---
title: "Organs at Work"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 49
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/49-organs-at-work
---

# Chapter 49 — Organs at Work

Sprint up three flights of stairs and take stock: [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) burning, [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) hammering, breath racing — and, oddly, [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) flushing and warm. Nobody ordered these changes; they ordered themselves, and in perfect proportion. This chapter measures what a working organ actually consumes, and how the body reorganizes its deliveries the moment work begins.

## 49.1 What a working muscle consumes

**Proposition 49.1 (The muscle’s bill).**

A working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) spends, and its spending can be measured. Compared with the same [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) at rest, a hard-working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle):

- takes far more *[oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration)* from the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) crossing it — several times its resting draw;
- takes far more *glucose* — the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) ’s ready [sugar-nutrient](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) , its everyday fuel;
- releases correspondingly more *[carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration)* — and *heat* .

This is [Proposition 46.5](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#prop-g7-what-is-respiration-power)’s cellular burning, read from an organ’s meters: fuel and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in, work, waste gas and warmth out.

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

**Example 49.2 (Measuring across a muscle).**

Physiologists compare [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) *entering* a [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) with [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) *leaving* it. At rest, the leaving [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) has given up a modest share of its [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [glucose](#prop-g7-organs-at-work-bill). In hard work the same comparison shows the leaving [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) stripped — most of its deliverable [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) taken — while [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) has jumped. Same organ, same meters: only the work changed, and the bill followed it.

**Remark 49.3 (The warmth is the receipt).**

The working body’s heat is not an accident: burning fuel always yields warmth, and [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) is the body’s furnace room. Shivering is the trick run deliberately — [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) set to work at nothing but warmth ([Example 18.2](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#ex-g3-balanced-diet-rest)’s standing charge, briefly doubled). The sprint’s flushed [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) is the cooling system carrying furnace heat away — next section’s business.

## 49.2 Deliveries on demand

**Proposition 49.4 (The body reorganizes its supply).**

The moment work begins, the supply lines reorganize to pay the bill ([Example 27.7](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#ex-g4-heart-and-blood-effort) announced it; here is the full dispatch):

1. *[breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration)* deepens and quickens — reloading the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) faster at the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) ( [Example 26.6](https://one-course.com/books/biology/1/en/chapter/26-breathing#ex-g4-breathing-running) );
2. the *[heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)* beats faster and harder — more rounds per minute ( [Proposition 27.6](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) );
3. the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) is *redirected* : the [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) serving the working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) (and the cooling [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) ) widen, while those of the resting [digestive tract](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) narrow — the same [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) , rerouted to the floor that is spending.

All three follow the work automatically, steered by the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system)’s unasked services ([Remark 33.4](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#rem-g5-brain-in-command-more)).

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

**Example 49.5 (The dispatch, felt from inside).**

The stair sprint’s stock-taking, explained line by line: racing breath — reloading; hammering [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) — faster rounds; burning [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) — the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’ bill being paid at the limit; flushed warm [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) — furnace heat carried out to the cooling surface. Even the classic stitch and the sluggishness after a big meal fit: [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) and sprint compete for the rerouted [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), which is why [Method 25.7](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#met-g4-journey-of-food-habits) advised against racing on a full [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract).

![A pulse through three minutes of hard effort: the climb with the work, the plateau near the maximum, and the slow return — recovery — as the body settles its accounts.](https://one-course.com/images/onecourse/chapters/biology-1/g7-organs-at-work/fig-cf70c059f8d8.svg)

*A [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) through three minutes of hard effort: the climb with the work, the plateau near the maximum, and the slow return — [recovery](#def-g7-organs-at-work-recovery) — as the body settles its accounts.*

**Definition 49.6 (Recovery).**

When the effort stops, [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) and [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) do not: they fall back only gradually — the *recovery* phase. The body is settling debts: reloading its [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) stores, clearing the effort’s wastes, shedding the accumulated heat. The fitter the body, the faster its recovery — which is why recovery time is itself a fitness measurement.

