Which statement describes lung function?

• A. They regulate pH
• B. They filter waste from the blood
• C. They produce red blood cells
• D. They help oxygen enter red blood cells

Answer: (D)

Gas exchange is the lungs’ central job. When you breathe in, oxygen diffuses from the air in the alveoli into the blood in the pulmonary capillaries. The alveolar-capillary membrane is ultra-thin, which allows oxygen to enter the bloodstream quickly, where it binds to hemoglobin in red blood cells for transport to tissues throughout the body. That direct process—oxygen entering red blood cells via the lungs—is what this statement describes.

The other options describe functions outside the lungs’ primary role. The lungs do influence pH by helping regulate carbon dioxide levels, but filtering waste from blood is the kidneys’ job, and producing red blood cells occurs in the bone marrow.