---
title: "Removing the Wastes"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 53
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes
---

# Chapter 53 — Removing the Wastes

Deliveries have dominated this year: [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in, [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) in, fuel to every working [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell). But every workshop makes waste, and a delivery service that never collects the rubbish soon poisons its own city. The [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) account is already settled — breathed out at the [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli). This closing chapter follows the *other* wastes, to the body’s discreetest organs: the [kidneys](#def-g7-removing-wastes-kidneys).

## 53.1 The wastes beyond carbon dioxide

**Proposition 53.1 (The cells’ other rubbish).**

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 and building ([Proposition 46.5](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#prop-g7-what-is-respiration-power), [Proposition 41.1](https://one-course.com/books/biology/1/en/chapter/41-matter-from-food#prop-g6-matter-from-food-transform)) yield more than [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). Using building-matter [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families), in particular, [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) a nitrogen-bearing waste — *urea* — which the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) collects from every tissue exactly as it collects [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). [Urea](#prop-g7-removing-wastes-rubbish) is harmless in passing traffic and poisonous piled up: it must leave, continuously.

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

**Example 53.2 (Two collection rounds, one blood).**

The same drop of [Example 52.5](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#ex-g7-heart-and-circulation-tour) runs both collections at once: at the thigh’s [capillaries](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) it takes on [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) *and* [urea](#prop-g7-removing-wastes-rubbish). The first it drops at the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), next circuit. The second rides on — past the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), which cannot touch it — until the body circuit carries it through the organs built for exactly this cargo.

## 53.2 The kidneys, the blood’s filters

**Definition 53.3 (Kidneys and the urinary system).**

The two *kidneys* — fist-sized, bean-shaped, low in the back — are blood-cleaning filters. Each receives a generous [artery](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) straight from the body circuit’s main line. Inside, the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) is filtered through about a million microscopic filter units; the cleaned [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) returns by the kidney’s [vein](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels), and the removed load — [urea](#prop-g7-removing-wastes-rubbish), surplus water, surplus salts — [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) as *urine*, trickling down two tubes to the *bladder*, the holding tank emptied at will.

**Proposition 53.4 (Filter, then reclaim).**

The [kidney](#def-g7-removing-wastes-kidneys)’s trick is generosity followed by thrift. The filter units first push a huge volume of the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s watery part — many tens of litres a day — through their fine sieves, [urea](#prop-g7-removing-wastes-rubbish), sugars, salts and all. Then, along each unit’s winding tube, the body *reclaims* what it wants back: nearly all the water, all the [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill), salts to measure — leaving the [urea](#prop-g7-removing-wastes-rubbish) and the surpluses behind. Of the tens of litres sieved, about a litre and a half [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) as [urine](#def-g7-removing-wastes-kidneys): filter everything, take back the good, and the waste has nowhere to hide.

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

![The kidney’s two-stage trick: sieve generously, reclaim thriftily. What is not taken back — urea and the surpluses — is the urine.](https://one-course.com/images/onecourse/chapters/biology-1/g7-removing-wastes/fig-a31c04384b0c.svg)

*The [kidney](#def-g7-removing-wastes-kidneys)’s two-stage trick: sieve generously, reclaim thriftily. What is not taken back — [urea](#prop-g7-removing-wastes-rubbish) and the surpluses — is the [urine](#def-g7-removing-wastes-kidneys).*

**Example 53.5 (Reading the thrift).**

The reclaiming stage explains the everyday observations. Drink much, and the [kidneys](#def-g7-removing-wastes-kidneys) reclaim less water: [urine](#def-g7-removing-wastes-kidneys) plentiful and pale. [Sweat](#rem-g7-removing-wastes-sweat) a hot match dry, and they reclaim nearly all: [urine](#def-g7-removing-wastes-kidneys) scant and dark — the body guarding its water ([Proposition 22.3](https://one-course.com/books/biology/1/en/chapter/22-what-plants-need#prop-g4-what-plants-need-needs) taught water’s value for [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant); the [kidney](#def-g7-removing-wastes-kidneys) is the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) side of the same economy). And [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) in the [urine](#def-g7-removing-wastes-kidneys), which the sieve lets through but reclaiming should recover completely, is a classic medical signal that something — reclaiming or [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) sugar — is out of order.

