---
title: "The Lungs and Gas Exchange"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 51
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange
---

# Chapter 51 — The Lungs and Gas Exchange

Every minute of your life, about eight litres of air pass through two organs you have never seen. [Chapter 26](https://one-course.com/books/biology/1/en/chapter/26-breathing#ch-g4-breathing) sketched the path and the swap; [Chapter 47](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#ch-g7-how-animals-breathe) placed the human machine among the kingdom’s four. This chapter completes the picture: the ventilation machinery, the air pockets by their proper name, the numbers of the exchange — and what smoke does to the whole works.

## 51.1 The ventilation machinery

**Proposition 51.1 (How air is moved).**

The [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) have no [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) of their own; the chest pumps them like a bellows:

- [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in is *work* : the [rib](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) lift and widen the cage, and the *diaphragm* — the broad [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) floor under the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) — flattens downward; the widened chest stretches the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) , and air flows in to fill them;
- [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) out, at rest, is mostly *release* : [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) relax, the stretched chest springs back, air flows out. (Blowing out candles adds [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) to the spring.)

About $15$ times a minute at rest ([Definition 26.1](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements)), unasked ([Remark 33.4](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#rem-g5-brain-in-command-more)) — and deepened and quickened on demand ([Proposition 49.4](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-supply)).

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

**Example 51.2 (Feeling the bellows).**

Hands on [ribs](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton), breathe in deep: the cage rises and widens under your palms ([Example 26.2](https://one-course.com/books/biology/1/en/chapter/26-breathing#ex-g4-breathing-watch)). Now pant like a dog: the quick shallow puffs are almost all [diaphragm](#prop-g7-lungs-and-gas-exchange-ventilation). Hiccups, for completeness, are the [diaphragm](#prop-g7-lungs-and-gas-exchange-ventilation)’s twitchy false starts — the bellows misfiring on its own schedule.

## 51.2 The alveoli and the exchange

**Definition 51.3 (Alveoli).**

The tiny air pockets of [Definition 26.3](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs) are the *alveoli*: hundreds of millions of thin-walled bubbles clustered at the airways’ finest tips, each wrapped in fine [blood vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels). Their summed surface is the half tennis court of [Remark 26.4](https://one-course.com/books/biology/1/en/chapter/26-breathing#rem-g4-breathing-sponge) — the human copy of [Proposition 47.1](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#prop-g7-how-animals-breathe-spec)’s large-thin-moist specification.

**Proposition 51.4 (The exchange, in numbers).**

Compare the air that enters with the air that [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts): inhaled air is about $21$ percent [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and almost free of [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration); exhaled air is down to about $16$ percent [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and up to about $4$ percent [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) (warm and moist besides — [Example 26.7](https://one-course.com/books/biology/1/en/chapter/26-breathing#ex-g4-breathing-mirror)). The differences are the exchange: across the alveolar walls, [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) crosses to the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) and [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) crosses back ([Proposition 26.5](https://one-course.com/books/biology/1/en/chapter/26-breathing#prop-g4-breathing-exchange)), every minute of every day.

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

![One alveolar cluster of the hundreds of millions: thin bubble walls, a wrap of fine vessels, and the two-way crossing that is the point of it all.](https://one-course.com/images/onecourse/chapters/biology-1/g7-lungs-and-gas-exchange/fig-169101ed141d.svg)

*One alveolar cluster of the hundreds of millions: thin bubble walls, a wrap of fine [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels), and the two-way crossing that is the point of it all.*

**Example 51.5 (The accounting checks).**

Eight litres a minute, a twentieth of each breath’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) kept: the arithmetic supplies roughly the resting body’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) bill — and in effort, with [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) tripled and deepened and the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s rounds accelerated ([Proposition 49.4](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-supply)), the delivery multiplies to match the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’ bill. The machine’s numbers and the body’s ledger balance; they are the same books kept at two counters.

## 51.3 The machinery abused: smoke

**Proposition 51.6 (What smoke does to the works).**

Tobacco smoke attacks every floor of this chapter ([Proposition 36.5](https://one-course.com/books/biology/1/en/chapter/36-healthy-choices#prop-g5-healthy-choices-substances) gave the warning; here is the engineering):

- its tars coat and irritate the airways: the cleaning surfaces falter and the “smoker’s cough” is the airways’ overloaded broom;
- alveolar walls, chronically irritated, break down: pockets merge into fewer, bigger, baggier spaces — *surface is lost* , and with it exchange (the specification, dismantled);
- one of smoke’s gases rides the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) ’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) seats, crowding [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) off its own transport;
- and the airways narrow — every breath through a smoker’s chest is a breath through a part-blocked bellows.

The damage is slow, cumulative and only partly reversible: the never-started [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) keeps its half tennis court whole ([Remark 36.6](https://one-course.com/books/biology/1/en/chapter/36-healthy-choices#rem-g5-healthy-choices-never)).

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

**Example 51.7 (Two stair tests).**

The three flights of [Chapter 49](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#ch-g7-organs-at-work), taken by two adults of equal age and training habits, one a long-term smoker: the smoker arrives gasping at a [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) the other reaches only sprinting. Less surface, seats taken, narrowed airways — the supply chain pays [Proposition 49.1](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill)’s bill through a throttled intake. Fitness tests read the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ history as surely as any scan.

