Breathing and gas exchange · System
Exercise, asthma and smoking
Three things that change how well you breathe, and they act on three different parts of the system. Naming which part is most of the answer.
Start here
An asthma inhaler works in a room full of perfectly good air.
Someone having an asthma attack is standing outdoors. The air around them is 21% oxygen, the same as everyone else's. They cannot get enough of it. A puff from a blue inhaler helps within minutes, and it contains no oxygen at all.
If the air is fine, what has gone wrong?
The route in has narrowed. Muscle in the walls of the bronchioles has contracted, the lining has swollen and produced extra mucus, and air cannot get through fast enough — with 21% oxygen sitting right outside the mouth. The inhaler relaxes that muscle. Nothing about the air needed fixing.
Locate the fault · three factors
Which part of the system does each one hit?
0 of 3 opened
Commit to a part of the system before opening each one. The four options are the same every time, and only one of the three factors damages more than one part.
Factor 1 · during and after hard exercise
Someone runs 400 metres flat out. Their breathing rate rises from about 14 breaths a minute to over 40, and each breath is far deeper. They keep breathing hard for several minutes after stopping.
Factor 2 · during an asthma attack
A pupil with asthma starts wheezing on a cold morning. Breathing out is harder than breathing in, there is a tight feeling in the chest, and a blue reliever inhaler helps within minutes.
Factor 3 · after years of smoking
An adult who has smoked for twenty-five years is breathless walking uphill, coughs most mornings, and a lung function test shows reduced gas transfer that does not improve with an inhaler.
Which part of the system is affected?
You located itNot the part you chose
The breathing muscles — working harder and faster.
- What actually changes
- The diaphragm and intercostal muscles contract more forcefully and more often. Nothing about the airways, the alveoli or the blood is altered.
- What triggers it
- Rising carbon dioxide in the blood, detected in the brain stem. Not a shortage of oxygen — the body monitors the waste product, not the fuel.
- Effect on gas exchange
- Alveolar air is refreshed more often, so the concentration difference across the alveolar wall stays steeper and more oxygen diffuses per second.
- Reversible?
- Completely, within minutes. The continued hard breathing after stopping is clearing the carbon dioxide backlog.
You located itNot the part you chose
The airways — narrowed bronchioles.
- What actually changes
- Three things at once: muscle in the bronchiole walls contracts, the lining swells, and extra mucus is produced. All three reduce the diameter of the tube.
- Why it matters so much
- Flow through a tube depends steeply on its radius — halve the radius and flow drops around sixteenfold. A modest narrowing has a dramatic effect.
- Effect on gas exchange
- The alveoli are undamaged and the blood is fine. Not enough air is reaching the exchange surface, which is a delivery problem, not an exchange problem.
- Reversible?
- Yes, with treatment. A reliever inhaler relaxes the airway muscle. An attack that does not respond to it is a medical emergency.
You located itNot the part you chose
The alveoli — and the airways and the blood as well.
- What actually changes
- Alveolar walls are broken down, merging many small alveoli into fewer large ones. The volume is similar; the surface area is not. This is emphysema.
- And elsewhere
- Cilia are paralysed and destroyed, so mucus must be coughed out instead of swept out — the morning cough. Carbon monoxide occupies haemoglobin, so the blood carries less oxygen.
- Effect on gas exchange
- Less surface, so less exchange per second, and less carrying capacity in the blood arriving. Two of the four requirements from the last lesson are damaged at once.
- Reversible?
- The cilia recover over months after stopping and the carbon monoxide clears within a day. The lost alveolar walls do not grow back — that part is permanent.
Four substances, four different damages
Cigarette smoke is not one thing doing one thing.
Tar
Airways and alveoli. A sticky mixture containing carcinogens. It coats the airway lining, paralyses and then destroys cilia, and its irritation drives the chronic inflammation that damages alveolar walls.
Carbon monoxide
The blood. Binds to haemoglobin about 200 times more strongly than oxygen and does not readily let go, taking red blood cells out of service. This is a circulation effect and it happens immediately, not after decades.
Nicotine
Blood vessels and heart. Narrows blood vessels and raises heart rate and blood pressure. It is also the substance that makes stopping physically difficult, which is why stopping is a medical matter and not simply a decision.
Particulates and heat
Airways. Irritate the lining directly, increasing mucus production while the cilia that would clear it are being disabled. The result is mucus accumulating with no way out except coughing.