**Remark 53.6 (The skin lends a hand).**

*Sweat* is a sideline of the same business: water and salts (with traces of [urea](#prop-g7-removing-wastes-rubbish)) released at the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) — though its first job is [Example 49.5](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#ex-g7-organs-at-work-felt)’s cooling. The [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), [kidneys](#def-g7-removing-wastes-kidneys) and [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) divide the body’s whole waste removal: [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) to the first, [urea](#prop-g7-removing-wastes-rubbish) and the surpluses to the second, heat and a salty trickle to the third. Nothing poisonous is left to pile up — in a healthy body, the rubbish never wins.

## 53.3 The year’s machine, complete

**Proposition 53.7 (The full supply-and-removal loop).**

With the [kidneys](#def-g7-removing-wastes-kidneys) in place, this year’s tour closes into one machine. The [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), pumped round the double circuit ([Definition 52.3](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-double)), loads [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) at the [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli) and [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) at the [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption); delivers both to every working [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) for burning and building; and collects the wastes — [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) to the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), [urea](#prop-g7-removing-wastes-rubbish) and surpluses to the [kidneys](#def-g7-removing-wastes-kidneys). Every organ of grades four and seven is a station of this single loop; every exchange crosses a [capillary](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) wall; and the whole runs on [Proposition 46.5](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#prop-g7-what-is-respiration-power)’s slow fire, [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) by [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

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

**Method 53.8 (The whole-body audit).**

For any substance, audit its passage:

1. entry: mouth and [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) , or nose and [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli) ;
2. transport: which circuit [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) , which special stops ( [liver](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) );
3. use: fuel burned, material built in, or neither;
4. exit: [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli) (gas), [kidneys](#def-g7-removing-wastes-kidneys) ( [urea](#prop-g7-removing-wastes-rubbish) , surpluses), [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) (sweat’s share) — or, never absorbed at all, the leftovers’ route ( [Proposition 25.5](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) ).

Run it on water, on [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill), on a salty crisp: the machine has a station for every step, which is the year’s finding in one exercise.

## 53.4 Exercises

**Exercise 53.1 ★.**

What is [urea](#prop-g7-removing-wastes-rubbish) — from what use does it arise, and why must it leave?

**Solution of Exercise 53.1.**

A nitrogen-bearing waste left by the use of building-matter [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) in the [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell). Harmless in passing traffic, it poisons if it piles up — so it must leave continuously.

**Exercise 53.2 ★.**

Which wastes leave at the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), which at the [kidneys](#def-g7-removing-wastes-kidneys), and which at the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)?

**Solution of Exercise 53.2.**

[Lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four): [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). [Kidneys](#def-g7-removing-wastes-kidneys): [urea](#prop-g7-removing-wastes-rubbish) and the surpluses of water and salts. [Skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense): heat, and sweat’s water and salts (with traces of [urea](#prop-g7-removing-wastes-rubbish)).

**Exercise 53.3 ★.**

Describe the [kidneys](#def-g7-removing-wastes-kidneys): number, size, place, and their plumbing to the [bladder](#def-g7-removing-wastes-kidneys).

**Solution of Exercise 53.3.**

Two, fist-sized and bean-shaped, low in the back, each fed by a generous [artery](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels). [Urine](#def-g7-removing-wastes-kidneys) trickles from each down its tube to the [bladder](#def-g7-removing-wastes-kidneys) — the holding tank emptied at will.

**Exercise 53.4 ★.**

State the filter-then-reclaim trick, with the two volumes that frame it.

**Solution of Exercise 53.4.**

First filter generously: many tens of litres of the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s watery part are pushed through the sieves daily, good and bad together. Then reclaim thriftily along the winding tubes. The frame: tens of litres sieved, about a litre and a half leaving as [urine](#def-g7-removing-wastes-kidneys).

**Exercise 53.5 ★.**

What is reclaimed completely, what to measure, and what not at all?

**Solution of Exercise 53.5.**

Completely: [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) (and nearly all the water). To measure: water and salts, tuned to the day’s needs. Not at all: [urea](#prop-g7-removing-wastes-rubbish) — the point of the whole exercise.

**Exercise 53.6 ★.**

Why is [urine](#def-g7-removing-wastes-kidneys) plentiful and pale one day, scant and dark another?

**Solution of Exercise 53.6.**

The reclaiming is tuned: after much drinking, less water is taken back — plentiful pale [urine](#def-g7-removing-wastes-kidneys); after sweating or little drinking, nearly all is reclaimed — scant dark [urine](#def-g7-removing-wastes-kidneys). The [kidney](#def-g7-removing-wastes-kidneys) guards the body’s water account.

**Exercise 53.7 ★★.**

Why can the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) not remove [urea](#prop-g7-removing-wastes-rubbish), and the [kidneys](#def-g7-removing-wastes-kidneys) not remove [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) fast enough? What does the divided labor say about the two wastes’ forms?

