Body Reps · Physiology

The lungs and breathing

Every breath you take on the gym floor has one job: get oxygen into your blood and get carbon dioxide out. This page follows the air in, shows you the muscles that move it, and ends where the swap actually happens: the alveoli.

You already know two of the players. Your ribs open and close every time you breathe. And the obliques you met on Day 2 help you breathe out hard.

Start training Try the interactives

Sources for this intro: [S5], [S6], [S7]

Route

The airway route

Air travels in this order:

nose or mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli

Everything up to the smallest bronchioles is plumbing. It moves the air and cleans, warms and moistens it on the way. Gas exchange only happens at the very end.

Original schematic. Gas exchange only happens at the alveoli.

Nose and mouth

The nose is the main way in and out. Hairs and sticky mucus in the nose trap dust. The lining warms and moistens the air. Air can also come in through your mouth, straight into the pharynx.

No name block here: these are everyday words. Source: [S5].

Pharynx

Say it: FA-rinks
Word parts: pharynx: Greek for throat.
Everyday: throat
What it is: The muscular tube at the back of the nose and mouth. It carries both air and food.
In the gym: Whether you breathe through your nose or your mouth on a hard set, the air passes through the pharynx.

Ph sounds like F, and F comes before L. Pharynx comes before larynx on the way down.

Larynx

Say it: LA-rinks
Word parts: larynx: Greek for the upper windpipe.
Everyday: voice box. Its biggest cartilage makes the Adam’s apple.
What it is: The cartilage box between the pharynx and the trachea. It holds the vocal cords. A flap called the epiglottis closes over it when you swallow.
In the gym: Your vocal cords sit here, so this is where you talk from. The talk test (energy systems page) uses that: the harder you’re breathing, the fewer words you can get out.

Hum, then touch the front of your throat. That buzz is your larynx, the voice box.

Trachea

Say it: truh-KEE-uh
Word parts: trachea: from Greek trakheia artēria, literally rough artery. It was named for its rings of cartilage.
Everyday: windpipe
What it is: The tube from the larynx down into the chest. C-shaped rings of cartilage hold it open.
In the gym: The rings stop it collapsing, so the main airway stays open while you breathe in and out.

The rough artery: the ancient Greeks named the windpipe for its bumpy rings.

Bronchi

Say it: BRONG-kye. One is a bronchus (BRONG-kus).
Word parts: bronchus: from Greek bronkhos, windpipe.
Everyday: main airways of the lungs
What it is: The two tubes the trachea splits into, one into each lung. They keep branching into the bronchial tree.
In the gym: Like the trachea, they are held open by cartilage and lined with mucus that traps dust and germs.

The windpipe splits like an upside-down Y: one bronchus for each lung.

Bronchioles

Say it: BRONG-kee-ohlz
Word parts: bronchiole: a little bronchus (-iole marks it as small).
Everyday: small airways
What it is: The small branches that come off the bronchi. They have muscle in their walls but no cartilage.
In the gym: Because their walls are muscle, they can widen or narrow to change how much air gets through. In an asthma attack they narrow, which is why breathing gets harder.

Bronchi-OLE: the “-ole” ending marks the little ones.

Alveoli

Say it: al-VEE-uh-lye. One is an alveolus (al-VEE-uh-lus).
Word parts: alveolus: Latin for a small hollow or cavity.
Everyday: air sacs
What it is: The tiny air sacs at the end of the airways, wrapped in capillaries. This is where gas exchange happens.
In the gym: This is the point of the whole system. Oxygen crosses into your blood here and carbon dioxide crosses out.

Bunches of grapes on the end of the branches. The alveoli are the grapes.

Sources for the airway: [S5]. Word parts: [S18].

The breathing muscles

The lungs have no muscle of their own to pull air in. They are stuck to the inside of your chest wall by a thin film of fluid, so when the chest gets bigger the lungs get bigger too. The muscles below do the moving.

Diaphragm

Say it: DYE-uh-fram
Word parts: dia: across. phragma: a fence or partition.
Everyday: none in common use. It’s the sheet of muscle under your lungs.
What it is: The main breathing muscle. A sheet of muscle that separates the chest from the abdomen.
In the gym: When it contracts it moves down, making more room in the chest so air flows in. It works on every breath, at rest and in a hard set.

A fence across your middle: chest above, belly below. Contract it and the fence drops.

External intercostals

Say it: ek-STUR-nul in-ter-KOS-tulz
Word parts: external: outer. inter: between. costal: from Latin costa, rib.
Everyday: the muscles between your ribs (outer layer)
What it is: The outer layer of muscles between the ribs. They lift the ribs up and out when you breathe in.
In the gym: They work with the diaphragm on every breath in. Put your hands on your sides and breathe in hard: that widening is them at work.

Externals help air Enter. E for E.

Internal intercostals

Say it: in-TUR-nul in-ter-KOS-tulz
Word parts: internal: inner. inter: between. costal: rib.
Everyday: the muscles between your ribs (inner layer)
What it is: The inner layer of muscles between the ribs. They pull the ribcage in when you breathe out hard.
In the gym: At rest they aren’t needed for breathing out. In a hard effort they help squeeze the ribcage smaller to force air out faster.

IN-ternals pull the ribs IN.

Sources for the muscles: [S6]. Word parts: [S18].

Breathing in and out

Inspiration

Say it: in-spih-RAY-shun
Word parts: in: in. spirare: to breathe. From Latin inspirare, to blow into.
Everyday: breathing in, inhaling
What it is: Breathing in. It is always active: muscles have to contract.
In the gym: At rest the diaphragm and external intercostals do it. When you work hard, extra muscles join in, including the scalenes in your neck, which lift the top of the ribcage.

Inspiration literally means breathing in, and it always takes muscle.

