Breathing and gas exchange

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Notes de leçon

Big idea: a system that swaps gases

Big idea (key concept): Systems. The breathing system is a set of organs that work together. The airways, the lungs, the ribs and the muscles each do one job, and together they bring oxygen in and take carbon dioxide out.
Related concept: Function. Every part of the system has a shape that suits its job. A tube kept open by rings, a muscle that flattens, a sac with a one-cell-thick wall: in each case, structure explains function.
Global context: Identities and relationships. Your breathing changes with who you are and what you do: asleep, running, anxious, or living with a condition such as asthma. Understanding the system helps us look after our own bodies and other people's.
Your cells need oxygen for respiration, and respiration makes carbon dioxide as a waste product. The breathing system exchanges these two gases with the air.
Breathing (ventilation) is moving air in and out of the lungs. Gas exchange is the swapping of oxygen and carbon dioxide between the air in the lungs and the blood. Respiration is the chemical reaction in cells that releases energy. They are three different things.

The breathing system

Air enters through the nose or mouth and passes down the trachea (windpipe). The nose has hairs and mucus that trap some dust.
The trachea is held open by C-shaped rings of cartilage. Without them it could collapse when the pressure inside it drops as you breathe in.
The trachea splits into two bronchi (one is a bronchus), one going to each lung. Each bronchus branches again and again into smaller tubes called bronchioles.
At the end of the smallest bronchioles are clusters of tiny air sacs called alveoli (one is an alveolus). There are hundreds of millions of them in your lungs, and this is where gas exchange happens.
The lungs are soft and spongy. They have no muscles of their own, so they cannot pull air in by themselves.
The lungs sit inside the thorax (chest), protected by the ribcage. The diaphragm, a sheet of muscle, forms the floor of the thorax. Intercostal muscles between the ribs move the ribcage.
The path of air is: nose or mouth, trachea, bronchi, bronchioles, alveoli. Used air takes the same path back out.

The human breathing system, from the nose to the alveoli

Ventilation: how air gets in and out

Air always moves from a place of higher pressure to a place of lower pressure. Breathing works by changing the volume of the thorax, which changes the pressure inside it.
Breathing in (inhaling): the intercostal muscles contract, so the ribcage moves up and out. The diaphragm contracts and flattens, moving down. The volume of the thorax increases, so the pressure inside falls below the air pressure outside. Air is pushed into the lungs by the higher pressure outside.
Breathing out (exhaling): the intercostal muscles relax, so the ribcage moves down and in. The diaphragm relaxes and returns to a dome shape, moving up. The volume of the thorax decreases, so the pressure inside rises above the air pressure outside. Air is forced out.
A common mistake is to say the lungs suck the air in. In fact the muscles change the size of the thorax, and the pressure difference moves the air.
At rest an adult breathes about 12 to 20 times per minute. During exercise both the rate and the depth of breathing go up.

Breathing in and breathing out: ribs, diaphragm and pressure

Gas exchange in the alveoli

Gas exchange happens by diffusion: particles spread from where there are lots of them (high concentration) to where there are fewer (low concentration), with no energy needed.
Air in an alveolus has more oxygen and less carbon dioxide than the blood arriving in the capillary around it. So oxygen diffuses into the blood and carbon dioxide diffuses out of the blood into the alveolus.
Inside the blood, oxygen joins with haemoglobin in red blood cells and is carried to the body. Carbon dioxide is carried back to the lungs, mostly dissolved in the plasma.
Alveoli are adapted for fast diffusion. They have a huge surface area (millions of alveoli), a very thin wall (one cell thick, so the diffusion distance is short), a moist lining (gases dissolve before they diffuse), and a dense network of capillaries.
Ventilation and blood flow keep the concentration gradient steep. Breathing brings fresh oxygen-rich air in and removes carbon dioxide, and the moving blood carries oxygen away and brings more carbon dioxide. If the gradient flattened, diffusion would slow down and stop.
Compare the air you breathe in with the air you breathe out. Inhaled air is about 21% oxygen and 0.04% carbon dioxide. Exhaled air is about 16% oxygen and 4% carbon dioxide. Only a part of the oxygen is taken out. Nitrogen (about 78%) does not change, and exhaled air also contains more water vapour.

