---
title: "The Heart and Circulation"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 52
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation
---

# Chapter 52 — The Heart and Circulation

[Chapter 27](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#ch-g4-heart-and-blood) introduced the pump and its delivery rounds; since then the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) has acquired responsibilities — [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) ([Proposition 50.6](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption)), [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) ([Proposition 51.4](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#prop-g7-lungs-and-gas-exchange-numbers)), bills at every working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) ([Proposition 49.1](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill)). Time to map the road network properly: the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s four rooms, the two circuits, and the three kinds of vessel.

## 52.1 The heart’s four rooms

**Definition 52.1 (Atria and ventricles).**

The [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s two sides ([Definition 27.5](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)) each hold two rooms: an upstairs *atrium* that receives incoming [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), and a downstairs *ventricle* — the thick-walled pumping room — that drives it out. Between and beyond them, *valves* — one-way doors — snap shut behind each surge; their paired snaps are exactly the “lub-dub” a stethoscope (or an [ear](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) on a chest) hears.

**Proposition 52.2 (One beat, in order).**

Each beat runs the same sequence on both sides at once: the atria contract, topping up the [ventricles](#def-g7-heart-and-circulation-rooms); the [ventricles](#def-g7-heart-and-circulation-rooms) contract, driving [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) out — right side toward the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), left side toward the body — while the entry [valves](#def-g7-heart-and-circulation-rooms) slam (lub); the [ventricles](#def-g7-heart-and-circulation-rooms) relax, the exit [valves](#def-g7-heart-and-circulation-rooms) slam (dub), and the rooms refill. Then again, $100000$ times a day ([Remark 27.9](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#rem-g4-heart-and-blood-rest)).

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

## 52.2 Two circuits, three vessels

**Definition 52.3 (The double circulation).**

The [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s road network is two loops through one pump ([Definition 27.5](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s two sides, now as circuits):

- the *[lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) circuit* : right [ventricle](#def-g7-heart-and-circulation-rooms) $\rightarrow$ [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) (loading [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) , unloading [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) ) $\rightarrow$ left [atrium](#def-g7-heart-and-circulation-rooms) ;
- the *body circuit* : left [ventricle](#def-g7-heart-and-circulation-rooms) $\rightarrow$ the whole body (delivering, collecting) $\rightarrow$ right [atrium](#def-g7-heart-and-circulation-rooms) .

Every drop alternates circuits: [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), body, [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), body — which is why the left [ventricle](#def-g7-heart-and-circulation-rooms), pushing round the far larger body loop, is the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s thickest-walled room.

**Definition 52.4 (Arteries, capillaries, veins).**

Three kinds of vessel make each loop ([Definition 27.3](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels), now named):

- *arteries* : thick-walled, elastic, carrying [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) *away* from the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) under pressure — the [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) ( [Proposition 27.6](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) ) is their stretch at each surge;
- *capillaries* : the hair-fine [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) threading every organ — walls one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) thin, where *all* the exchanges happen: [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) , [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) , [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) ;
- *veins* : wide, thinner-walled, returning [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) *to* the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) at low pressure, with one-way [valves](#def-g7-heart-and-circulation-rooms) of their own — the bluish lines of [Remark 27.4](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#rem-g4-heart-and-blood-see) .

![The double circulation: right side to the lungs and back to the left; left side round the body and back to the right. Red arrows carry the oxygen-rich stretches, blue the returning ones.](https://one-course.com/images/onecourse/chapters/biology-1/g7-heart-and-circulation/fig-ad470fd1d1b2.svg)

*The double circulation: right side to the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) and back to the left; left side round the body and back to the right. Red arrows carry the oxygen-rich stretches, blue the returning ones.*

**Example 52.5 (One drop, one full tour).**

[Example 27.2](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#ex-g4-heart-and-blood-round)’s delivery round, with the map’s names: right [ventricle](#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) [capillaries](#def-g7-heart-and-circulation-vessels) ([oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) aboard 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), bright red) — left [atrium](#def-g7-heart-and-circulation-rooms), left [ventricle](#def-g7-heart-and-circulation-rooms) — [artery](#def-g7-heart-and-circulation-vessels) — a thigh [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’s [capillaries](#def-g7-heart-and-circulation-vessels) ([oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) off, [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) on, darker) — [vein](#def-g7-heart-and-circulation-vessels), [valves](#def-g7-heart-and-circulation-rooms) easing the uphill trip — right [atrium](#def-g7-heart-and-circulation-rooms). Two circuits, under a minute, no wrong turns available: the [valves](#def-g7-heart-and-circulation-rooms) see to that.