**Method 51.8 (Keeping the machinery).**

[Method 26.8](https://one-course.com/books/biology/1/en/chapter/26-breathing#met-g4-breathing-care), upgraded with this chapter’s reasons:

1. no smoke — yours or others’: surface lost is barely regained;
2. air the rooms ( [Exercise 26.10](https://one-course.com/books/biology/1/en/chapter/26-breathing#exo-g4-breathing-10) ) and prefer clean outdoor air for sport;
3. exercise the bellows: [deep-breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) effort builds the [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) and keeps the full surface ventilated;
4. [nose-breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in cold and dust ( [Exercise 26.7](https://one-course.com/books/biology/1/en/chapter/26-breathing#exo-g4-breathing-7) ) — the filter protects the broom.

## 51.4 Exercises

**Exercise 51.1 ★.**

Describe [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in and [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) out as work and release: which [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), which movements?

**Solution of Exercise 51.1.**

[Breathing](https://one-course.com/books/biology/1/en/chapter/26-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) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) lift and widen the cage while the [diaphragm](#prop-g7-lungs-and-gas-exchange-ventilation) flattens downward; the stretched chest pulls the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) wide and air flows in — muscular work. [Breathing](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements) out at rest: the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) relax and the stretched chest springs back, pushing air out — release, nearly free.

**Exercise 51.2 ★.**

What is the [diaphragm](#prop-g7-lungs-and-gas-exchange-ventilation), and where does it sit?

**Solution of Exercise 51.2.**

The broad [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) floor under the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), closing the chest off below; its flattening is the deep half of every breath in.

**Exercise 51.3 ★.**

What are the [alveoli](#def-g7-lungs-and-gas-exchange-alveoli) — how many, how walled, wrapped in what?

**Solution of Exercise 51.3.**

The [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ air pockets: hundreds of millions of thin-walled bubbles at the airways’ finest tips, each wrapped in fine [blood vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels).

**Exercise 51.4 ★.**

Give the round numbers: [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in inhaled air, in exhaled air; [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in exhaled air. What are the differences?

**Solution of Exercise 51.4.**

Inhaled: about $21$ percent [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), almost no [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). Exhaled: about $16$ percent [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and about $4$ percent [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). The differences — five points of [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) taken, four of [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) added — are the exchange itself.

**Exercise 51.5 ★.**

Which specification do the [alveoli](#def-g7-lungs-and-gas-exchange-alveoli) meet, and with which two other organs of this year do they share it?

**Solution of Exercise 51.5.**

The large-thin-moist, blood-lined crossing surface. Shared with the [gills](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) ([Proposition 47.1](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#prop-g7-how-animals-breathe-spec)) and the intestine’s [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) ([Example 50.7](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#ex-g7-digestion-and-nutrients-spec)).

**Exercise 51.6 ★.**

Name three of smoke’s four attacks on the machinery.

**Solution of Exercise 51.6.**

Tars coating and irritating the airways (the overloaded broom and its cough); alveolar walls breaking down — surface lost; a smoke gas riding the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) seats; narrowed airways throttling the bellows.

**Exercise 51.7 ★★.**

Why is [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in work while resting [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) out is free? Where does the outward push come from?

**Solution of Exercise 51.7.**

In must stretch chest and [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) against their spring — that stretching is the work. The resting out is the spring itself returning: the widened chest and stretched [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) recoil, and the recoil pushes the air.

**Exercise 51.8 ★★.**

Explain why lost alveolar surface cannot be compensated by [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) faster — which factor of the exchange is missing?

**Solution of Exercise 51.8.**

The exchange needs surface as much as fresh air: crossing happens only where air meets blood-lined wall. Faster bellows through fewer, baggier pockets ventilates surface that no longer exists — the missing factor is the crossing area itself.

**Exercise 51.9 ★★.**

Interpret the two stair tests of [Example 51.7](#ex-g7-lungs-and-gas-exchange-stairs) attack by attack.

**Solution of Exercise 51.9.**

The smoker climbs with narrowed airways (throttled intake), a reduced alveolar surface (less loading per breath), and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) seats occupied by smoke’s gas (less carried per round). To pay the same muscular bill, the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) must run more rounds — hence the gasping arrival at a sprint-level [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse).

**Exercise 51.10 ★★.**

A diver breathes out slowly while surfacing; a trumpet player holds long steady notes. Which halves of [Proposition 51.1](#prop-g7-lungs-and-gas-exchange-ventilation) are they each training or using?

**Solution of Exercise 51.10.**

Both are trained control of the *outward* phase: adding measured [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) to what is normally a passive spring — the diver to vent expanding air steadily, the player to meter one long controlled outflow. (Their deep intakes train the inward work too.)

**Exercise 51.11 ★★.**

Reconcile [Example 51.5](#ex-g7-lungs-and-gas-exchange-accounting) with [Proposition 49.4](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-supply): which three adjustments multiply delivery in effort, and at which counters?

