Describe how oxygen in the air reaches the capillaries surrounding the alveoli in the lungs.

Oxygen moves down the trachea, into the bronchi and into the smaller bronchioles, down a pressure gradient. Once the oxygen reaches the alveoli it moves down a diffusion gradient across the alveolar epithelium into the surrounding fluid. It then moves across the capillary epithelium into the capillary, once again down a diffusion gradient, thus entering the blood.