## 49.3 Training, and the limits of the machine

**Proposition 49.7 (Training raises the ceiling).**

Exercised regularly, the whole supply chain strengthens ([Remark 27.9](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#rem-g4-heart-and-blood-rest), [Method 26.8](https://one-course.com/books/biology/1/en/chapter/26-breathing#met-g4-breathing-care)): the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) moves more [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) per beat, 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) deepen, the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) themselves grow and store more fuel, and their fine [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) multiply. Result: the same effort costs a trained body a lower [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse), and its ceiling — the hardest effort it can pay for — rises. The machine is rebuilt by its own use.

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

**Example 49.8 (Two runners, one hill).**

Untrained and trained run the same hill. Untrained: [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) at $180$ halfway, gasping, forced to walk — the supply lines cannot pay the bill, and the debt calls a halt. Trained: [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) $150$ at the top, speech possible, [recovery](#def-g7-organs-at-work-recovery) in three minutes. Same hill, same bill — different ceilings, built by months of exactly such hills taken gently.

**Method 49.9 (Reading your own machine).**

A self-experiment with a watch ([Method 27.8](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#met-g4-heart-and-blood-pulse) supplies the [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) technique):

1. record the resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) , seated and calm;
2. do three minutes of steady effort — step-ups, skipping;
3. record the [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) at once, then each minute until it returns to rest: the [recovery](#def-g7-organs-at-work-recovery) time;
4. repeat weekly through a term of regular exercise: watch resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) , effort [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) and [recovery](#def-g7-organs-at-work-recovery) time all drift down — the rebuild of [Proposition 49.7](#prop-g7-organs-at-work-training) , in your own data.

## 49.4 Exercises

**Exercise 49.1 ★.**

List what a working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) takes more of, and what it releases more of.

**Solution of Exercise 49.1.**

More [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and more [glucose](#prop-g7-organs-at-work-bill) taken from the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb); more [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and more heat released.

**Exercise 49.2 ★.**

What is [glucose](#prop-g7-organs-at-work-bill), and where does the working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) get it?

**Solution of Exercise 49.2.**

The [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s ready [sugar-nutrient](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) — the everyday fuel of the [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’ burning. The [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) takes it from the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) flowing through it (stocked, further back, by [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion)’s deliveries).

**Exercise 49.3 ★.**

Describe the entering-versus-leaving [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) comparison, and what it shows in rest against effort.

**Solution of Exercise 49.3.**

[Blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) entering the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) is compared with [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) leaving it. At rest the leaving [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) has given up only a modest share of [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [glucose](#prop-g7-organs-at-work-bill); in hard work it [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) stripped of most deliverable [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), and much richer in [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). The difference is the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’s bill.

**Exercise 49.4 ★.**

Name the three supply reorganizations of [Proposition 49.4](#prop-g7-organs-at-work-supply).

**Solution of Exercise 49.4.**

[Breathing](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements) deepens and quickens; the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) beats faster and harder; the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) is redirected — [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) of working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) and cooling [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) widen while the resting [digestive tract](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s narrow.

**Exercise 49.5 ★.**

Why does hard work warm you — and what is shivering, in this chapter’s terms?

**Solution of Exercise 49.5.**

Burning fuel always yields warmth, and working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) is the body’s furnace. Shivering is [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) set to work producing nothing but that warmth — heating on demand.

**Exercise 49.6 ★.**

What is [recovery](#def-g7-organs-at-work-recovery), and what debts is the body settling during it?

**Solution of Exercise 49.6.**

The gradual return of [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) and [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) after effort ends. The body is reloading [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) stores, clearing the effort’s wastes, and shedding accumulated heat.

**Exercise 49.7 ★★.**

Read the [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) graph: resting value, plateau value, and roughly how long [recovery](#def-g7-organs-at-work-recovery) takes. What would training change in each?

