Check yourself
What happens to the diaphragm when you breathe in?
It contracts and flattens, moving downwards. This makes the chest bigger, so air is drawn into the lungs.
Name the gas taken into the blood and the gas removed at the alveoli.
Oxygen passes from the air in the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be breathed out.
Give two features of alveoli that make gas exchange fast.
A very large surface area and thin (one-cell-thick) walls. A good blood supply also keeps the concentration difference high so gases diffuse quickly.