**Remark 52.6 (Why capillaries are the point).**

[Arteries](#def-g7-heart-and-circulation-vessels) and [veins](#def-g7-heart-and-circulation-vessels) are only highways; every actual delivery — every exchange this year has studied — happens across [capillary](#def-g7-heart-and-circulation-vessels) walls. The specification of [Proposition 47.1](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#prop-g7-how-animals-breathe-spec) lives here too: [one-cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) walls (thin), threading every tissue by the tens of thousands of kilometres ([Definition 27.3](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) — large), bathed in the body’s moisture. The [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) exists to keep [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) moving *through [capillaries](#def-g7-heart-and-circulation-vessels)*; all the rest is plumbing to and from.

## 52.3 Reading and keeping the network

**Example 52.7 (Measurements at the roadside).**

Three everyday readings of the network: the *[pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse)* — [artery](#def-g7-heart-and-circulation-vessels) stretch, counting the surges ([Method 27.8](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#met-g4-heart-and-blood-pulse)); *[blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) pressure*, the cuff’s two numbers — the [artery](#def-g7-heart-and-circulation-vessels) pressure at surge and between surges; and the stethoscope’s *lub-dub* — the [valves](#def-g7-heart-and-circulation-rooms)’ doors heard closing. Three windows, no incision.

![The consultation’s two classics: the model showing the rooms and vessels, the stethoscope that hears their doors close.](https://one-course.com/images/onecourse/chapters/biology-1/g7-heart-and-circulation/img-94d33a114b56.jpg)

*The consultation’s two classics: the model showing the rooms and [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels), the stethoscope that hears their doors close.*

**Proposition 52.8 (Keeping the network).**

The network’s long-term enemies and friends are known:

- *exercise* strengthens the pump ( [Proposition 49.7](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-training) ) and keeps the highways supple;
- *balanced eating* ( [Method 18.8](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#met-g3-balanced-diet-check) ) protects the [artery](#def-g7-heart-and-circulation-vessels) walls, which too-rich [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) slowly [furs](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) up and stiffens;
- *smoke* attacks [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) as well as [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) ( [Proposition 51.6](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#prop-g7-lungs-and-gas-exchange-smoke) ) — narrowing, stiffening, seat-stealing;
- the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) ’s own supply — it feeds by its own small [arteries](#def-g7-heart-and-circulation-vessels) , not from the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) in its rooms — depends on all three.

The habits of [Proposition 36.1](https://one-course.com/books/biology/1/en/chapter/36-healthy-choices#prop-g5-healthy-choices-contract) are, concretely, vessel maintenance.

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

**Method 52.9 (Mapping any organ into the network).**

For any organ this book meets:

1. place it on the body circuit: [artery](#def-g7-heart-and-circulation-vessels) in, [vein](#def-g7-heart-and-circulation-vessels) out, [capillaries](#def-g7-heart-and-circulation-vessels) within;
2. list what crosses its [capillary](#def-g7-heart-and-circulation-vessels) walls, in and out ( [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) : [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) in; [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) : fuel and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in, [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) out; [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) : heat out);
3. note its special connection, if any (intestine’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) calls at the [liver](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) first; the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) sit on their own circuit);
4. check the flows against the organ’s job — the network diagram should retell the organ’s chapter.

## 52.4 Exercises

**Exercise 52.1 ★.**

Name the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s four rooms and each one’s role.

**Solution of Exercise 52.1.**

Right [atrium](#def-g7-heart-and-circulation-rooms): receives the body’s returning [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb). Right [ventricle](#def-g7-heart-and-circulation-rooms): pumps it to the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four). Left [atrium](#def-g7-heart-and-circulation-rooms): receives the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ oxygen-rich [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb). Left [ventricle](#def-g7-heart-and-circulation-rooms): pumps it round the whole body — the thickest-walled of the four.

**Exercise 52.2 ★.**

What are the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s [valves](#def-g7-heart-and-circulation-rooms) for, and what is the lub-dub?

**Solution of Exercise 52.2.**

They are one-way doors that stop [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) surging backward. The lub-dub is their paired closings: entry [valves](#def-g7-heart-and-circulation-rooms) slamming as the [ventricles](#def-g7-heart-and-circulation-rooms) drive (lub), exit [valves](#def-g7-heart-and-circulation-rooms) slamming as they relax (dub).

