What you need to know
- Identify the trachea, bronchi, bronchioles, alveoli, lungs, ribs, intercostal muscles and diaphragm.
- Explain inhalation and exhalation using volume and pressure changes.
- Distinguish ventilation, gas exchange and cellular respiration.
- Relate alveolar structure to rapid diffusion.
- Explain effects of exercise and smoking and evaluate breathing measurements.
Airways and lungs
The lungs lie in the thorax. The ribs and intercostal muscles form and move the chest wall, while the dome-shaped diaphragm separates the thorax from the abdomen.
How ventilation works
| Change | Inhalation | Exhalation at rest |
|---|---|---|
| External intercostal muscles | Contract | Relax |
| Rib cage | Moves up and out | Moves down and in |
| Diaphragm | Contracts and flattens | Relaxes and becomes domed |
| Thoracic volume | Increases | Decreases |
| Pressure in lungs | Falls below atmospheric pressure | Rises above atmospheric pressure |
| Air movement | Into lungs | Out of lungs |
The movement of air into and out of the lungs. It is not the same process as respiration, which releases energy in cells.
During forced exhalation, internal intercostal and abdominal muscles can contract to reduce thoracic volume more strongly.
Gas exchange at the alveoli
Oxygen concentration is higher in alveolar air than in deoxygenated blood arriving at the lungs, so oxygen diffuses into the blood. Carbon dioxide concentration is higher in the arriving blood, so carbon dioxide diffuses into the alveoli.
- Millions of alveoli create a large surface area.
- Alveolar and capillary walls are each one cell thick, giving a short diffusion distance.
- A moist lining allows gases to dissolve before diffusing.
- A dense capillary network carries oxygen away and brings carbon dioxide.
- Ventilation refreshes alveolar air, maintaining steep concentration gradients.
Say which gas moves, in which direction, and down which concentration gradient.
Inhaled and exhaled air
| Gas | Inhaled air, approximately | Exhaled air, approximately | Reason for difference |
|---|---|---|---|
| Oxygen | 21% | 16% | Oxygen enters blood and is used in respiration. |
| Carbon dioxide | 0.04% | 4% | Respiration produces carbon dioxide, which enters the alveoli. |
| Nitrogen | About 79% | About 79% | Little nitrogen is exchanged. |
| Water vapour | Variable | Usually higher | Air is humidified by moist respiratory surfaces. |
Exhaled air still contains substantial oxygen because not all inhaled oxygen is absorbed and fresh air mixes with air already in the lungs.
Exercise and smoking
During exercise, muscle cells respire faster. Oxygen demand and carbon dioxide production rise, so breathing becomes faster and deeper. After exercise, elevated ventilation helps restore resting conditions and supports processes associated with recovery.
Tobacco smoke contains several harmful components:
- Tar damages cilia, increases mucus accumulation and contains carcinogens.
- Damage to alveolar walls in emphysema reduces surface area and elastic recoil.
- Carbon monoxide binds strongly to haemoglobin, reducing oxygen transport.
- Nicotine is addictive and affects the cardiovascular system.
Smoking raises risk; it does not mean every smoker develops the same disease.
Measuring breathing
Breathing rate can be counted as breaths per minute before and after standardised exercise. Control exercise duration and intensity, posture, room conditions and time of measurement. Repeat trials and calculate means.
A spirometer can estimate lung volumes, but not every volume can be measured directly with simple equipment. Participants should not share mouthpieces without appropriate hygiene, and strenuous exercise may be unsuitable for some people.
Breathing rate alone ignores breath depth. Two people taking the same number of breaths may ventilate different volumes of air.
Quick retrieval check
1. What happens to the diaphragm during inhalation?
2. Why does air enter when thoracic volume increases?
3. Give three alveolar adaptations.
4. How does carbon monoxide reduce oxygen transport?
Exam connection
Explain why severe emphysema can cause breathlessness during light exercise.
Show the mark points
- Alveolar walls are damaged, reducing gas-exchange surface area.
- Elastic recoil may be reduced, making ventilation less effective.
- Less oxygen diffuses into the blood per unit time.
- Muscles receive less oxygen for aerobic respiration as demand rises.