Gas exchange at an alveolus

Lung volume, smoking and asthma

Tidal volume is the volume of air moved in or out in one normal breath, about 500 cm³ at rest. Vital capacity is the largest volume you can breathe out after breathing in as deeply as possible. Even then, some air always stays in the lungs. A spirometer is used to measure these volumes.
The airways are lined with cilia (tiny hairs) and cells that make mucus. Mucus traps dust and bacteria and the cilia sweep it upwards to be swallowed or coughed out.
Tobacco smoke contains three harmful things. Tar coats the airways, damages the cilia and contains chemicals that cause cancer. Nicotine is an addictive drug. Carbon monoxide joins to haemoglobin instead of oxygen, so the blood carries less oxygen.
Smoking can lead to chronic bronchitis (a long-lasting cough from inflamed airways full of mucus), emphysema (the walls of the alveoli break down, so the surface area for gas exchange falls and a person becomes short of breath) and lung cancer.
Asthma is a condition in which the airways become inflamed and narrow. The muscles in the walls of the bronchioles tighten and more mucus is made, so less air can pass and breathing out becomes hard.
Common asthma triggers are pollen, dust mites, cold air, smoke and exercise. A reliever inhaler contains a drug that relaxes the muscles in the airway walls so they widen again.

Cilia and mucus lining the airway

Think like a scientist: does exhaled air contain more carbon dioxide?

Limewater is a clear solution that turns cloudy (milky) when carbon dioxide is bubbled through it. The faster it goes cloudy, the more carbon dioxide is in the gas.
In the apparatus shown, the tubes are set up so that inhaled air bubbles through tube A only, and exhaled air bubbles through tube B only. You breathe in and out through one mouthpiece.
The independent variable is the type of air (inhaled or exhaled). The dependent variable is the time taken for the limewater to go cloudy. For a fair test, keep the volume and concentration of limewater, the size of the tubes and the number of breaths the same.
A prediction should come from the science: exhaled air should turn tube B cloudy faster because respiration in cells makes carbon dioxide, and some of it is breathed out.
Inquiry task 1: write a method to test whether breathing rate changes with exercise. Say what you will change, what you will measure, three variables you will control and how many times you will repeat it.
Inquiry task 2: a student tests only herself and says “exercise doubles breathing rate for everyone”. Evaluate this conclusion and suggest two improvements.

Testing inhaled and exhaled air with limewater

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Questions d'entraînement

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1.Which tube carries air from the nose and mouth down towards the lungs?
Easy
- AThe diaphragm
- BA bronchiole
- CThe oesophagus
- DThe trachea
2.What are the tiny air sacs at the ends of the airways called?
Easy
- ACapillaries
- BAlveoli
- CBronchioles
- DBronchi
3.Which muscle forms the floor of the thorax and helps you breathe?
Easy
- AThe diaphragm
- BThe stomach wall
- CThe heart
- DThe trachea
4.Which gas do body cells need to take in from the air for respiration?
Easy
- ANitrogen
- BCarbon dioxide
- CHydrogen
- DOxygen
5.Which gas moves from the blood into the alveoli so that it can be breathed out?
Easy
- AOxygen
- BNitrogen
- CHydrogen
- DCarbon dioxide
6.What happens to the ribcage when you breathe in?
Easy
- AIt moves up and in
- BIt moves down and in
- CIt moves up and out
- DIt stays completely still
7.Which gas makes limewater turn cloudy?
Easy
- ACarbon dioxide
- BNitrogen
- COxygen
- DWater vapour
8.Which substance in cigarette smoke is addictive?
Easy
- AOxygen
- BCarbon monoxide
- CNicotine
- DTar