**Exercise 52.3 ★.**

Trace the two circuits, naming start, destination and return for each.

**Solution of Exercise 52.3.**

[Lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) circuit: right [ventricle](#def-g7-heart-and-circulation-rooms) to the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ [capillaries](#def-g7-heart-and-circulation-vessels) (load [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), unload [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration)), returning to the left [atrium](#def-g7-heart-and-circulation-rooms). Body circuit: left [ventricle](#def-g7-heart-and-circulation-rooms) to the whole body’s [capillaries](#def-g7-heart-and-circulation-vessels) (deliver, collect), returning to the right [atrium](#def-g7-heart-and-circulation-rooms).

**Exercise 52.4 ★.**

Give the three vessel kinds with one structural feature and one job each.

**Solution of Exercise 52.4.**

[Arteries](#def-g7-heart-and-circulation-vessels): thick, elastic walls — carry [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) away from the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) under pressure. [Capillaries](#def-g7-heart-and-circulation-vessels): walls one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) thin — the site of every exchange. [Veins](#def-g7-heart-and-circulation-vessels): wide, thinner walls with one-way [valves](#def-g7-heart-and-circulation-rooms) — return [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) to the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) at low pressure.

**Exercise 52.5 ★.**

Why is the left [ventricle](#def-g7-heart-and-circulation-rooms) the thickest-walled room?

**Solution of Exercise 52.5.**

It drives the body circuit — the far larger loop, out to toes and [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) and back — so it must push hardest, and its wall is built to match.

**Exercise 52.6 ★.**

Where — in which vessel kind — do all the body’s exchanges happen? Name three organs’ [capillary](#def-g7-heart-and-circulation-vessels) exchanges.

**Solution of Exercise 52.6.**

In the [capillaries](#def-g7-heart-and-circulation-vessels). [Lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ [capillaries](#def-g7-heart-and-circulation-vessels): [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in, [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) out; [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption)’s: [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) in; [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)’: fuel and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) off, [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) on ([skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)’s: heat out).

**Exercise 52.7 ★★.**

Run [Example 52.5](#ex-g7-heart-and-circulation-tour) for a drop serving the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) instead of the thigh. What does the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system)’s [capillary](#def-g7-heart-and-circulation-vessels) exchange take and leave? ([Proposition 33.9](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#prop-g5-brain-in-command-protect) lists its appetites.)

**Solution of Exercise 52.7.**

Right [ventricle](#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) [capillaries](#def-g7-heart-and-circulation-vessels) ([oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) aboard) — left [atrium](#def-g7-heart-and-circulation-rooms) — left [ventricle](#def-g7-heart-and-circulation-rooms) — neck [arteries](#def-g7-heart-and-circulation-vessels) — the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system)’s [capillaries](#def-g7-heart-and-circulation-vessels) — neck [veins](#def-g7-heart-and-circulation-vessels) — right [atrium](#def-g7-heart-and-circulation-rooms). The [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system)’s exchange takes a large, steady share of [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) — the demanding commander’s standing order.

**Exercise 52.8 ★★.**

The [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) is felt at [arteries](#def-g7-heart-and-circulation-vessels), never at [veins](#def-g7-heart-and-circulation-vessels). Explain with the two [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels)’ builds and pressures.

**Solution of Exercise 52.8.**

The [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) is the [artery](#def-g7-heart-and-circulation-vessels) wall’s elastic stretch at each ventricular surge — high pressure meeting a stretchy thick wall. By the [veins](#def-g7-heart-and-circulation-vessels) the surge has been spent crossing the [capillaries](#def-g7-heart-and-circulation-vessels): low, smooth pressure in a soft wide tube — nothing left to throb.

**Exercise 52.9 ★★.**

[Veins](#def-g7-heart-and-circulation-vessels) have one-way [valves](#def-g7-heart-and-circulation-rooms); [arteries](#def-g7-heart-and-circulation-vessels) none. What problem do the [vein](#def-g7-heart-and-circulation-vessels) [valves](#def-g7-heart-and-circulation-rooms) solve — and why do soldiers standing stock-still sometimes faint?

**Solution of Exercise 52.9.**

The uphill return: from the [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), venous [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) must climb against gravity at low pressure, and the [valves](#def-g7-heart-and-circulation-rooms) hold each gain. The still-standing soldier’s calf [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) never squeeze the [veins](#def-g7-heart-and-circulation-vessels), the climb stalls, [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) pools in the [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system)’s supply dips — the faint. Marching feet are [vein](#def-g7-heart-and-circulation-vessels) pumps.

