How are gasses exchanged in the alveoli?

Answer 1

Oxygen moves into the blood because the partial pressure of oxygen is higher in air and lower in blood. Carbon dioxide moves out of the blood because the particle pressure of carbon dioxide is higher in blood and lower in air.

Answer 2

okay, to put it simply, when oxygen from the air outside enters the human lungs it enters the numerous, one cell thick alveoli. these tiny air sacs are very thin, have a large surface area and also have plenty of capillaries surrounding them. This means that Diffusion of the oxygen from the alveoli into the bloodstream is very quick. As might be expected the exchange of CO2(the waste product of respiration) is just the reverse, cells to bloodstream to alveoli to the air outside.

hope this helps