Question

how does the exchange of gases takes place across the alveoli

Answer 1

The exchange takes place in the millions of alveoli in the lungsand the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.

Answer 2

The exchange actually takes place through diffusion. The oxygen present in inhaled air is taken up by the haemoglobin present in the rbc and co2 is taken to the lungs through water since the diffusion process is very slow.