**Exercise 52.10 ★★.**

Apply [Method 52.9](#met-g7-heart-and-circulation-map) to the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract), special connection included.

**Solution of Exercise 52.10.**

[Artery](#def-g7-heart-and-circulation-vessels) in from the body circuit; within, the [villi](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption)’s [capillaries](#def-g7-heart-and-circulation-vessels) take up the meal’s [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) ([Proposition 50.6](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption)); the special connection: its [vein](#def-g7-heart-and-circulation-vessels) runs first to the [liver](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#prop-g7-digestion-and-nutrients-absorption) for sorting and banking, and only then does the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) rejoin the general return to the right [atrium](#def-g7-heart-and-circulation-rooms). The diagram retells the [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) chapter.

**Exercise 52.11 ★★.**

Why does the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) need its own small feeding [arteries](#def-g7-heart-and-circulation-vessels), with its rooms full of [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)? And why does their furring matter so much?

**Solution of Exercise 52.11.**

The rooms’ [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) rushes through; the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s own thick walls, working without rest, need their own [capillary](#def-g7-heart-and-circulation-vessels) delivery like any [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) — so small feeding [arteries](#def-g7-heart-and-circulation-vessels) [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) over the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) itself. Furred, they throttle the pump’s own fuel line: the one organ that can never pause is the one their blockage starves.

**Exercise 52.12 ★★★.**

“The [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) is plumbing in the service of [capillaries](#def-g7-heart-and-circulation-vessels).” Defend the claim using [Remark 52.6](#rem-g7-heart-and-circulation-capillaries) and two exchanges from earlier chapters.

**Solution of Exercise 52.12.**

Every exchange the body lives by — [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), [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), fuel and wastes at every [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) — happens only across [capillary](#def-g7-heart-and-circulation-vessels) walls; [arteries](#def-g7-heart-and-circulation-vessels) and [veins](#def-g7-heart-and-circulation-vessels) merely bring [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) to those walls and take it away, and the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) merely keeps it moving. Pump and highways exist for the sake of the hair-fine lanes: stop the [capillary](#def-g7-heart-and-circulation-vessels) traffic anywhere and that organ dies, however sound the plumbing.

## 52.5 Problem: The Cardiologist’s Morning

**Problem 52.1.**

Weekend problem — four consultations, one map of the network

Shadow a cardiologist for a morning of four (invented) consultations, map in hand.

**Part I — The healthy baseline.**

1. Patient 1, a fit teenager, gets the three roadside readings of [Example 52.7](#ex-g7-heart-and-circulation-measures) . Name each, and what part of the machinery each reads.
2. Her resting [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) is $58$ . The cardiologist smiles. Why ( [Exercise 27.11](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#exo-g4-heart-and-blood-11) )?
3. Her lub-dub is crisp and double. What exactly snapped, twice, in every beat?
4. Sketch her [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) ’s path from right [atrium](#def-g7-heart-and-circulation-rooms) round both circuits and back, in eight named stops.

**Part II — Three troubles.**

5. Patient 2 has a leaky valve between left [atrium](#def-g7-heart-and-circulation-rooms) and [ventricle](#def-g7-heart-and-circulation-rooms) : some [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) surges backward at each beat, with a whooshing murmur. Predict the consequences for the body circuit’s deliveries and the patient’s stamina.
6. Patient 3, a heavy smoker with a rich diet, has furred, stiffened [arteries](#def-g7-heart-and-circulation-vessels) . Which readings of consultation 1 will have worsened, and why does the left [ventricle](#def-g7-heart-and-circulation-rooms) work overtime?
7. Patient 3’s furring includes the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) ’s own feeding [arteries](#def-g7-heart-and-circulation-vessels) . Explain the cardiologist’s special concern in one sentence.
8. Patient 4 has swollen [ankles](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) after long standing days: the [veins](#def-g7-heart-and-circulation-vessels) ’ uphill return is struggling. Which structures are failing, and why does walking — calf [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) squeezing the [veins](#def-g7-heart-and-circulation-vessels) — relieve it?