Expiration

Say it: ek-spih-RAY-shun
Word parts: ex: out. spirare: to breathe. From Latin exspirare, to breathe out.
Everyday: breathing out, exhaling
What it is: Breathing out. At rest it is passive. In hard exercise it becomes active.
In the gym: Quiet breathing out needs no muscle effort: the diaphragm and intercostals relax and the stretchy lungs spring back. In a hard effort, the internal intercostals and your abdominal muscles, including the obliques, contract to push air out faster.

At rest, breathing out is free: let go and the lungs spring back like a stretched resistance band.

Summary: who does what

	At rest (quiet breathing)	Hard exercise (forced breathing)
Breathing in	Active. Diaphragm + external intercostals.	Active. Diaphragm + external intercostals + extra muscles such as the scalenes in the neck.
Breathing out	Passive. Muscles relax, lungs recoil.	Active. Internal intercostals + abdominal muscles, including the obliques.

Breathing in · at rest

Active. Diaphragm + external intercostals.

Breathing in · hard exercise

Active. Diaphragm + external intercostals + extra muscles such as the scalenes in the neck.

Breathing out · at rest

Passive. Muscles relax, lungs recoil.

Breathing out · hard exercise

Active. Internal intercostals + abdominal muscles, including the obliques.

Sources: [S6]. Word parts: [S18].

Pressure and volume, in plain words

Air always moves from where the pressure is higher to where it is lower.

Bigger space, lower pressure. When the diaphragm drops and the ribs lift, your chest gets bigger. The air inside your lungs has more room, so its pressure falls below the air outside. Air rushes in.
Smaller space, higher pressure. When the muscles relax (or squeeze, in a hard effort), your chest gets smaller. The pressure inside rises above the air outside. Air is pushed out.

Think of a pair of bellows by a fire. Pull the handles apart and air is sucked in. Push them together and it’s blown out. Your chest is the bellows. Your lungs just follow along.

This rule has a name, Boyle’s law: when the space a gas is in gets bigger, its pressure drops, and when the space gets smaller, its pressure rises.

Source: [S6]

Gas exchange at the alveoli

Breathing gets fresh air to the alveoli. The swap itself happens by diffusion.

Air in the alveoli is high in oxygen. Blood arriving from the heart is low in oxygen. So oxygen moves from the alveoli into the blood.
Blood arriving from the body is high in carbon dioxide. Air in the alveoli is lower in it. So carbon dioxide moves from the blood into the alveoli, and you breathe it out.
No energy is needed. Each gas simply moves from where there’s more of it to where there’s less.

Three things make it fast:

Thin walls. The alveolus wall and the capillary wall together are extremely thin.
Close contact. Each alveolus is wrapped in capillaries, so blood passes right next to the air.
Huge surface area. There are a great many alveoli across both lungs.

When you breathe harder, the air in the alveoli is refreshed faster, so oxygen gets in and carbon dioxide gets out more quickly. Your breathing rate and depth go up mainly because carbon dioxide in your blood rises, not because oxygen falls.

The same swap happens in reverse at your muscles. Oxygen moves out of the blood into the working muscle, and carbon dioxide moves out of the muscle into the blood.

Diffusion

Say it: dih-FYOO-zhun
Word parts: from Latin diffundere: dis, apart or in every direction, and fundere, to pour.
Everyday: spreading out
What it is: The movement of a substance from where there is more of it to where there is less, with no energy needed.
In the gym: It’s how oxygen gets from your lungs into your blood, and from your blood into your working muscles.

Spray deodorant in one corner of the changing room and soon the whole room smells of it. Gas spreads from crowded to empty.

Capillaries

Say it: kuh-PIL-uh-reez
Word parts: capillary: from Latin capillaris, of hair.
Everyday: the smallest blood vessels
What it is: The smallest, thinnest-walled blood vessels. They link arteries to veins. In the lungs they wrap around the alveoli.
In the gym: Capillaries are where blood actually swaps gases with the air in your lungs and with your working muscles.

Capillary comes from the Latin for hair. Picture hair-thin tubes.

Sources: [S1], [S3], [S5], [S6], [S7]. Word parts: [S18].

Try it

Interactives

Order the airway

Seven stops. Tap them in the order air travels.

Your route so far

Tap the next stop

Tap the next stop in order.

Drag the gases across

Move O₂ from the alveolus into the blood, and CO₂ from the blood into the alveolus. Drag with mouse or finger, or tap a token then tap the destination.

Alveolus (air)

Capillary (blood)

Drag or tap: O₂ into blood, CO₂ into alveolus.

Recall

Quick check

Type each answer from memory. No list.

Sources

[S1] OpenStax, 19.1 Heart Anatomy (systemic capillary exchange) — openstax.org
[S3] British Heart Foundation, How your heart works — bhf.org.uk
[S5] OpenStax, 22.1 Organs and Structures of the Respiratory System — openstax.org
[S6] OpenStax, 22.3 The Process of Breathing — openstax.org
[S7] OpenStax, 22.4 Gas Exchange — openstax.org
[S18] Online Etymology Dictionary (Etymonline) — etymonline.com

All accessed 6 October 2026.

The airway route

Nose and mouth

Pharynx

Larynx

Trachea

Bronchi

Bronchioles

Alveoli

The breathing muscles

Diaphragm

External intercostals

Internal intercostals

Breathing in and out

Inspiration

Expiration

Summary: who does what

Breathing in · at rest

Breathing in · hard exercise

Breathing out · at rest

Breathing out · hard exercise

Pressure and volume, in plain words

Gas exchange at the alveoli

Diffusion

Capillaries

Interactives

Order the airway

Drag the gases across

Alveolus (air)

Capillary (blood)

Quick check

Sources

Train the names