Key fact
Exercise changes how fast you ventilate and is fully reversible. An asthma attack narrows the airways and is reversible with treatment. Smoking damages the cilia, the alveolar walls and the blood, and the alveolar damage is permanent.
Think again
“Being out of breath means your lungs cannot hold enough air.”
Lung capacity is almost never the limit. During hard exercise your muscles are respiring far faster, producing far more carbon dioxide, and it is the carbon dioxide in your blood that your brain measures and responds to — not a shortage of oxygen and not the size of your lungs. That is why holding your breath becomes unbearable long before your blood oxygen has fallen dangerously, and why breathing hard after a sprint continues after you have stopped running: there is a backlog of carbon dioxide to clear. Trained athletes do not generally have much larger lungs than untrained people; what they have is a heart, a circulation and a set of muscles that use oxygen better once it arrives.
“During an asthma attack there is not enough oxygen in the air.”
The air is unchanged — 21% oxygen, exactly as it was a minute earlier. What has changed is the diameter of the bronchioles, and a small change in diameter has a very large effect on flow: halving the radius of a tube reduces the flow through it by about a factor of sixteen. That is why an attack can go from mild to serious quickly, why a blue inhaler containing a muscle relaxant helps rather than an oxygen cylinder, and why the correct first response is the inhaler and, if it does not work, emergency help. Handing someone oxygen while their airway is shut solves nothing.
“Tar is the harmful part of cigarette smoke.”
Tar is one of at least four separate problems, and it is not the one that acts fastest. Carbon monoxide binds to haemoglobin roughly 200 times more strongly than oxygen does, so it takes red blood cells out of service immediately — a heavy smoker may have a tenth of their haemoglobin unavailable at any moment, which is a circulation problem rather than a lung one. Nicotine narrows blood vessels and raises heart rate. The heat and irritants paralyse and destroy the cilia, so mucus is no longer swept out and has to be coughed instead. And the enzymes released during the resulting chronic inflammation break down alveolar walls, merging many small alveoli into few large ones and destroying exchange surface that never grows back. Four mechanisms, four organs affected, one of them irreversible.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · What drives faster breathing
What does your body actually detect that makes you breathe faster during exercise?
Rung 2 · The one that catches people
Which of these effects of smoking is permanent?
Rung 3 · Explain the inhaler
Explain why a reliever inhaler helps during an asthma attack even though it contains no oxygen, and why giving someone oxygen instead would not fix the underlying problem.
Rung 4 · Take it somewhere new
A patient with emphysema has almost normal lung volume but greatly reduced gas transfer. Explain how both can be true, using what you learnt about alveoli, and say which of the four requirements for a good exchange surface has been lost.
Key note
Exercise raises breathing rate and depth, triggered by rising carbon dioxide, and is fully reversible. Asthma narrows the bronchioles through muscle contraction, swelling and mucus, and is treated by relaxing that muscle. Smoking paralyses and destroys cilia, introduces carbon monoxide that occupies haemoglobin, and destroys alveolar walls — reducing exchange surface permanently.
Going further
The link between smoking and lung cancer was established without any experiment on humans, which is a genuinely interesting problem in how science proves things. Richard Doll and Austin Bradford Hill began by surveying hospital patients in 1950, then followed 40 000 British doctors for decades — watching who smoked, who stopped, and who died of what. Correlation alone is never proof, so the case was built from several independent strands: the risk rose with the number smoked, fell when people stopped, appeared in every country studied, and had a mechanism identifiable in the chemistry of tar. Doll himself smoked when the study began and gave up two years in, on his own evidence. That is what changing your mind in response to data actually looks like, and it is rarer than it should be.
Before this lesson
- This follows on from Alveoli: built for exchange.
Connects to
- Stomata and gas exchange in plants
- Alveoli: built for exchange
- Antagonistic muscle pairs
Where the diaphragm and intercostals' pulling — and only pulling — is the whole subject.
At GCSE this becomes
- Risk factors for non-communicable disease, correlation and cause, and the effect of exercise on respiration.
Where to next
Ask Mr Badmus AI
Want to check which part of the system each factor affects?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential.
This lesson describes the biology of asthma and of smoking. It is not medical advice and does not replace an asthma plan: if you have asthma, follow the plan your doctor or nurse gave you, and treat a reliever inhaler that is not working as an emergency. For help stopping smoking, a GP or pharmacist is the right place to start.
Lesson content © MrBadmusAI.