When you breathe in, your diaphragm contracts and moves down and your ribs move up and out. The chest gets bigger, pressure drops, and air rushes into the lungs. Air passes down the trachea, into the bronchi and bronchioles, and into tiny air sacs called alveoli. Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses out.
The heart pumps blood in a double circulation: the right side pumps blood to the lungs to collect oxygen, and the left side pumps oxygen-rich blood round the body. Arteries have thick, muscular walls to carry blood under high pressure. Veins have valves to stop blood flowing backwards. Capillaries have walls one cell thick so substances can pass in and out. Smoking damages the alveoli and makes this exchange harder.
Worked example
Why are alveoli good at gas exchange?
- They give a huge surface area.
- Their walls are very thin and next to capillaries.
Answer:So oxygen and carbon dioxide can diffuse across quickly.
Key words
- alveoli
- tiny air sacs in the lungs
- diaphragm
- a sheet of muscle below the lungs
- gas exchange
- oxygen in, carbon dioxide out
- capillary
- a tiny blood vessel with very thin walls
New questions every time.
NGSS-referenced: MS-LS1-3