Can someone please explain to me the definition of partial pressure in a very simple simple way and how it applies to us?
Like the partial pressures of different gases in our bodies.. And why people get decompression sickness when they scuba dive.. And why having helium lessens the risk. Etc. Why are partial pressures important?
TIA!!
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Partial pressure is how much of the pressure a certain substance is making up. So for example in sea level air you typically have a pressure of 100kPa. And since 20% of the air is oxygen the partial pressure of oxygen in the atmosphere is 20kPa. Partial pressure of nitrogen is 78kPa, and so on.
The reason we talk about partial pressure so often is that this is the effective concentration of that substance. If you think of the atmosphere as a bunch of molecules floating around with little interaction to each other. Then you put up a plate and count the number of oxygen molecules which hits it. You do not care about the nitrogen, water or anything else in the atmosphere. Then the rate at which oxygen molecules hit this plate corresponds to the partial pressure. If you for example add double the amount of nitrogen to the air making the pressure 180kPa then the number of oxygen molecules is still the same so the number of molecules that hit the plate is still the same. You can even remove all the nitrogen and make the pressure as low as 20kPa, close to vacuum, and there will still be just as many oxygen molecules hitting the plate at the same rate as before. The nitrogen or any other thing in the atmosphere just does not matter. Only the amount of oxygen matter and this is what partial pressure measures.
So say for example that the plate is made of a material which reacts with oxygen. The rate at which it reacts is only dependent on the partial pressure of the oxygen. This is important in for example diving and space travel because humans need a certain amount of oxygen to survive but too much oxygen is toxic to us. So we would like to keep a partial pressure of 20kPa at all times. However our lungs can handle much higher and lower pressures. So we use other gasses then oxygen to adjust the pressure and keep the partial pressure of oxygen the same. For saturation diving you might use nitrogen or even better helium as these are not toxic. For space travel we use nothing, or pretty close to nothing anyway, leaving only oxygen and some humidity.
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