Plus de questions

9.What does a reliever inhaler do for a person having an asthma attack?
Easy
- AIt relaxes the airway muscles so the airways widen
- BIt adds oxygen straight into the blood
- CIt makes the alveoli bigger so more air fits
- DIt stops the body making mucus for life
10.Which of these is the correct path of air into the lungs?
Easy
- AAlveoli, bronchioles, bronchi, trachea
- BBronchi, trachea, alveoli, bronchioles
- CTrachea, bronchi, bronchioles, alveoli
- DTrachea, bronchioles, bronchi, alveoli
11.Breathing and respiration mean exactly the same thing.
Easy
True or false?
12.The lungs contain muscles that pull air in.
Easy
True or false?
13.When you breathe out at rest, the diaphragm relaxes and moves up.
Easy
True or false?
14.Most of the air we breathe in is nitrogen.
Easy
True or false?
15.Complete the sentence.
Easy
The trachea divides into two tubes called the ____, one for each lung.
16.Complete the sentence.
Easy
A condition in which the airways become narrow and breathing is difficult is called ____.
17.Complete the sentence.
Easy
The muscles between the ribs are called the ____ muscles.
18.Match each part of the breathing system to its job.
Easy
- Trachea
- Alveoli
- Diaphragm
- Intercostal muscles
- Exchange gases with the blood
- Muscle that flattens when you breathe in
- Move the ribs up and out
- Carries air to the bronchi
19.Put these parts in the order that air passes through them when you breathe in.
Easy
- Bronchioles
- Trachea
- Alveoli
- Bronchi
20.Why is it useful that the wall of an alveolus is only one cell thick?
Medium
- AIt makes the alveolus stronger
- BIt lets the alveolus hold more air
- CGases have only a short distance to diffuse
- DIt stops the blood from leaving the capillary
21.Why do the lungs contain millions of alveoli rather than one big air sac?
Medium
- ATogether they give a huge surface area for diffusion
- BThey produce extra oxygen for the blood
- CThey make the lungs light enough to float
- DThey keep the air warm and dry inside
22.What is the job of the rich blood supply around the alveoli?
Medium
- AIt warms the air in the alveoli before it is breathed out
- BIt carries oxygen away, keeping the gradient steep
- CIt makes the walls of the alveoli thicker and stronger
- DIt pushes air out of the alveoli when the muscles relax
23.Which process moves oxygen from the alveoli into the blood?
Medium
- AFiltration
- BContraction
- CDigestion
- DDiffusion
24.When you breathe in, what happens to the pressure inside the thorax?
Medium
- AIt becomes zero
- BIt rises above the air pressure outside
- CIt stays exactly the same
- DIt falls below the air pressure outside
25.
Here are the approximate gas percentages in inhaled and exhaled air.
Gas Inhaled air (%) Exhaled air (%)
Nitrogen 78 78
Oxygen 21 16
Carbon dioxide 0.04 4
Which gas shows no real change?
Medium
- AThey all change
- BOxygen
- CCarbon dioxide
- DNitrogen
26.
Using the same table of inhaled and exhaled air, which statement is correct?
Gas Inhaled air (%) Exhaled air (%)
Nitrogen 78 78
Oxygen 21 16
Carbon dioxide 0.04 4
Medium
- AExhaled air has less carbon dioxide than inhaled air
- BExhaled air is mostly carbon dioxide
- CExhaled air still contains a lot of oxygen
- DExhaled air contains no oxygen
27.What is the job of the C-shaped rings of cartilage in the trachea?
Medium
- AThey stop the trachea collapsing when pressure falls
- BThey push air along the trachea
- CThey trap dust and bacteria in the airway
- DThey exchange gases with the blood
28.What is the job of mucus and cilia in the airways?
Medium
- AMucus traps dirt and microbes and cilia sweep it up
- BMucus makes oxygen and cilia pump the blood
- CCilia push air down into the alveoli
- DMucus and cilia warm the air in the alveoli
29.How does carbon monoxide in cigarette smoke harm the body?
Medium
- AIt joins to haemoglobin, so the blood carries less oxygen
- BIt sticks to the cilia and stops them moving
- CIt makes the walls of the alveoli much thicker
- DIt is an addictive drug that affects the brain
30.Why does tar in smoke make a smoker cough more?
Medium
- AIt stops the diaphragm contracting properly
- BIt damages cilia, so mucus and dirt build up
- CIt makes the lungs grow bigger and rougher
- DIt gives the blood extra oxygen to carry
31.What is tidal volume?
Medium
- AThe largest volume of air you can breathe out
- BThe volume of air moved in or out in one normal breath
- CThe number of breaths in one minute
- DThe volume of air that always stays in the lungs
