B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

The Respiratory System: Breathing and Gas Exchange

B3.4 B3.3 B2.1 A1.1 A1.5 A1.6 A1.8 A1.13About 75 minutes

Learning goals

I can trace the path of air from the nose to the alveoli and describe the job of each structure.
I can explain how the diaphragm and rib muscles cause breathing, using volume and pressure.
I can explain how gas exchange happens by diffusion in the alveoli, and how alveoli are suited to it.
I can design a fair test of how exercise affects breathing rate, and analyse the data.

Words to know

respiratory system
The organ system that brings oxygen into the body and removes carbon dioxide.

trachea
The windpipe: a tube held open by rings of cartilage that carries air to the bronchi.

bronchi (singular: bronchus)
The two large tubes that branch from the trachea, one into each lung.

bronchioles
Smaller and smaller tubes that branch from the bronchi inside the lungs.

alveoli (singular: alveolus)
Tiny air sacs at the ends of the bronchioles where gas exchange happens.

diaphragm
A dome-shaped sheet of skeletal muscle below the lungs that contracts to make you breathe in.

gas exchange
The movement of oxygen into the blood and carbon dioxide out of the blood by diffusion.

breathing vs. cellular respiration
Breathing moves air in and out of the lungs; cellular respiration is the chemical reaction in mitochondria that uses oxygen to release energy.

Why can't you hold your breath for long?

Try it (sitting down): hold your breath. After some seconds, the urge to breathe becomes very strong. Interestingly, that urge comes mostly from carbon dioxide building up in your blood, not from running out of oxygen. Every cell in your body is doing cellular respiration (you studied it in Grade 9), using oxygen and producing carbon dioxide. The respiratory system's job is to bring in oxygen and get rid of carbon dioxide, all day and all night.

Breathing is not the same as cellular respiration

Breathing (ventilation): the physical movement of air into and out of the lungs.
Gas exchange: oxygen diffuses from the air in the lungs into the blood; carbon dioxide diffuses the other way.
Cellular respiration: the chemical reaction in mitochondria: glucose + oxygen → carbon dioxide + water + energy.

The respiratory system handles the first two; the circulatory system connects them to the third.

The path of air

Nose and nasal cavity: hairs and sticky mucus trap dust and microbes; blood vessels warm the air; the moist lining adds water vapour. (Breathing through your mouth skips this filtering.)
Pharynx (throat): shared by air and food.
Larynx (voice box): contains the vocal cords. The epiglottis covers it when you swallow.
Trachea (windpipe): held open by C-shaped rings of cartilage so it doesn't collapse. It is lined with ciliated epithelial cells and mucus-making cells: the cilia sweep mucus with trapped particles up to the throat, where it is swallowed or coughed out.
Bronchi: the trachea splits into two bronchi, one to each lung.
Bronchioles: the bronchi branch again and again into thousands of tiny tubes, like the branches of a tree.
Alveoli: each bronchiole ends in clusters of tiny air sacs. The two lungs together contain hundreds of millions of alveoli, giving a gas-exchange surface of tens of square metres, many times the area of your skin.

The human respiratory system (simplified, not to scale). Branching tubes carry air to millions of alveoli, shown here as a tiny cluster at the end of one bronchiole. The diaphragm below the lungs powers breathing.

Gas exchange in the alveoli

Each alveolus is wrapped in a net of capillaries. Gases move by diffusion, from where they are more concentrated to where they are less concentrated:

Air in the alveolus has more oxygen than the blood arriving from the body, so oxygen diffuses into the blood, where red blood cells pick it up.
Blood arriving from the body has more carbon dioxide than the air in the alveolus, so carbon dioxide diffuses out and is breathed out.

Alveoli are built for fast diffusion (structure fits function again):

Huge surface area (hundreds of millions of alveoli).
Very thin walls: the alveolus and capillary walls are each made of one layer of flat (squamous) epithelial cells.
Moist lining, so gases dissolve.
Rich blood supply and constant breathing keep the concentration differences high.

Gas exchange at one alveolus. Oxygen and carbon dioxide each diffuse from high to low concentration across two thin walls. Blood leaving the lungs is rich in oxygen; air breathed out is richer in carbon dioxide.

How breathing works: volume and pressure

Your lungs have no muscles of their own. They are pulled open and pushed closed by the chest around them.

Inhaling: the diaphragm contracts and flattens (moves down), and the muscles between the ribs pull the ribs up and out. The chest cavity gets bigger, so the air pressure inside the lungs drops below the air pressure outside. Air flows in.
Exhaling (at rest): the diaphragm and rib muscles relax. The diaphragm domes back up and the ribs fall. The chest gets smaller, pressure inside rises above the outside pressure, and air flows out.

Air always flows from higher pressure to lower pressure.

Breathing in and out (simplified side view of the chest). The diaphragm (thick curved line) flattens to breathe in and domes up to breathe out. Changing the volume of the chest changes the pressure, and air moves to even it out.

Gas	Inhaled air (approx.)	Exhaled air (approx.)
Nitrogen	78%	78%
Oxygen	21%	16%
Carbon dioxide	0.04%	4%
Water vapour	varies	more (saturated)

Exhaled air still contains a lot of oxygen (which is why rescue breathing works), but about 5% of the air has been swapped from oxygen to carbon dioxide.

Worked example

Let's calculate how much air and oxygen you use

At rest, Ali breathes 14 times per minute and moves 0.5 L of air with each breath. Use the table above to estimate (a) the air he breathes per minute and (b) the volume of oxygen his body takes in per minute.

