The Respiratory System

Moving air, and exchanging gas.

The respiratory system delivers oxygen to the blood and removes carbon dioxide. Air travels nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.

Gas exchange

Exchange happens only at the alveoli — hundreds of millions of thin-walled sacs wrapped in capillaries. Everything above them is plumbing. Diffusion runs down partial pressure gradients in both directions at once.

Mechanics

The diaphragm contracts and flattens, enlarging the chest cavity and lowering pressure so air flows in. Quiet exhalation is passive elastic recoil.