Breathing and gas exchange
Air passes through the trachea, bronchi and bronchioles to millions of alveoli, where oxygen diffuses into the blood and CO₂ diffuses out. Alveoli have a large surface area, thin walls (one cell thick), a rich capillary network and are ventilated, keeping concentration gradients steep. To inhale, the external intercostal muscles contract (ribs up and out) and the diaphragm contracts and flattens: volume increases, pressure decreases, air flows in. Exhaling reverses this. Expired air has less oxygen, more CO₂ and more water vapour. Smoking damages cilia and alveoli and causes cancer.
≈ 40 min