Final answer:
The thin-walled air sacs in the lungs where gas exchange occurs are called alveoli. Alveoli are contained within alveolar sacs and are surrounded by capillaries, making them crucial for the diffusion of oxygen into the blood and carbon dioxide out of it.
Step-by-step explanation:
The microscopic, thin-walled air sacs where the actual exchange of oxygen and carbon dioxide occurs are known as alveoli. Alveoli are clustered in groups within alveolar sacs, which are attached to the end of alveolar ducts connected to respiratory bronchioles. Each alveolus is approximately 200 to 300 microns in diameter and is surrounded by a network of capillaries. This proximity facilitates the diffusion of gases—oxygen into the blood and carbon dioxide out of it.
The alveoli have elastic walls that allow them to stretch during air intake, maximizing the available surface area for gas exchange. This elasticity, combined with their thin-walled parenchymal cell structure and extensive capillary network, makes alveoli effectively the primary site for gas exchange in the respiratory system. With a vast combined surface area—approximately 75 m² in humans—alveoli and their sacs ensure a large and efficient interface for pulmonary gas exchange.