(a) Air per minute = breaths per minute × volume per breath = 14 × 0.5 L = 7.0 L/min.
(b) Oxygen breathed in each minute: 21% of 7.0 L = 0.21 × 7.0 = 1.47 L. Oxygen breathed out: 16% of 7.0 L = 0.16 × 7.0 = 1.12 L.
Oxygen taken into the body = 1.47 − 1.12 = 0.35 L/min. (Shortcut: 5% of 7.0 L = 0.35 L.)
These are estimates: real percentages and volumes vary from person to person, so we round to 2 significant figures.

(a) 7.0 L of air per minute; (b) about 0.35 L of oxygen per minute absorbed.

Worked example

Let's design a fair test

Question: How does exercise affect breathing rate? Write a hypothesis, identify the variables and describe how to make the test fair and reliable.

Hypothesis: If a person exercises, then their breathing rate will increase, because working muscle cells do more cellular respiration, using more oxygen and producing more carbon dioxide.
Independent variable: activity level (resting, after 1 minute of walking on the spot, after 1 minute of jumping jacks). Dependent variable: breaths per minute.
Controlled variables: the same person, the same length of each activity, counting for the same time (for example, 30 s × 2), counting immediately after stopping, sitting in the same position to count.
Reliability: do 3 trials of each level, resting until breathing returns to normal between trials, and calculate the mean.
Safety: stop if dizzy or short of breath; people with asthma or a heart condition should only do gentle activity, with a grown-up's OK.

A fair test changes only the activity level, measures breaths per minute the same way each time, repeats each level 3 times and uses the mean.

Just for fun

What did the alveolus say to the capillary?

"I think we should exchange something. Maybe some gases?"

Hands-on activity

Breathing-rate investigation

You need: a timer, a partner (or count your own breaths), paper for a data table, and safe space. Safety: if you have asthma or a heart or breathing condition, only do gentle activity, and stop if you feel dizzy or unwell.

Sit quietly for 2 minutes. Count your breaths for 30 seconds and multiply by 2. Repeat to get 3 resting trials.
Walk on the spot for 1 minute, sit, and count immediately. Rest until your breathing is back to normal. Repeat 3 times.
Do jumping jacks (or fast stepping) for 1 minute and count again, 3 trials.
Make a table (activity, trial 1, trial 2, trial 3, mean) and a bar graph of the means.
Write a conclusion: does your evidence support the hypothesis? Name one source of error.

What to look for: resting rates are often around 12 to 20 breaths per minute for teens and adults, and rates rise with harder activity. A common source of error is that breathing starts slowing the moment you stop, so counting must start right away. Another: knowing you are being counted can change how you breathe.

Extension (with a grown-up nearby): take a normal breath and breathe out fully into a balloon, then measure the balloon's width. Balloons are a choking hazard for small children; keep them, and any broken pieces, away from little ones and people with latex allergies.

Your turn

List, in order, the structures air passes through from the nose to the alveoli.

What keeps the trachea from collapsing?

Explain what happens to the diaphragm, chest volume and pressure when you inhale.

Give three features of alveoli that make gas exchange efficient.

Why does oxygen move from the alveoli into the blood?

What is the difference between breathing and cellular respiration?

Exhaled air has 16% oxygen and 4% carbon dioxide. Explain the change from inhaled air.

More practice: warm-up

Where does gas exchange happen?

What do cilia in the trachea do?

Which flap covers the larynx when you swallow?

About what percentage of inhaled air is oxygen?

Which muscle sheet below the lungs is the main breathing muscle?

More practice: on your own

A person breathes 18 times a minute, 0.4 L per breath. How much air do they breathe in one hour?

Why is it better to breathe through your nose than your mouth on a cold winter day?

Explain why the walls of alveoli are made of squamous (flat) epithelial cells, using structure and function.

A student's resting breathing counts are 15, 16 and 14 breaths per minute. Find the mean, and say why three trials are better than one.

In a model of the lungs, a balloon inside a bottle inflates when a rubber sheet at the bottom is pulled down. Explain which body parts each item represents and why the balloon inflates.

Challenge

Smoking damages the cilia in the airways. Predict two effects on the respiratory system and explain why.

In emphysema, the walls between alveoli break down, forming fewer, larger air sacs. Explain why a person with emphysema becomes short of breath, using surface area.

During exercise, a student's breathing rises to 40 breaths per minute with 2.0 L per breath, and 4% of the air is swapped from O₂ to CO₂. How much oxygen does she absorb per minute? Compare it to Ali's 0.35 L/min at rest.

Check your understanding

Quiz

Where in the lungs does gas exchange take place?

When you inhale, what happens to the pressure inside your lungs?

Oxygen moves from the alveoli into the blood by...

Which tissue holds the trachea open?

A person breathes 12 times per minute, 0.5 L per breath. How much air per minute?

Which statement is correct?

For parents and teachers

Big idea: expectation B3.4 asks students to explain the primary functions of animal systems, such as "the respiratory system supplies oxygen to and removes carbon dioxide from the organism." Students should be able to trace the airway, explain breathing using volume and pressure, and explain gas exchange by diffusion.

How to help: Do the breathing-rate investigation together; you can be the timer. Check the data table has units and a mean. If your teen has asthma, keep the activity gentle and have their inhaler nearby.

Common difficulties:

Saying the lungs "suck in" air by themselves (the diaphragm and ribs change the chest's volume).
Confusing breathing with cellular respiration.
Thinking exhaled air has no oxygen.
Mixing up which way each gas diffuses.