**

About how many times a minute does a calm resting child breathe? What happens to that number during a sprint, and why?

**Solution of Exercise 26.3.**

About $15$ times a minute. In a sprint it rises to $40$ or more, deeper too — the hard-working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) need oxygen faster, so the [lungs](#def-g4-breathing-organs) load it faster.

**Exercise 26.4 ★.**

Which gas does the body take from the air, and which does it return? Where exactly does the swap happen?

**Solution of Exercise 26.4.**

It takes oxygen and returns carbon dioxide. The swap happens in the [lungs](#def-g4-breathing-organs)’ tiny air pockets, across their thin walls, with the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb).

**Exercise 26.5 ★.**

How does exhaled air differ from inhaled air? Name three differences.

**Solution of Exercise 26.5.**

Exhaled air is warmer, moister (the misted mirror), poorer in oxygen and richer in carbon dioxide.

**Exercise 26.6 ★.**

Why are the [lungs](#def-g4-breathing-organs) built as millions of tiny pockets rather than two simple bags? Which other organ of this year uses the same trick?

**Solution of Exercise 26.6.**

Because the [air-blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) swap needs surface: millions of pocket walls add up to half a tennis court of meeting surface folded into the chest. The [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) plays the same trick with its folds for absorbing [nutrients](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion).

**Exercise 26.7 ★.**

Why is it better to breathe through the nose on a freezing or dusty day?

**Solution of Exercise 26.7.**

The nose warms, moistens and filters the air on its way in — cold, dry, dusty air reaches the delicate [lungs](#def-g4-breathing-organs) already prepared.

**Exercise 26.8 ★.**

Try it: sitting calmly, count your breaths for one minute. Then do twenty jumping jacks and count again. Report both numbers and explain the change.

**Solution of Exercise 26.8.**

Open answer. Typically about $15$ at rest and $25$–$40$ after the jumping jacks: the working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) demanded more oxygen, so [breathing](#def-g4-breathing-movements) sped up and deepened to supply it.

**Exercise 26.9 ★★.**

Meals can wait hours; [breathing](#def-g4-breathing-movements) cannot wait a minute. What does this say about how much oxygen the body keeps in store, compared with its stores of food?

**Solution of Exercise 26.9.**

That the body stores food generously (hours’ or days’ worth) but keeps almost no store of oxygen at all — barely a minute’s. Oxygen must therefore be delivered continuously, breath by breath, which is why [breathing](#def-g4-breathing-movements) can never pause long.

**Exercise 26.10 ★★.**

A classroom feels “stuffy” by late morning. What has changed in its air, by [Proposition 26.5](#prop-g4-breathing-exchange), and what does [Method 26.8](#met-g4-breathing-care) prescribe?

**Solution of Exercise 26.10.**

Twenty-five breathers have been taking oxygen from the room’s air and returning carbon dioxide all morning: its air has grown poorer in one and richer in the other. The prescription is rule 1: open the windows and air the room.

**Exercise 26.11 ★★★.**

The trout has no [lungs](#def-g4-breathing-organs): water flows over its feathery gills, and the oxygen *dissolved in the water* passes into its [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) there. What job do gills and [lungs](#def-g4-breathing-organs) share? And why does a trout suffocate in air, where oxygen is plentiful? (Hint: out of water, the feathery gill surfaces collapse and stick together.)

**Solution of Exercise 26.11.**

Both are the meeting place of [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) and oxygen: huge, thin, blood-lined surfaces where oxygen crosses in and carbon dioxide crosses out — gills taking oxygen dissolved in water, [lungs](#def-g4-breathing-organs) taking it from air. In air the trout’s feathery gill surfaces collapse and stick together: the great crossing surface shrinks to almost nothing, and no crossing surface means no oxygen, however rich the air around it.