Final answer:
Air pressure depends on altitude and the number of air molecules in a given volume, and it is key for the ability to breathe.
Step-by-step explanation:
The two factors air pressure depends on are the altitude (or height) above sea level and the amount of air molecules (or gas molecules) present in a given volume. Atmospheric pressure decreases with an increase in altitude because there is less weight of the air above. This relationship can be seen when comparing atmospheric pressure at sea level, which is approximately 101 kPa or 760 mm Hg, to the much lower pressure found on the summit of Mt. Everest. Additionally, according to Boyle's law, at a constant temperature, the pressure of a gas is inversely proportional to the volume it occupies. If the volume increases, the pressure decreases, and vice versa, which is crucial for the ability to breathe.