32.Which of these are adaptations of the alveoli for gas exchange? (select all that apply)
Medium
- AA thick muscular wall
- BA very thin wall
- CRings of cartilage
- DA large total surface area
- EA rich supply of capillaries
33.Which of these can trigger an asthma attack? (select all that apply)
Medium
- APollen
- BDrinking water
- CCold air
- DEating a balanced diet
- ETobacco smoke
34.Oxygen is carried around the body by haemoglobin inside red blood cells.
Medium
True or false?
35.Match each substance in tobacco smoke to its effect.
Medium
- Tar
- Nicotine
- Carbon monoxide
- Makes smoking addictive
- Reduces oxygen carried by the blood
- Damages cilia and can cause cancer
36.Put these steps of breathing in into the correct order.
Medium
- Air flows into the lungs
- The volume of the thorax increases
- The intercostal muscles and diaphragm contract
- The pressure inside the thorax falls
37.Complete the sentence.
Medium
Oxygen is carried in red blood cells by a protein called ____.
38.Complete the sentence.
Medium
Gases move into and out of the blood by ____, from a high to a low concentration.
39.
Use the table to work out how many percentage points the oxygen level falls between inhaled and exhaled air.
Gas Inhaled air (%) Exhaled air (%)
Oxygen 21 16
Carbon dioxide 0.04 4
Medium
40.Describe what happens to the diaphragm and the ribcage when you breathe in.
Medium
41.A student counts her breaths in one minute at rest and again after two minutes of jogging. What is the independent variable?
Medium
- AThe student's name
- BResting or jogging
- CThe length of the student's lungs
- DThe number of breaths per minute
42.In the same investigation, which of these must be controlled to make it a fair test?
Medium
- AWhether the student is resting or jogging
- BThe number of breaths counted
- CThe student's breathing rate
- DThe time over which breaths are counted
43.Why does the breathing system need to keep ventilating the lungs for gas exchange to continue?
Hard
- AThe lungs would otherwise run out of nitrogen and collapse
- BFresh air makes the walls of the alveoli thinner each time
- CFresh air keeps oxygen in the alveoli higher than in the blood
- DAir pressure pushes oxygen through the alveoli walls
44.In emphysema the walls between alveoli break down. Why does this make a person short of breath?
Hard
- AThe blood stops carrying haemoglobin altogether
- BThe trachea becomes too narrow for air to pass through
- CThere is less surface area for gas exchange, so less oxygen enters
- DThe diaphragm can no longer contract at all
45.
A student measured her breathing rate during exercise.
Time spent jogging (min) Breaths per minute
0 14
1 22
2 28
3 33
Which conclusion is best supported by these results?
Hard
- ABreathing rate rose as the time spent jogging increased
- BJogging makes lungs bigger
- CEveryone's breathing rate doubles after two minutes
- DBreathing rate falls when you exercise
46.In the investigation above only one student was tested, once. What is the best improvement?
Hard
- ATest the student for only 30 seconds
- BRepeat with several people and calculate a mean for each time
- CAsk the student to jog as fast as she can each time
- DUse a heavier student
47.During an asthma attack it is often breathing out that feels hardest. What is the best explanation?
Hard
- AThe alveoli fill with water and swell
- BThe trachea loses its rings of cartilage
- CNarrowed airways make it hard to push air out
- DThe diaphragm stops relaxing completely
48.The carbon dioxide in exhaled air is made in the alveoli.
Hard
True or false?
49.Which of these happen when you breathe out at rest? (select all that apply)
Hard
- AThe volume of the thorax increases
- BThe diaphragm relaxes and moves up
- CThe ribcage moves down and in
- DThe intercostal muscles contract strongly
- EThe pressure in the thorax rises above the pressure outside
50.Put these steps in the order taken by an oxygen molecule from the air to a body cell.
Hard
- The blood carries it around the body
- It reaches an alveolus in inhaled air
- It is released to a respiring cell
- It joins with haemoglobin in a red blood cell
- It dissolves in the moist lining and diffuses across the alveolus wall
51.Explain how the structure of an alveolus makes gas exchange fast.
Hard
52.Explain why a long-term smoker may get out of breath much more easily than a non-smoker.
Hard
53.Plan a fair test to find out whether breathing rate changes with exercise. Say what you would change, what you would measure and two things you would keep the same.
Hard

