Lesson focus: alveolusA tiny air sac in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out. exchange gases efficiently because area, distance and gradients work together.
Explain oxygen and carbon-dioxide diffusionThe net movement of particles from an area of high concentration to an area of low concentration, like the smell of perfume spreading across a room. across an alveolus and capillaryThe smallest kind of blood vessel, with a wall just one cell thick so substances can pass through..
Build on earlier learning
Movement of substancesApply diffusion factors to a specialised exchange surface.
You will be successful when you can…
- Trace oxygen and carbon dioxide directions.
- Identify alveolar and capillary structures.
- Explain large area and short distance.
- Explain how ventilation and blood flow maintain gradients.
Words that carry the science
- alveolus
- Tiny air sac providing a gas-exchange surface in the lungs.
- capillary
- Narrow blood vessel with a thin wall.
- ventilation
- Movement of air into and out of lungs.
- gas exchange
- Diffusion of oxygen and carbon dioxide across a respiratory surface.
- concentration gradient
- Difference in concentration driving net diffusion.
Explain gas exchange through surface, distance and gradients
01Gases move in opposite directions
Oxygen concentration is higher in fresh alveolar air than in deoxygenated blood, so oxygen diffuses into blood. Carbon dioxide is higher in arriving blood, so it diffuses into alveolar air.
Both movements occur across the alveolar epithelium and capillaryThe smallest kind of blood vessel, with a wall just one cell thick so substances can pass through. endothelium.
02Structure supports diffusion
Millions of alveolusA tiny air sac in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out. give a large area. Their walls and capillary walls are each one cell thick, producing a short diffusion pathway. A moist surface allows gases to dissolve. In short: there are millions of alveoli, which gives a huge surface area for oxygen to pass through. Their walls are very thin (one cell thick), so oxygen only has a short way to go to get into the blood.
03Damage changes a factor
Thickening or fluid increases diffusion distance. Loss of alveolar walls reduces area. Reduced ventilation or blood flow weakens gradients. Different problems can therefore lower exchange for different reasons.
Gas exchange in an alveolus
Oxygen diffuses from the air in the alveolus into the blood, and carbon dioxide diffuses the other way. Breathing and blood flow keep the concentration gradients steep.
Oxygen Carbon dioxide
Left: air in the alveolus. Right: blood in a capillary, flowing down.
Controls
Values are illustrative. A thin wall, a big surface and a good blood supply all make diffusion faster.
Predict a thickened barrier
A condition doubles the diffusion distance while area and gradients are initially unchanged.
- Identify the changed factor: distance.
- Particles take longer on average to cross a longer pathway.
- Gas diffusion rate falls.
- Blood leaving the lungs may gain oxygen less rapidly.
How to make the reasoning visible: Name the altered diffusion factor before predicting the body-level effect.
Exchange-factor diagnosis
Match each change to its most direct effect.
| Change | Factor affected | Predicted effect |
|---|---|---|
| Alveolar walls destroyed | Surface area decreases | Lower exchange capacity |
| Fluid around surface | Distance increases | Slower diffusion |
| Shallow ventilation | Gradient weakens | Less net oxygen diffusion |
| Reduced capillary flow | Gradient weakens | Slower gas transfer |
Why does blood flow help oxygen diffusion?
Which direction does carbon dioxide move?
Use the table. Fluid builds up around the alveoli. Why does this slow gas exchange?
Check your understanding
Answer from memory, then check against the suggested answer.
Where does oxygen diffuse?
Why are alveolar walls thin?
What maintains gradients?
Why are there millions of alveoli?
Which gas diffuses out of the blood into the alveoli?
Why does one large air sac exchange less effectively than many tiny alveoli of the same total volume?
Explain why faster breathing alone may not fully restore gas exchange if capillary flow is severely reduced.
Smoking damages the walls of the alveoli so they break down. Why does this make the person short of breath?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Oxygen diffuses from the alveoli (tiny air sacs in the lungs) into the blood. Answer the parts to explain how two features help.
How many alveoli are there? What does this give?
(1)How does this help oxygen get into the blood?
(1)How thick are the walls of the alveoli and the capillaries?
(1)How does this help oxygen get into the blood?
(1)Total for this question: 4 marks
Command word: For each feature, name it, then say how it helps.