Pressure in liquids and gases - OCR 21st CenturyAtmospheric pressure
Every living thing on Earth is in balance with the pressure of the air or water around it. Pressure helps blood to move around the body and allows organisms to breathe.
The atmosphere is the layer of air around the Earth. Key features of the atmosphereThe layers of gases that surround the Earth. The important gases in the atmosphere are nitrogen, oxygen and carbon dioxide.:
it is thin compared to the size of the Earth
it becomes less dense as the altitudeA measure of an area's height above sea level. increases
Pressure
Air molecules colliding with a surface cause atmospheric pressure. Atmospheric pressure at sea level is about 101,000 Pa (101 kPa) but just 27,000 Pa (27 kPa) at the cruising altitude of a passenger plane.
Air pressure is due to the weight of air in the atmosphere vertically above the area where the pressure is being measured. At sea level, there is 101,000 N of air above every metre squared of the Earth’s surface.
Atmospheric pressure decreases as the height of a surface above ground level increases. This is because, as the altitude increases:
the number of air molecules decreases
there is less air above a surface
the weightThe force acting on an object due to the pull of gravity from a massive object like a planet. The force acts towards the centre of the planet and is measured in newtons (N). of the air decreases
This is why aircraft that fly at high altitudes must be pressurised. If the air pressure is too low, humans cannot take in oxygen quickly enough to meet their bodies’ needs.