**Solution of Exercise 49.7.**

Rest about $72$; plateau near $160$; [recovery](#def-g7-organs-at-work-recovery) roughly five to six minutes on the graph. Training lowers the resting value, lowers the plateau for the same effort, and shortens the [recovery](#def-g7-organs-at-work-recovery).

**Exercise 49.8 ★★.**

Explain the after-lunch sluggishness and the sprinter’s stitch with the rerouting rule.

**Solution of Exercise 49.8.**

Both are rerouting effects. After lunch the [digestive tract](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) holds a large share of the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), so the rest of the body runs sluggish; sprinting then forces the two floors to compete for deliveries — the stitch and the old rule against racing on a full [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract).

**Exercise 49.9 ★★.**

Why is the flushed [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) of effort a *delivery* change, not a malfunction? Whose problem is it solving?

**Solution of Exercise 49.9.**

Widened [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) are the dispatch of furnace heat to the body’s radiator surface: a delivery of heat outward, solving the working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’ overheating problem. The flush is the cooling system running, not failing.

**Exercise 49.10 ★★.**

List three rebuilds by which training raises the ceiling, and the everyday sign of each.

**Solution of Exercise 49.10.**

The [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) moves more [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) per beat — everyday sign, a lower resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse). The [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) grow and store more fuel — the same hill feels easier. The supply network’s fine [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) multiply and [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) deepens — speech stays possible during effort, and [recovery](#def-g7-organs-at-work-recovery) shortens.

**Exercise 49.11 ★★.**

Two students run [Method 49.9](#met-g7-organs-at-work-read): resting [pulses](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) $68$ and $86$; recoveries three and eight minutes. Which machine is the trained one, and what exactly do the two numbers each certify?

**Solution of Exercise 49.11.**

The $68$-rest, three-minute machine is the trained one. The resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) certifies a [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) that moves more per beat; the [recovery](#def-g7-organs-at-work-recovery) time certifies supply lines that settle the effort’s debts fast. Together they are the two ends of [Proposition 49.7](#prop-g7-organs-at-work-training)’s rebuild.

**Exercise 49.12 ★★★.**

Connect three chapters in one paragraph: the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’s bill ([Proposition 49.1](#prop-g7-organs-at-work-bill)), [respiration](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration)’s purpose ([Proposition 46.5](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#prop-g7-what-is-respiration-power)), and the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s delivery service ([Proposition 27.1](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-carries)) — as one account of a single sprint.

**Solution of Exercise 49.12.**

In a sprint, the [leg](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’ [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) burn [glucose](#prop-g7-organs-at-work-bill) with [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) to power contraction — [respiration](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration)’s purpose — and their bill (more fuel, more [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in; more [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), heat out) must be paid at once. The [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) is the paying service: loaded at [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) and intestine, pumped faster by the racing [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), rerouted to the [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), it delivers the fuel and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and carts off the [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) — one sprint, three chapters, a single account.

## 49.5 Problem: The School Fitness Study

**Problem 49.1.**

Weekend problem — a term of pulse data, read like a physiologist

The class runs [Method 49.9](#met-g7-organs-at-work-read) every Monday for a term: resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse), [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) after three minutes of step-ups, and [recovery](#def-g7-organs-at-work-recovery) time. Two anonymized records: student A — week 1: rest $84$, effort $168$, [recovery](#def-g7-organs-at-work-recovery) $9$ minutes; week 12: rest $72$, effort $150$, [recovery](#def-g7-organs-at-work-recovery) $5$ minutes. Student B — week 1: rest $70$, effort $150$, [recovery](#def-g7-organs-at-work-recovery) $4$ minutes; week 12: rest $69$, effort $148$, [recovery](#def-g7-organs-at-work-recovery) $4$ minutes.

