What is the primary purpose of capillary networks across alveoli in the lungs?

• A. Blood filtration
• B. Gas exchange
• C. Heat exchange
• D. Immune filtration

Answer: (B)

Gas exchange is the main purpose of the capillary networks surrounding the alveoli. These tiny blood vessels sit right next to extremely thin alveolar walls, creating a large surface area and a very short diffusion distance. Oxygen moves from the air in the alveoli into the blood where its partial pressure is lower, while carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air to be exhaled. This setup—thin respiratory membrane plus abundant capillaries—facilitates rapid diffusion and oxygenation of blood as it travels to the heart and onward to the rest of the body. Other options describe functions not primarily served by these capillaries in the lungs, such as filtration or heat exchange, which are not their main role here.