Gas Exchange
Structure of the Thorax
The thorax is the chest cavity enclosed by the ribcage and diaphragm.
| Structure | Description / Function |
|---|---|
| Trachea | Main airway from mouth/nose to lungs; held open by C-shaped cartilage rings |
| Bronchi (singular: bronchus) | Two branches of the trachea, one entering each lung |
| Bronchioles | Smaller branches of the bronchi throughout the lungs |
| Alveoli (singular: alveolus) | Tiny air sacs at the ends of bronchioles; site of gas exchange |
| Pleural membranes | Two membranes surrounding the lungs; pleural fluid reduces friction |
| Diaphragm | Muscular sheet below the lungs; contracts to increase thorax volume |
| Intercostal muscles | Muscles between the ribs; contract/relax to move ribcage |
Ventilation (Breathing)
Ventilation moves air into and out of the lungs by changing the volume and pressure of the thorax.
Inhalation (breathing in):
- Diaphragm contracts → flattens downward
- External intercostal muscles contract → ribs move up and out
- Volume of thorax increases
- Pressure inside falls below atmospheric pressure
- Air rushes in (down the pressure gradient)
Exhalation (breathing out):
- Diaphragm relaxes → domes upward
- Internal intercostal muscles contract; external intercostals relax → ribs move down and in
- Volume of thorax decreases
- Pressure inside rises above atmospheric pressure
- Air is pushed out
Pressure and Volume
Volume ↑ → pressure ↓ → air flows in. Volume ↓ → pressure ↑ → air flows out. This relationship follows Boyle’s Law.
Alveoli: Adaptations for Gas Exchange
| Adaptation | How it helps |
|---|---|
| Enormous number (~300 million per lung) | Very large total surface area |
| Thin walls (one cell thick) | Short diffusion distance for O₂ and CO₂ |
| Rich capillary network surrounding each alveolus | Maintains steep concentration gradients; removes O₂ and delivers CO₂ constantly |
| Moist lining | Gases dissolve before diffusing across |
| Large surface area:volume ratio | Maximises rate of gas exchange |
Gas exchange at the alveolus:
- O₂ diffuses from alveolus (high O₂) → into blood capillary (low O₂)
- CO₂ diffuses from blood capillary (high CO₂) → into alveolus (low CO₂)
- Both move down their concentration gradients (passive diffusion)
Effects of Smoking
Smoking damages several structures in the gas exchange system:
| Consequence | Cause | Effect |
|---|---|---|
| Lung cancer | Carcinogens in tar cause mutations in lung cells | Uncontrolled cell division; tumour growth |
| Chronic bronchitis | Smoke irritates airways; increased mucus production; cilia damaged | Persistent cough; mucus accumulates; increased infection risk |
| Emphysema | Toxins destroy alveolar walls | Fewer, larger air sacs; reduced surface area; less efficient gas exchange; breathlessness |
| Cardiovascular disease | Nicotine and CO raise heart rate and blood pressure; CO reduces O₂ carrying capacity of blood | Increased risk of heart attack and stroke |
CO and Haemoglobin
Carbon monoxide (CO) binds irreversibly to haemoglobin with a higher affinity than oxygen. This permanently reduces the blood’s oxygen-carrying capacity, causing oxygen deficiency in tissues.
Practical: Investigate Breathing (Spec 2.50)
Measuring breathing rate:
- Count the number of breaths per minute at rest.
- Exercise for a set time, then recount.
- Record recovery time (time for breathing rate to return to resting level).
Using a bell jar model of the lungs:
- Balloons = lungs
- Glass tube/Y-piece = trachea/bronchi
- Rubber sheet at base = diaphragm
- Bell jar = thorax
- Pull rubber sheet down → volume increases → balloons inflate (inhalation)
- Push rubber sheet up → volume decreases → balloons deflate (exhalation)
Exam Questions
Q: Describe how the diaphragm and intercostal muscles bring about inhalation.
During inhalation, the diaphragm contracts and flattens, and the external intercostal muscles contract, pulling the ribcage upward and outward. This increases the volume of the thorax, causing the pressure inside to fall below atmospheric pressure. Air flows into the lungs down the pressure gradient.
Q: Explain how alveoli are adapted for efficient gas exchange.
Alveoli have a very large total surface area due to their enormous number. Their walls are only one cell thick, minimising the diffusion distance. Each alveolus is surrounded by a dense network of capillaries, which maintains a steep concentration gradient by continuously removing O₂ and supplying CO₂. The moist lining allows gases to dissolve before diffusing.
Q: Explain how smoking causes emphysema and how this affects gas exchange.
Toxins in cigarette smoke destroy the thin walls between alveoli. The alveoli merge into fewer, larger sacs. This greatly reduces the total surface area available for gas exchange, so less oxygen can diffuse into the blood per breath. The person becomes breathless, especially during exercise.
Q: Explain why carbon monoxide in cigarette smoke is dangerous.
Carbon monoxide binds irreversibly to haemoglobin in red blood cells, forming carboxyhaemoglobin. This reduces the amount of haemoglobin available to carry oxygen, so less oxygen is transported to tissues. This can cause oxygen deficiency and increases the risk of heart disease.
Exam Tip
When explaining ventilation, always link the muscle action → volume change → pressure change → air movement. Examiners award marks for each step in the chain. Don’t skip the pressure step.