Notes de révision

## The big idea - The breathing system is a set of organs working together to bring oxygen in and take carbon dioxide out (**Systems**, **Function**). - **Breathing** moves air, **gas exchange** swaps gases with the blood, **respiration** happens in cells. ## Structure - Nose or mouth, **trachea**, **bronchi**, **bronchioles**, **alveoli**. - The **diaphragm** and **intercostal muscles** change the size of the **thorax**; the lungs have no muscles. ## Ventilation - **In:** muscles contract, ribs up and out, diaphragm flattens, volume up, pressure down. - **Out:** muscles relax, ribs down and in, diaphragm domes, volume down, pressure up. ## Gas exchange - Oxygen and carbon dioxide **diffuse** across the thin wall of the alveolus. - Adaptations: large **surface area**, thin wall, moist lining, rich blood supply; ventilation keeps the **gradient** steep. - Inhaled air is about 21% oxygen and 0.04% carbon dioxide; exhaled air is about 16% oxygen and 4% carbon dioxide. ## Health - **Smoking**: tar, nicotine and carbon monoxide. **Asthma**: narrow airways, relieved by an inhaler. ## Investigating - Change one variable, measure one, control the rest, repeat and take a mean.

Breathing (ventilation): Moving air into and out of the lungs.
Gas exchange: The swapping of oxygen and carbon dioxide between the air in the alveoli and the blood.
Trachea: The windpipe: a tube kept open by C-shaped rings of cartilage that carries air to the bronchi.
Bronchus: One of the two tubes that branch from the trachea, one into each lung.
Bronchiole: A very small airway that branches from a bronchus and ends at alveoli.
Alveolus: A tiny air sac in the lungs where gas exchange takes place.
Diaphragm: A sheet of muscle below the lungs that flattens when you breathe in.
Intercostal muscles: Muscles between the ribs that move the ribcage up and out or down and in.
Diffusion: The net movement of particles from a region of high concentration to a region of low concentration.
Concentration gradient: The difference in concentration between two places; a steeper gradient makes diffusion faster.
Haemoglobin: The protein in red blood cells that carries oxygen.
Tidal volume: The volume of air moved in or out of the lungs in one normal breath.
Vital capacity: The largest volume of air that can be breathed out after breathing in as deeply as possible.
Cilia: Tiny hairs on the airway lining that sweep mucus upwards.
Emphysema: A lung disease in which the walls of the alveoli break down, reducing the surface area for gas exchange.
Asthma: A condition in which the airways become inflamed and narrow, making breathing difficult.

Cartes mémo

What is the difference between breathing and respiration?

Breathing moves air in and out of the lungs. Respiration is a chemical reaction in cells that releases energy.

Give the path of air into the lungs.

Nose or mouth, trachea, bronchi, bronchioles, alveoli.

Why does the trachea have C-shaped rings of cartilage?

To keep it open when the pressure inside falls as you breathe in.

What happens when you breathe in?

Intercostal muscles and diaphragm contract, ribs go up and out, volume increases, pressure falls and air flows in.

What happens when you breathe out?

The muscles relax, ribs go down and in, volume decreases, pressure rises and air is forced out.

Why do the lungs not need their own muscles?

The diaphragm and intercostal muscles change the volume of the thorax, and the pressure change moves the air.

What is gas exchange?

Oxygen diffuses into the blood and carbon dioxide diffuses out, across the walls of the alveoli.

Name four adaptations of alveoli.

Large surface area, wall one cell thick, moist lining, rich capillary supply.

How do breathing and blood flow keep diffusion going?

They keep the concentration gradients steep by renewing the air and moving the blood.

What carries oxygen in the blood?

Haemoglobin in red blood cells.

What are the approximate percentages of oxygen and carbon dioxide in inhaled air?

About 21% oxygen and 0.04% carbon dioxide.

What are they in exhaled air?

About 16% oxygen and 4% carbon dioxide.

How do you test for carbon dioxide?

Bubble the gas through limewater; it turns cloudy.

What do cilia and mucus do?

Mucus traps dust and bacteria, and cilia sweep it up the airway.

Name the three harmful substances in tobacco smoke and one effect of each.

Tar damages cilia and causes cancer, nicotine is addictive, carbon monoxide reduces oxygen carried by blood.

What is emphysema?

Damage to the alveolus walls that reduces surface area, so a person is short of breath.

What happens in an asthma attack?

The airways narrow because the muscles tighten and the lining swells; a reliever inhaler relaxes the muscles.

What makes an investigation a fair test?

Change only one variable, measure one, and keep all the others the same.

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Gas	Inhaled air (%)	Exhaled air (%)
Nitrogen	78	78
Oxygen	21	16
Carbon dioxide	0.04	4

Gas	Inhaled air (%)	Exhaled air (%)
Nitrogen	78	78
Oxygen	21	16
Carbon dioxide	0.04	4

Time spent jogging (min)	Breaths per minute
0	14
1	22
2	28
3	33