**Solution of Exercise 53.7.**

[Carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) is a gas, and gases cross at an air border — the [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli); [urea](#prop-g7-removing-wastes-rubbish) is not a gas and cannot leave with the breath, but it rides [dissolved](https://one-course.com/books/biology/1/en/chapter/48-oxygen-in-the-water#def-g7-oxygen-in-the-water-dissolved) and filters beautifully at a liquid border — the sieves. Each waste exits by the border its form suits: the division of labor is chemistry matched to architecture.

**Exercise 53.8 ★★.**

Explain why [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) in the [urine](#def-g7-removing-wastes-kidneys) alarms a doctor, through the sieve-and-reclaim stages.

**Solution of Exercise 53.8.**

The sieve passes [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) freely — that is normal; the reclaiming stage should then recover every unit of it. [Glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) arriving in the [urine](#def-g7-removing-wastes-kidneys) means reclaiming was overwhelmed or broken: either [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) sugar so high the [recovery](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#def-g7-organs-at-work-recovery) cannot keep pace, or the [recovery](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#def-g7-organs-at-work-recovery) itself failing — both worth a doctor’s attention.

**Exercise 53.9 ★★.**

Follow one [urea](#prop-g7-removing-wastes-rubbish) unit from a thigh [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’s [capillary](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) to the [bladder](#def-g7-removing-wastes-kidneys), station by station.

**Solution of Exercise 53.9.**

[Muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) [capillary](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) — [vein](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) — right [atrium](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-rooms) and [ventricle](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-rooms) — [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) circuit (the [urea](#prop-g7-removing-wastes-rubbish) rides on; the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) cannot touch it) — left [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) — body [artery](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) to the [kidney](#def-g7-removing-wastes-kidneys) — sieved at a filter unit — *not* reclaimed — down the collecting tube, down to the [bladder](#def-g7-removing-wastes-kidneys).

**Exercise 53.10 ★★.**

Run [Method 53.8](#met-g7-removing-wastes-audit) on a glass of water drunk after sport.

**Solution of Exercise 53.10.**

Entry: no dismantling, crossing at the [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) into the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb). Transport: [liver](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) first, then both circuits. Use: the medium of every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s wet chemistry, and the cooling system’s supply. Exit: shared among [urine](#def-g7-removing-wastes-kidneys) (tuned by reclaiming), sweat (the sporty share), breath’s moisture, and a little in the leftovers.

**Exercise 53.11 ★★.**

[Kidney](#def-g7-removing-wastes-kidneys) patients whose filters fail visit a machine that cleans their [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) through an artificial [membrane](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts). Which stage of [Proposition 53.4](#prop-g7-removing-wastes-filter) does the machine copy, and why must sessions recur every few days?

**Solution of Exercise 53.11.**

The sieve — filtering wastes from [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) across a fine [membrane](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts); the machine is an external filter unit. Sessions recur because the [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) never stop making [urea](#prop-g7-removing-wastes-rubbish) ([Proposition 53.1](#prop-g7-removing-wastes-rubbish)): a filter that runs twice a week must catch up on days of accumulation, where the [kidneys](#def-g7-removing-wastes-kidneys)’ sieves run every minute.

**Exercise 53.12 ★★★.**

Write the year’s summary paragraph promised by [Proposition 53.7](#prop-g7-removing-wastes-complete): one [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), two circuits, four borders ([alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli), [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption), [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) [capillaries](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels), [kidney](#def-g7-removing-wastes-kidneys) filters) — and the slow fire that the whole machine serves.

**Solution of Exercise 53.12.**

For example: one [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), driven round the [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) and body circuits by the four-roomed [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), loads [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) at the [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli) and [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) at the [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption), and delivers both to every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s [capillaries](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels), where the slow fire of [respiration](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) burns fuel with [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) to power all work. The same [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) collects the fire’s ashes — [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) to the [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli), [urea](#prop-g7-removing-wastes-rubbish) and surpluses to the [kidney](#def-g7-removing-wastes-kidneys)’s sieve-and-reclaim filters — so the four borders serve one economy. Every organ of the year is a station of this loop, and the loop exists so that the fire in the [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) never lacks fuel, [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), or rubbish collection.

## 53.5 Problem: The Day of a Water Molecule

**Problem 53.1.**

Weekend problem — one glass of water, audited through the whole machine

Follow a glass of water drunk at breakfast on a sports day, as the year’s grand audit.

