IGCSE Biology / Breathing and gas exchange
The parts of the breathing system
Edexcel International GCSE Biology (4BI1), chapter 3: Breathing and gas exchange, specification point 2.46. Free lesson.
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The breathing system carries air from the nose to the alveoli, where oxygen and carbon dioxide are exchanged with the blood. Knowing the parts and their jobs helps you distinguish breathing, which moves air, from gas exchange.
The main parts of the breathing system
Air travels down a series of tubes that branch into smaller and smaller passages. The thorax is the chest area containing the lungs and the structures around them. Trachea: the windpipe, held open by rings of cartilage. Bronchi: the trachea splits into two, with one bronchus leading to each lung. Bronchioles: each bronchus branches repeatedly into narrower tubes. Alveoli: tiny air sacs at the ends of the smallest bronchioles.
The ribs, muscles and pleural membranes
The ribs form a protective cage around the lungs, with the intercostal muscles between them. The diaphragm is a sheet of muscle beneath the lungs, separating the thorax from the abdomen. These structures help move air in and out, but they do not exchange gases. The pleural membranes are two thin layers around the lungs. Fluid between the membranes lets the lungs slide against the ribs without friction.
How oxygen travels from the nose to the blood
Follow one oxygen molecule to see how the narrowing tubes connect the outside air to the blood. At the alveolus, oxygen crosses the alveolus wall and the capillary wall. Each wall is one cell thick, so oxygen crosses just two cell walls in total. The nose warms and moistens the air. Mucus traps dust. Air passes down the trachea, then into one of the bronchi and through the branching bronchioles. An alveolus is an air sac one cell thick, where gas exchange happens. Gas exchange is oxygen and carbon dioxide crossing the alveolus wall. It is different from breathing, which moves air…
Why the trachea has cartilage rings
The trachea carries all the air going to and from the lungs. When you breathe in hard, the pressure drop could make it collapse, so rings of cartilage hold it open. Each bronchiole carries much less air, so the pressure change in it is small. Bronchioles do not have cartilage rings. Their flexibility lets them change diameter.
What to remember about gas exchange
The tubes become narrower and more numerous as they branch from the trachea towards the alveoli. Gas exchange happens only in the alveoli, across two cell walls into or out of the blood. Remember the order: trachea, bronchi, bronchioles, alveoli. The ribs, intercostal muscles and diaphragm move air. They do not exchange it.