**Part III — The wall chart.**

9. The clinic’s wall chart shows the double loop. Explain to a waiting patient why [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) must visit the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) between every two body tours.
10. Add the vessel legend: [artery](#def-g7-heart-and-circulation-vessels) , [capillary](#def-g7-heart-and-circulation-vessels) , [vein](#def-g7-heart-and-circulation-vessels) — one line each on build, pressure and job.
11. The chart’s footer reads: “all treatment serves the [capillaries](#def-g7-heart-and-circulation-vessels) .” Justify it with [Remark 52.6](#rem-g7-heart-and-circulation-capillaries) .
12. Close the morning with the prevention prescription the cardiologist hands every patient — three habits, each tied to its mechanism in [Proposition 52.8](#prop-g7-heart-and-circulation-keep) .

**Solution of Problem 52.1.**

**1.** The [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse): the [arteries](#def-g7-heart-and-circulation-vessels)’ surge count — the pump’s rate read at a [wrist](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint). [Blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) pressure: the [artery](#def-g7-heart-and-circulation-vessels) pressure at surge and between — the highways’ load. The stethoscope’s lub-dub: the [valves](#def-g7-heart-and-circulation-rooms) heard closing — the doors’ soundness.

**2.** 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) in a fit patient means a strong [ventricle](#def-g7-heart-and-circulation-rooms) moving 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 trained [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) idles calm, with reserve in hand.

**3.** The valve doors: entry [valves](#def-g7-heart-and-circulation-rooms) at the [ventricles](#def-g7-heart-and-circulation-rooms)’ drive (lub), exit [valves](#def-g7-heart-and-circulation-rooms) at their relaxation (dub) — four doors closing in two pairs, cleanly.

**4.** Right [atrium](#def-g7-heart-and-circulation-rooms), right [ventricle](#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) [capillaries](#def-g7-heart-and-circulation-vessels), left [atrium](#def-g7-heart-and-circulation-rooms), left [ventricle](#def-g7-heart-and-circulation-rooms), body [arteries](#def-g7-heart-and-circulation-vessels), body [capillaries](#def-g7-heart-and-circulation-vessels) (say, a [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)), [veins](#def-g7-heart-and-circulation-vessels) — and home to the right [atrium](#def-g7-heart-and-circulation-rooms).

**5.** Each beat loses part of its surge backward through the leak, so the body circuit’s deliveries per beat shrink; the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) compensates with more beats and harder work, and stamina drops — the murmur is delivery lost as backwash.

**6.** [Blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) pressure up (stiff, narrowed highways resist the surge) and the [pulse](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse)’s [artery](#def-g7-heart-and-circulation-vessels) stretch changed; the left [ventricle](#def-g7-heart-and-circulation-rooms), pushing every round against the stiffened circuit, works overtime and thickens.

**7.** Furring in the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s own feeding [arteries](#def-g7-heart-and-circulation-vessels) throttles the fuel line of the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) that can never rest — the classic path to a [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) attack.

**8.** The [veins](#def-g7-heart-and-circulation-vessels)’ one-way [valves](#def-g7-heart-and-circulation-rooms) and the missing muscle-pump of movement: standing still, the calves never squeeze, the [valves](#def-g7-heart-and-circulation-rooms)’ gains stall, and the [ankles](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) pool. Walking squeezes the [veins](#def-g7-heart-and-circulation-vessels) rhythmically — the relief is mechanical.

**9.** Because the body’s tour spends 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): only the [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) circuit can reload it. Skipping the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) would send spent [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) on deliveries with an empty van — the double loop is refuel-then-deliver, enforced by the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart)’s two sides.

**10.** [Artery](#def-g7-heart-and-circulation-vessels): thick, elastic, high pressure — outbound highways. [Capillary](#def-g7-heart-and-circulation-vessels): one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) thin, threading everything — the delivery lanes where all exchanges happen. [Vein](#def-g7-heart-and-circulation-vessels): wide, valved, low pressure — the return.

**11.** Every consultation’s trouble mattered exactly where it touched [capillary](#def-g7-heart-and-circulation-vessels) service: the leak starved deliveries, the furring throttled supply lines, the pooling stalled returns. Readings, [valves](#def-g7-heart-and-circulation-rooms) and highways are means; [capillary](#def-g7-heart-and-circulation-vessels) traffic — the exchanges — is the end.

**12.** Move daily — the pump strengthens and the highways stay supple; eat balanced — the [artery](#def-g7-heart-and-circulation-vessels) walls stay clear of furring; no smoke — [vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) neither narrow nor stiffen, and the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s seats stay free for [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration).