Explanation

Breathing: diaphragm and intercostal muscles change chest volume. Inspiration: diaphragm flattens, ribs rise, volume increases, pressure decreases, air enters.

Gas exchange: occurs in alveoli – thin walls, large surface area, moist, surrounded by capillaries. O₂ diffuses into blood, CO₂ diffuses out.

Oxygen transported by hemoglobin in red blood cells. CO₂ carried as bicarbonate ions in plasma. Breathing rate controlled by brain responding to CO₂ levels.