|MadSci Network: General Biology|
Glenn, You are correct in saying that too much oxygen can be poisonous. Breathing 100% oxygen at sea level pressure can cause oxygen toxicity in approximately 1 hour. One of the first noticeable symptoms of oxygen toxicity is seizure, so it can be very dangerous, especially for divers. Much if my information I attained from the following web-sight which is a book by Dr. Lawrence Martin, M.D. on diving. http://www .mtsinai.org/pulmonary/books/scuba/contents.htm I was unable to confirm the use of 100% oxygen on the Apollo flights but I used the information from the dive sight to figure out how they would do it. By the way, I am guessing that the reason they used pure oxygen was for weight savings. Very important in space flight (well, at least the launch part!) Air is approximately 21% oxygen, so using air instead of oxygen would have weighed nearly 5 times that of the pure oxygen. To get back to your question, the important rule is Henry's Law as quoted from Dr. Martin's book. (Section D "An Explanation of Pressure and the Laws of Dalton, Boyle, Charles and Henry") "Henry's law states: The amount of any gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in that particular liquid. Simplified: As the pressure of any gas increases, more of that gas will dissolve into any solution with which it is in free contact. Mathematically, where VG is the volume of a particular gas, VL is the volume of a particular liquid, a is the solubility coefficient for the gas in that liquid, and Pg is the pressure of the gas in contact with the liquid. Taken together, Henry and Dalton's laws predict two very important consequences: 1) When ambient pressure is lowered (as at altitude), the partial pressure of oxygen and nitrogen in the body must fall, and there will be less molecules of each gas dissolved in the blood and tissues. 2) When ambient pressure is raised (as when diving), the partial pressure of oxygen and nitrogen in the body must rise, and there will be more molecules of each gas dissolved in the blood and tissues." Your blood carries oxygen in two ways, attached to the hemoglobin of the red blood cells and dissolved in the plasma. Each red blood cell carries at max 4 oxygen molecules so increasing or decreasing the pressure won't affect that much, but changing the pressure will affect the amount of gas in the plasma. This has serious consequences for divers but is also how the astronauts were able to breathe 100% oxygen without toxic effects. At sea level we live at approximately 14.7 psi of pressure from the weight of the air above us (Also known as 1 atmosphere). At this pressure our plasma has a certain amount of oxygen and nitrogen (as well as CO2 and other gasses) dissolved in it. Henry's law shows that if we increase the ambient pressure (the pressure that your body and lungs feel) then the plasma is able to hold more molecules of each gas. This can be serious for divers. If someone dives to 33 feet under water they are at approximately 2 atmospheres, or twice the pressure. Now I don't think the plasma can directly hold exactly twice as many molecules, but it certainly can hold more. Why is this dangerous? If a diver has been under greater than atmospheric pressure for an extended time to allow more gas to dissolve into the blood than suddenly goes from high pressure to a lower pressure, the plasma can no longer hold as much gas molecules and they will form back into a gas (while still in you). This condition is known as 'the bends' to divers. A great example of this is to open a soda can. Soda is under a positive pressure, which allows the pop to hold more molecules of CO2 than when the top is released and the liquid is now at atmospheric pressure and CO2 is released. So now we have an example of increasing pressure affecting the gas in the blood, but Henry's law also shows that decreasing the pressure will decrease the amount of oxygen in the blood. Here in Colorado we are at 1 mile above sea level (about .75 Atmospheres) and I will have less oxygen dissolved in my blood then you will there in Connecticut. (I believe my body compensates by increasing the number of red blood cells, but don't quote me on that because I didn't look it up). But this is because we breathe the same air with the same percentage of oxygen. If I were to increase the percentage of oxygen, I would soon be able to match the amount of dissolved oxygen that you at sea level would have. At this lesser pressure I will have less total molecules of dissolved air, but a greater percentage of these dissolve molecules will be oxygen. This is how the Apollo space program was able to give the astronauts 100% oxygen for an extended period of time. At the low pressure the plasma in the blood will hold a lot less total molecules than you and I, but nearly all of them will be oxygen, unlike us air breathers with nearly 78% of the total dissolved molecules being nitrogen. Hope this made sense. The given website is an excellent source if you have more questions about blood-gas and other respiratory questions. Good luck. BK
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