**Solution of Exercise 51.11.**

Deeper-and-faster [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) multiplies loading at the alveolar counter; the faster, harder [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) multiplies rounds at the transport counter; rerouting widens the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’ local [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) at the delivery counter. Three adjustments, one multiplied delivery ([Example 51.5](#ex-g7-lungs-and-gas-exchange-accounting)’s balanced books).

**Exercise 51.12 ★★★.**

“The [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) is a border of half a tennis court folded into a shoebox, and smoke is a slow fire at the border.” Justify both images with this chapter’s numbers and mechanisms.

**Solution of Exercise 51.12.**

The border: hundreds of millions of [alveoli](#def-g7-lungs-and-gas-exchange-alveoli) sum to about half a tennis court of exchange surface, folded into a chest — across which five percentage points of every breath’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) cross inward and four of [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) cross out. The slow fire: smoke’s tars and irritation consume that border year by year — coating its approaches, collapsing its pockets, seizing its transport seats — cumulative, quiet, and only partly reversible, like a border burned faster than it can be rebuilt.

## 51.5 Problem: The Breathing Machine Dossier

**Problem 51.1.**

Weekend problem — an engineer’s inspection of the human bellows

An engineering class “inspects” the human [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) machine on paper: intake, pump, exchange surface, transport connection, and maintenance record.

**Part I — The pump.**

1. Draw up the pump’s cycle: the two phases, the movers of each, and which phase costs [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) at rest.
2. The pump has no contact with the pumped organ’s own [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) — the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) have none. How does the chest’s widening move air at all?
3. Give the pump’s resting rate and its two effort adjustments ( [Proposition 49.4](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-supply) ).
4. Panting, sighing, blowing out candles: assign each to its place in the cycle’s mechanics.

**Part II — The exchange surface.**

5. Specify the surface: name, number, wall, wrapping, summed size.
6. Verify the specification against [Proposition 47.1](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#prop-g7-how-animals-breathe-spec) , item by item.
7. The intake air and exhaust air differ by the chapter’s percentages. Compute the [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) difference as a fraction of the intake’s $21$ percent — roughly what share of inhaled [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) is kept?
8. Why must the exhaust stay warm and moist — which property of the surface is that the price of?

**Part III — The maintenance record.**

9. File smoke’s four attacks under: intake, pump, surface, transport. One line each.
10. The dossier notes: “surface losses are capital losses.” Explain, against the coughs and colds that are running repairs.
11. Write the maintenance schedule — [Method 51.8](#met-g7-lungs-and-gas-exchange-keep) — as four engineer’s directives with reasons.
12. Conclude the inspection: one sentence on why this machine, alone of the body’s pumps and borders, is half under the owner’s voluntary control — and what that control is for (speech, song, the diver’s slow exhale).

**Solution of Problem 51.1.**

**1.** Phase in: [rib](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) lift and widen the cage, [diaphragm](#prop-g7-lungs-and-gas-exchange-ventilation) flattens — costs [energy](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs). Phase out (rest): [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) release, chest and [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) spring back — nearly free. Cycle about $15$ times a minute.

**2.** The [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) cling to the chest’s inner wall: widen the chest and the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) are stretched wide with it, and the outside air flows in to fill the stretched space. The pump works by reshaping the room, not squeezing the organ.

**3.** About $15$ cycles a minute at rest; in effort the cycles quicken *and* deepen — more breaths, and more air per breath.

**4.** Panting: quick shallow cycles, nearly all [diaphragm](#prop-g7-lungs-and-gas-exchange-ventilation). Sighing: one extra-deep stretch and long release — a reset of the spring. Candles: the outward phase with [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) added to the spring.

**5.** The [alveoli](#def-g7-lungs-and-gas-exchange-alveoli): hundreds of millions of thin-walled bubbles, each wrapped in fine [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels), summing to about half a tennis court.

**6.** Large: the summed court. Thin: bubble walls a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s breadth. Moist: their lining. Blood-lined: the wrapping [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels). Specification met on all four counts.

**7.** Five points kept out of $21$ — roughly a quarter of the inhaled [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) is taken per breath.

**8.** Of the surface’s moisture: air leaving has swept across wet walls (and warm ones), so exhaust carries away water and warmth — the misted mirror is the surface’s upkeep cost, paid with every breath.

**9.** Intake: tars coat and irritate the airways. Pump: narrowed airways throttle every cycle. Surface: alveolar walls break down and merge — area lost. Transport: smoke’s gas occupies the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) seats.

**10.** Coughs and colds inflame and clear — the machinery repairs and returns to service. Broken alveolar walls do not rebuild: each merged pocket is exchange area struck permanently off the books — capital, not running costs.

**11.** (1) Admit no smoke: capital losses are forever. (2) Run on clean air: filters and brooms have finite capacity. (3) Exercise the pump under load: deep effort maintains [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) and ventilates the whole surface. (4) Intake through the nose in cold and dust: pre-conditioning protects the works.

**12.** Alone among the body’s pumps, the bellows answers both the automatic services and the will — because the same outflow that vents [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) is the wind of speech and song, and a machine lent to language must accept the speaker’s hand on the controls.