The shape of the lesson
By the end of the lesson, learners can
- Watch gases cross the wall: Red dots are oxygen and grey dots are carbon dioxide.
- Muscle that narrows, fibres that recoil: Press Breathe and read the ALVEOLI read-out as the alveoli stretch and recoil.
- Area: millions of alveoli, each wrapped in capillaries.
- Distance: squamous epithelium plus endothelium, under 1 μm.
- Gradient: ventilation in, blood flow away.
- Tissues: cartilage supports, smooth muscle narrows, elastic fibres recoil.
One picture every idea returns to
In the alveoli, oxygen diffuses from the air (high partial pressure, about 13 kPa) into the blood in the capillaries (about 5.3 kPa), and carbon dioxide diffuses from the blood into the alveolar air, each down its own concentration gradient. Diffusion is fast because the surface area of the millions of alveoli is huge, the diffusion distance is less than 1 μm (the squamous epithelium of the alveolus plus the capillary endothelium), and the gradients are kept steep by ventilation, which replaces the alveolar air, and blood flow, which carries oxygenated blood away. Cartilage holds the trachea and bronchi open; smooth muscle contracts or relaxes to change the diameter of the airways, especially the bronchioles; elastic fibres stretch as the alveoli fill and recoil to help push air out.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Thin walls, steep gradients, tissues that flex.. Oxygen and carbon dioxide cross the alveolar wall by diffusion. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Watch gases cross the wall | Red dots are oxygen and grey dots are carbon dioxide. Project tab 4 of the lab (Gas exchange lab, gas exchange in the alveoli) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Muscle that narrows, fibres that recoil | Press Breathe and read the ALVEOLI read-out as the alveoli stretch and recoil. Project tab 5 of the lab (Gas exchange lab, smooth muscle and elastic fibres) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Muscle that narrows, fibres that recoil, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Gases cross by diffusion, not active transport, and each gas moves down its own gradient. | Say down a concentration (partial pressure) gradient, and name both layers of cells: the alveolar epithelium and the capillary endothelium. |
| Bronchioles have no cartilage, so it is the smooth muscle and elastic fibres in their walls that control their width. | Cartilage does not contract; muscle does. Elastic fibres do not contract either: they recoil passively after being stretched. |
Alveoli, gas exchange and the airway tissues: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
State the functions of cartilage, smooth muscle and elastic fibres in the gas exchange system.
Describe how oxygen moves from the air in an alveolus into a red blood cell in a capillary.
Explain how ventilation and blood flow maintain a steep concentration gradient for oxygen across the alveolar wall.
The alveolar wall is 0.2 μm thick and the capillary endothelium is 0.3 μm thick. Calculate the total diffusion distance between alveolar air and blood plasma in nanometres, and comment on its size.
Suggest why a person with asthma finds it harder to breathe out than to breathe in during an attack.
In the lab's alveolus model, alveolar pO2 starts at 13.3 kPa and blood pO2 at 5.3 kPa. When ventilation is switched off, both lines move towards each other. Explain this result.
Answer key and marking notes
- Cartilage: supports the trachea and bronchi, keeping them open so they do not collapse during inhalation.
- Smooth muscle: contracts or relaxes to change the diameter of the airways, controlling airflow.
- Elastic fibres: stretch during inhalation and recoil during exhalation, helping push air out of the alveoli.
- Oxygen dissolves in the moisture lining the alveolus.
- It diffuses down its partial pressure (concentration) gradient, from higher in the alveolar air to lower in the blood.
- It crosses the squamous epithelium of the alveolus and the endothelium of the capillary, a distance of less than 1 μm.
- It diffuses through the plasma into the red blood cell and combines with haemoglobin.
- Ventilation replaces alveolar air with fresh air, keeping the oxygen concentration in the alveoli high.
- It also removes carbon dioxide, keeping its concentration in the alveoli low.
- Blood flow carries oxygenated blood away from the alveoli.
- And brings deoxygenated blood with a low oxygen concentration, so oxygen keeps diffusing in.
- 0.2 + 0.3 = 0.5 μm = 500 nm.
- Less than 1 μm, a very short diffusion distance, so diffusion is fast.
- The smooth muscle in the bronchiole walls contracts, so the airways are narrowed.
- Breathing out relies on elastic recoil and the bronchioles are compressed further as the chest gets smaller, so they narrow more.
- Breathing in widens the airways as the lungs expand, so air enters more easily than it leaves.
- Without ventilation, the oxygen that diffuses into the blood is not replaced in the alveolus, so alveolar pO2 falls.
- Blood still takes up oxygen, so its pO2 rises.
- The gradient shrinks, so the rate of diffusion falls towards zero.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the function of the cartilage in the walls of the trachea and bronchi?
