Breathing and the Lungs
Year 7 ๐ซ Physiology & Health The respiratory system, gas exchange, and lung structure.
๐ซ Structure of the Respiratory System
Air passes through the trachea โ bronchi โ bronchioles โ alveoli.
- Trachea โ reinforced with C-shaped cartilage rings (keeps airway open); lined with ciliated epithelium and goblet cells (mucus)
- Bronchi (ร2) โ one to each lung; same structure as trachea
- Bronchioles โ smaller airways; smooth muscle allows diameter adjustment
- Alveoli โ tiny air sacs; site of gas exchange; ~300 million per lung = ~70 mยฒ surface area
๐ซ Breathing Mechanism
Breathing (ventilation) is driven by pressure changes caused by muscle contractions.
| Inhalation | Exhalation | |
|---|---|---|
| Diaphragm | Contracts โ moves down and flattens | Relaxes โ moves up (dome shape) |
| Intercostal muscles (external) | Contract โ ribs move up and out | Relax โ ribs move down and in |
| Thorax volume | Increases | Decreases |
| Lung pressure | Decreases (below atmospheric) | Increases (above atmospheric) |
| Air movement | Air flows IN | Air flows OUT |
๐ฌ Alveoli โ Adaptations for Gas Exchange
The structure of alveoli maximises the rate of gas exchange by diffusion.
- Huge surface area โ ~70 mยฒ of alveolar surface
- Thin walls โ one cell thick (short diffusion distance = fast diffusion)
- Moist lining โ gases dissolve before crossing membrane
- Rich blood supply โ capillaries surrounding each alveolus; blood flow maintains concentration gradient
- Ventilation โ fresh air continuously delivered; steep Oโ gradient maintained
Inhaled air: 21% Oโ, 0.04% COโ. Exhaled air: ~16% Oโ, ~4% COโ. (Exhaled air still contains significant oxygen.)
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Interactive Demo โ Breathing and the Lungs
Gas Volume Calculator