Pressure also cause by the weight of the air in the atmosphere. When you go into the atmosphere ,air pressure will gradually decreases. The air also gets thinner when you are on the Earth's surface, it is like being at the bottom of a lake of air .When you breathe in, the air pressure will force the air into your lungs.
How the Earth's atmosphere exerts pressure. The pressure exerted by a liquid or gas acts in all difference directions in the liquid or gas but not just downwards. It is proven that pressure increases with depth. The Earth's atmosphere is about 80 Km deep and it exerts a huge pressure on our own bodies. In outer space there is no air pressure so the volume consist of nothing is called vacuum.
Air pressure forces air in all directions. It will try to force air into any place where there is no air. Another way to say this is that air will try to fill up a vacuum.
When there is a straw in a liquid, the air inside the straw will be at the same pressure as the air outside the surface area.