- Ait keeps the airways open when air pressure inside them falls
- Bit contracts to narrow the airways and control airflow
- Cit recoils after stretching to push air out of the lungs
- Dit secretes mucus to trap dust and bacteria
Which two layers of cells must oxygen cross to move from the air in an alveolus into the blood plasma?
- Asquamous epithelium of the alveolus and capillary endothelium
- Bciliated epithelium of the alveolus and capillary endothelium
- Csquamous epithelium of the alveolus and a layer of smooth muscle
- Dcapillary endothelium and the membrane of a red blood cell
Which change would decrease the rate at which oxygen diffuses from the alveoli into the blood?
- Athickening of the alveolar walls by scar tissue
- Ban increase in the rate of ventilation
- Can increase in the rate of blood flow
- Dan increase in the number of alveoli
In emphysema, many elastic fibres in the alveolar walls are destroyed. Why do people with emphysema find it hard to breathe out fully?
- Athe alveoli do not recoil, so less air is pushed out
- Bthe bronchioles lose cartilage and collapse inwards
- Cthe smooth muscle can no longer relax the airways
- Dthe squamous epithelium becomes too thick for air to leave
During exercise the smooth muscle in the walls of the bronchioles relaxes. What is the effect of this?
- Athe bronchioles widen, so more air reaches the alveoli
- Bthe bronchioles narrow, so air moves faster
- Cthe alveoli stretch further, so they hold more air
- Dthe cartilage loosens, so the airways expand
The graph shows the partial pressure of oxygen in blood as it flows along a capillary next to an alveolus, in a person at rest. What does the graph show?
- Athe blood reaches the alveolar pO2 well before the end of the capillary
- Boxygen diffuses at the same rate all along the capillary
- Cthe blood leaving the capillary has a higher pO2 than the alveolar air
- Dno oxygen diffuses into the blood in the first half of the capillary
The rate of diffusion is inversely proportional to the diffusion distance. In a lung disease the distance between alveolar air and plasma increases from 0.5 μm to 2.0 μm, with no other change. By what percentage does the rate of oxygen diffusion fall?
- A75%
- B25%
- C50%
- D300%
A person holds their breath for 30 seconds while blood keeps flowing through the lungs. Why does the rate of carbon dioxide diffusion from blood to alveoli decrease?
- Aalveolar pCO2 rises towards the blood value, so the gradient shrinks
- Bcarbon dioxide diffuses only while the alveoli are being stretched
- Cthe blood stops carrying carbon dioxide when oxygen is low
- Dalveolar pO2 rises, which blocks carbon dioxide leaving
Quiz key
Cartilage supports the trachea and bronchi so they do not collapse when pressure inside falls during inhalation. Contracting to narrow the airways is the job of smooth muscle. Recoiling after being stretched is the job of elastic fibres. Mucus is secreted by goblet cells and mucous glands.
Oxygen crosses the thin squamous epithelium of the alveolus and then the endothelium of the capillary to reach the plasma. Alveoli are not lined with ciliated epithelium. There is no smooth muscle between alveolus and capillary. A red blood cell membrane is crossed after the plasma, and it is not a layer of cells.
Thicker alveolar walls increase the diffusion distance, so diffusion is slower. Faster ventilation keeps alveolar oxygen high and faster blood flow keeps blood oxygen low, so both steepen the gradient and speed diffusion up. More alveoli means a larger surface area, which also increases the rate.
Elastic fibres stretch as the alveoli fill and recoil to help push air out; without them, air stays trapped in the alveoli. Bronchioles have no cartilage to lose. Smooth muscle is not destroyed in emphysema. Squamous epithelium affects gas diffusion, not the bulk flow of air out of the alveoli.
Relaxed smooth muscle lets the bronchioles widen, reducing resistance so more air ventilates the alveoli and the oxygen gradient stays steep. Narrowing happens when the muscle contracts, as in asthma. Smooth muscle in the bronchioles does not control how far alveoli stretch; elastic fibres and lung volume do. Bronchioles contain no cartilage.
The blood pO2 rises steeply at first and levels off at the alveolar value within the first third or so, leaving spare length that allows full loading even when blood flows faster during exercise. The rate is not constant: it is fastest at the start, where the gradient is steepest, and falls to zero once pressures are equal. Diffusion cannot raise blood pO2 above the alveolar value. Most of the oxygen diffuses in during the first half, not none.
The distance is four times greater, so the new rate is 1/4 of the original: a fall of 75%. 25% is the new rate as a percentage of the original, not the fall. 50% would need the distance only to double. 300% is the percentage increase in distance, and a rate cannot fall by more than 100%.
With no ventilation, carbon dioxide arriving from the blood accumulates in the alveolar air, so alveolar pCO2 rises towards the blood value, the gradient becomes smaller and net diffusion slows. Diffusion does not depend on the alveoli being stretched. Blood keeps carrying carbon dioxide from respiring tissues. Alveolar pO2 falls, not rises, during breath-holding, and the two gases diffuse independently.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.