A level Biology B · Edexcel 9BI0 · Topic 4C
Gas exchange
Key points
- In mammals, air passes trachea → bronchi → bronchioles → alveoli. Alveoli have walls one cell thick, a huge surface area, a rich capillary network and a moist lining.
- Ventilation: breathing in, the external intercostal muscles and diaphragm contract, volume rises, pressure falls and air enters. Breathing out at rest is mostly passive.
- Fish gills have filaments and lamellae; countercurrent flow (blood and water in opposite directions) keeps a diffusion gradient along the whole lamella.
- Insects use spiracles and a system of tracheae and tracheoles delivering air directly to the tissues.
- In leaves, gases diffuse through stomata and air spaces in the spongy mesophyll; guard cells control the stomata.
- Lung measurements: tidal volume, vital capacity, and ventilation rate = tidal volume × breathing rate.
Topic notesPremium
Gas exchange surfaces let oxygen in and carbon dioxide out fast enough for an organism's needs. Mammals, fish, insects and plants each meet Fick's law in a different way, so comparing them is a favourite exam question.
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