IB Biology HLGas Exchange SystemsPaper 1 & 2~11 min read
Adaptations of Mammalian Lungs
A lung is not a bag of air. It is a tree of tubes that splits again and again until it ends in millions of tiny damp bubbles, each one wrapped in blood vessels. Everything about its design comes back to the same four rules: big area, short distance, moist, permeable.
📚 What you need to know
Air travels nose or mouth → trachea → bronchi → bronchioles → alveoli.
The trachea is held open by rings of cartilage; trachea and bronchi are lined with ciliated epithelium that sweeps out trapped mucus.
Bronchioles have smooth muscle but no cartilage, so they can widen and narrow to change airflow.
Many small alveoli give a very large surface area, and the branched network spreads them evenly through the lung.
Each alveolus is wrapped in an extensive capillary bed, which keeps the concentration gradient steep.
The alveolar wall is a single layer of flat epithelial cells, giving a very short diffusion distance.
Surfactant lowers surface tension so alveoli do not collapse and stick together when you breathe out.
The road in: from your nose to an alveolus
Air enters through the nose and mouth, where it is warmed and filtered, then goes down the trachea. The trachea has to stay open all the time – a collapsed windpipe is fatal – but it also has to bend when you turn your head. It solves both problems with C-shaped rings of cartilage: rigid enough to hold the tube open, separate enough to let it flex.
The trachea splits into two bronchi, one to each lung. Their walls also contain cartilage plus a layer of smooth muscle that can contract or relax to change the diameter of the airway. Both the trachea and the bronchi are lined with ciliated epithelium. Goblet cells make mucus that traps dust and bacteria, and the cilia beat the sticky mucus upwards, away from the lungs.
Each bronchus branches into narrow bronchioles. These have smooth muscle but no cartilage, so they can dilate when you need more air and constrict when something irritating, such as an allergen, is in the air. At the end of the smallest bronchioles sit groups of alveoli, and that is where the actual gas exchange happens.
Only the last stop does any gas exchange. Everything above the alveoli is plumbing that delivers, cleans and warms the air.
A neat way to keep the order: trachea, bronchi, bronchioles, alveoli – the words get longer as the tubes get smaller. Cartilage disappears at the bronchioles, and cilia disappear before the alveoli, because you do not want mucus sitting on your exchange surface.
How the alveoli are adapted
An alveolus is basically a moist bubble one cell thick with blood on the other side. Small, and there are a lot of them. Here is the point students often miss: lots of small bubbles have far more surface area than one big bag of the same volume. That is the whole reason for splitting the lung up.
Blue is deoxygenated blood arriving, red is oxygenated blood leaving. The blood only has to cross two thin cell layers to reach the air.
Adaptation
What it does
Factor improved
Many small alveoli
Splits the same volume into a vast number of surfaces
Large surface area
Branched bronchiole network
Spreads alveoli evenly through the whole lung, so no region is wasted
Large surface area
Extensive capillary bed
Blood is in contact with almost every alveolus
Surface area and steep gradient
Continuous blood flow
Deoxygenated blood in from the pulmonary artery, oxygenated blood out in the pulmonary vein
Maintains the concentration gradient
Alveolar wall one cell thick
Flattened epithelial cells, so gases cross in a fraction of a second
Short diffusion distance
Moist lining
Oxygen dissolves before it diffuses into the blood
Makes diffusion possible
Surfactant
Lowers surface tension so alveoli do not collapse or stick shut
Keeps the surface area available
Surfactant: the part that gets skipped
A wet bubble wants to close. Water molecules pull on each other, and in a tiny damp sac that pull, called surface tension, is strong enough to squeeze the alveolus shut when you breathe out. If that happened, you would have to force each alveolus open again from scratch on every breath.
Cells in the alveolar wall release surfactant, a substance that lowers surface tension. The alveoli stay open, they do not stick together, and breathing in stays easy. Premature babies sometimes have not made enough surfactant yet, which is why their breathing is so hard work.
Say the right wall. The alveolar wall is a single layer of epithelial cells in your lungs. A cell wall is the rigid box around a plant cell. Examiners see these swapped every year, and it costs the mark.
Worked examples
WE 1
The path of a molecule of oxygen
Outline the path taken by a molecule of oxygen from the air outside the body until it reaches the blood. (4 marks)
Step 1: into the body
Through the nose or mouth, where the air is warmed and filtered, then down the trachea.
Step 2: the split
Into one of the two bronchi, one leading to each lung.
Step 3: the small tubes
Along the branching bronchioles to a group of alveoli at the end.
Step 4: the crossing
It dissolves in the moist lining, then diffuses across the alveolar wall and the capillary wall into the blood.
Trachea → bronchus → bronchiole → alveolus → capillarythe last mark is usually for naming the two thin walls the gas crosses, not just for the list of tubes
WE 2
Explaining the adaptations
Explain three ways in which mammalian lungs are adapted for efficient gas exchange. (3 marks)
Point 1: area
There are many small alveoli, giving a very large surface area, so many molecules can diffuse at the same time.
Point 2: distance
The alveolar wall is one flattened cell thick, so the diffusion distance is short and diffusion is fast.
Point 3: gradient
A dense capillary network with continuously flowing blood removes oxygen as it arrives, keeping the concentration gradient steep.
Big area, short distance, steep gradientevery adaptation mark needs the “so that” part; the feature on its own is only half the answer
WE 3
The job of surfactant
Explain the importance of surfactant in the alveoli. (2 marks)
Point 1: what it does
Surfactant is secreted by cells of the alveolar wall and lowers the surface tension of the moist lining.
Point 2: why that matters
Without it the alveoli would collapse and stick together as air leaves during expiration, so surface area for gas exchange would be lost and breathing in would take much more effort.
Less surface tension, so the alveoli stay openlink it back to surface area and you turn a description into an explanation
💡 Exam tips
Learn the order of the airways and be able to write it in one line under pressure.
Cartilage in the trachea and bronchi, smooth muscle in the bronchioles. Do not put cartilage in a bronchiole.
For any adaptation question use the pattern: feature → effect → effect on rate of diffusion.
Name the vessels properly: deoxygenated blood arrives in the pulmonary artery, oxygenated blood leaves in the pulmonary vein.
Alveolar wall, not cell wall. Say epithelial cells.
If a diagram question asks you to label the gas exchange system, include the diaphragm and intercostal muscles – they are part of it.
⚠ Common mistakes
Saying alveoli are large. They are tiny; it is their number that makes the total area huge.
Confusing alveolar wall with cell wall. Mammals have no cell walls.
Writing that cilia line the alveoli. Cilia and mucus are in the trachea and bronchi only.
Saying the pulmonary artery carries oxygenated blood. It carries deoxygenated blood to the lungs.
Describing lungs as muscular. Lungs contain no muscle that moves them; the diaphragm and intercostal muscles do the work.
Listing adaptations with no explanation. A list of features with no “so that” rarely scores full marks.
Up next: How Ventilation Works – the muscles and pressure changes that pull fresh air into all those alveoli, and let it out again.
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