To breathe in, the diaphragm flattens and the ribs rise — chest volume increases, pressure drops, and air rushes in. To breathe out, they relax — volume falls, pressure rises, air is pushed out. ▶ Play to watch the lungs fill and empty.
Each alveolus is wrapped in capillaries. O₂ diffuses from the air into the blood and binds haemoglobin; CO₂ diffuses from the blood into the alveolus to be breathed out. The wall is thin, moist and large for fast diffusion.
Leaves swap gases through stomata, each opened by two guard cells. By day the leaf takes in CO₂ and releases O₂ (photosynthesis > respiration); at night it's the reverse. ▶ Play to open and close the pore.