**Part I — Inside one session.**

1. During the step-ups, list what is happening to a thigh [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) ’s [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [glucose](#prop-g7-organs-at-work-bill) draw, and to its [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and heat output.
2. Explain why [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) climb *together* during the minutes of effort.
3. A student feels warm and flushed by minute three. Give the furnace-and-cooling account.
4. Why does the [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) not drop to resting the moment the step-ups stop?

**Part II — Reading the term.**

5. Describe A’s changes across the term, and name the rebuilds of [Proposition 49.7](#prop-g7-organs-at-work-training) behind each number.
6. B’s numbers barely moved. Give the two honest readings — and which extra fact about B’s life outside class would decide between them.
7. Week 5, a student measures right after a hurried lunch and gets odd numbers. Which rule of [Proposition 49.4](#prop-g7-organs-at-work-supply) interfered, and what should the protocol say about measuring times?
8. Why must each student always measure the same way — same effort, same timing? (An old friend among methods; name it.)

**Part III — The physiologist’s letter.**

9. Write the two-sentence summary of A’s term for the class notice board, using *ceiling* and *[recovery](#def-g7-organs-at-work-recovery)* .
10. A parent asks whether a low resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) is worrying. Answer with [Exercise 27.11](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#exo-g4-heart-and-blood-11) ’s cyclist.
11. The class wants one number to track next term. Argue for [recovery](#def-g7-organs-at-work-recovery) time over effort [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) , using [Definition 49.6](#def-g7-organs-at-work-recovery) .
12. Close with the study’s moral in one sentence: what rebuilt A’s machine?

**Solution of Problem 49.1.**

**1.** Its [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [glucose](#prop-g7-organs-at-work-bill) draw from the passing [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) multiplies; its [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) output and heat output rise in step — the bill of hard work.

**2.** Because they are two stages of one supply line: [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) reloads the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) at the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) moves the reloaded [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) to the [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle). Demand raised at one end raises the pace of both.

**3.** Working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) are furnaces; their heat must leave. The [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) carries it to the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), whose widened [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) flush and radiate — furnace inside, radiator at the surface.

**4.** Because debts remain: [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) stores to reload, wastes to clear, heat to shed. The [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) falls only as the accounts settle — that falling is [recovery](#def-g7-organs-at-work-recovery).

**5.** Rest $84 \to 72$: a [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) rebuilt to move more per beat. Effort $168 \to 150$: the same step-ups now cost less — stronger [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), richer fine-vessel supply. [Recovery](#def-g7-organs-at-work-recovery) $9 \to 5$ minutes: the whole chain settles its debts faster. The term’s exercise did the rebuilding.

**6.** Either B began the term already trained — numbers near their trained floor move little — or B skipped the term’s exercise and merely kept a stable untrained machine. Deciding fact: B’s activity outside class (sport elsewhere would confirm the first reading; none, the second — and B’s good week-1 numbers lean that way already).

**7.** The rerouting rule: after a meal the [digestive tract](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) claims the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), so resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) reads high and effort tolerance low. Protocol: measure before lunch, or a good two hours after — always the same slot.

**8.** The fair test: only measurements taken the same way can be compared, week against week and student against student.

**9.** “Twelve weeks raised A’s ceiling: the same effort now costs $150$ beats instead of $168$, from a resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) eight beats lower. And A’s machine settles its debts in five minutes where September needed nine — [recovery](#def-g7-organs-at-work-recovery) is the clearest gain of all.”

**10.** Usually the opposite of worrying: a trained [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) moves more [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) per beat and so idles low — the cyclist’s calm $50$ doing an untrained $75$’s work. (Any concern belongs to a doctor, but low-and-fit is the common story.)

**11.** [Recovery](#def-g7-organs-at-work-recovery) time measures the whole chain — [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels), [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) — settling real debts, and it improves steadily and visibly with training; effort [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) depends more on the day’s exact effort and mood. As a single tracked number, [recovery](#def-g7-organs-at-work-recovery) is the honester summary.

**12.** Its own use: effort, repeated gently and regularly, is what rebuilt the machine that performs it.