**Part I — Entry and transport.**

1. Water needs no dismantling ( [Definition 50.4](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) ). Name its entry crossing and the vessel network that receives it.
2. Its [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) makes the standard first stop. Which organ, and why is the routing sensible for [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) even if water itself needs no sorting?
3. By mid-morning the water is everywhere the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) goes. Name the two circuits it now rides in alternation.
4. Some of it is soon inside working [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) . List two jobs water does there (recall that the [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ’ chemistry — juices, burning — runs wet).

**Part II — The afternoon match.**

5. During the match, part of the water [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) by a border that is not an excretory organ first. Name it, its cargo besides water, and its real job that day.
6. The match over, the player is thirsty and her [urine](#def-g7-removing-wastes-kidneys) that evening is scant and dark. Explain both with the reclaiming stage.
7. Meanwhile the match’s hard-worked [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) made extra [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [urea](#prop-g7-removing-wastes-rubbish) . Assign each its exit border, and the circuit [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) that took it there.
8. Why did the [kidneys](#def-g7-removing-wastes-kidneys) keep working right through the match, though [urine](#def-g7-removing-wastes-kidneys) production fell? What were they still removing?

**Part III — The audit closed.**

9. Next morning, some of the glass has left in breath. Explain how water exits at the [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli) ( [Example 26.7](https://one-course.com/books/biology/1/en/chapter/26-breathing#ex-g4-breathing-mirror) showed it years ago).
10. Draw up the glass’s full ledger: entry, stores and uses, and its four exits — [urine](#def-g7-removing-wastes-kidneys) , sweat, breath, and the small share in the leftovers.
11. The audit shows one [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) serving [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) , [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) , [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) , [kidneys](#def-g7-removing-wastes-kidneys) and [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) in turn. State, in two sentences, why this single-network design makes every organ’s health every other organ’s business.
12. Close the year: which single idea — name it — did every station of this audit turn out to serve? ( [Proposition 46.5](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#prop-g7-what-is-respiration-power) holds the answer.)

**Solution of Problem 53.1.**

**1.** Crossing at the [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) of the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract), into the [capillaries](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) — the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) network receives it.

**2.** The [liver](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption). The routing serves the [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) sharing the same [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) — sorting and banking — and water simply rides the standard line through it.

**3.** The [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) circuit and the body circuit, in strict alternation through the four-roomed [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart).

**4.** It is the medium 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)’ chemistry — juices and burning run [dissolved](https://one-course.com/books/biology/1/en/chapter/48-oxygen-in-the-water#def-g7-oxygen-in-the-water-dissolved) — and the carrier of everything the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) transports; it also feeds the cooling reserve the afternoon will spend as sweat.

**5.** The [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense): sweat — water and salts — whose real job that day is cooling the furnace-hot working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) ([Remark 49.3](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#rem-g7-organs-at-work-warmth)).

**6.** Sweating spent the water account; the [kidneys](#def-g7-removing-wastes-kidneys)’ reclaiming stage answered by taking nearly all sieved water back — scant, concentrated, dark [urine](#def-g7-removing-wastes-kidneys) — while thirst demanded the account be refilled.

**7.** [Carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration): [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) [capillary](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels), [vein](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels), right [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), [alveoli](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#def-g7-lungs-and-gas-exchange-alveoli) — breathed out. [Urea](#prop-g7-removing-wastes-rubbish): the same return, then past the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), left [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), [kidney](#def-g7-removing-wastes-kidneys) [artery](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels), sieved and sent bladder-ward.

**8.** [Urea](#prop-g7-removing-wastes-rubbish) and the salts of sweat-changed [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb): waste production never pauses, so the sieves never may. [Urine](#def-g7-removing-wastes-kidneys) *volume* fell — reclaiming kept the water — but the cleaning ran on.

**9.** The alveolar surface is moist, and each breath sweeps warmed, moistened air out — the misted mirror’s water. Part of any glass [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) as breath’s [humidity](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions).

**10.** Entry: [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption). Stores and uses: [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) volume, [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) chemistry, cooling reserve. Exits: [urine](#def-g7-removing-wastes-kidneys) (reclaim-tuned), sweat (the match’s share), breath’s moisture, and a small share never absorbed, out with the leftovers.

**11.** Because every organ draws supplies from, and unloads wastes into, the one shared [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb): a failing [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) starves every [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), a failing [kidney](#def-g7-removing-wastes-kidneys) poisons every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), a furred [artery](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels) throttles whichever station lies beyond it. One network means one fate, station by station.

**12.** [Respiration](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) — the slow fire in every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): entry, transport, burning, building and every waste border exist to feed it, supply it, and carry away its ashes.