Final answer:
Gas exchange in the respiratory system occurs in the alveoli during external respiration and in the tissues during internal respiration, facilitated by simple diffusion across thin and permeable membranes.
Step-by-step explanation:
The site in the repertory system where gas exchange occurs is the alveoli of the lungs. Here, external respiration takes place, which involves the exchange of oxygen and carbon dioxide between the air in the alveoli and the blood in the pulmonary capillaries. The process of gas exchange also occurs at the tissues where internal respiration happens, and oxygen is released from the blood to the cells while carbon dioxide is picked up from the cells to be transported back to the lungs.
Gas exchange is facilitated by simple diffusion across the respiratory membrane, which does not require energy but relies on pressure gradients to allow these gases to move. The large surface area of the alveoli, coupled with highly permeable and thin membranes, maximizes the diffusion of gases, making the respiratory system efficient at exchanging gases.