The physics behind SCUBA diving and the physical effects on the body.
Scuba diving is a sport that many people enjoy but hardly understand the physics behind. The journey from the pressure of the atmosphere to the deeper depths of the seas is not just a trip that involves breathing air out of a tank and looking at fish. If we were to go on an imaginary journey beginning at the surface and traveling to the bottom of a 300 foot lake your body would under go a change in pressure nearly of 10 times. The physics behind diving starts is with the initial change in pressure on your body, and progresses to taking your first breath underwater, to you body being saturated with gasses and finally at great depths the very oxygen that keeps you alive begins to poison you.
The very beginning of a journey to the depths begins at the change from the surface to the water. At the most basic and simple level the first thing that happens when you change over from air to water is the rate in which you change pressure. The density of water is almost 800 times that of air. So when an altitude change of a couple hundred feet in air is almost negligible, a change of a couple feet in water is very drastic. To double the pressure in air one would have to come down from 18,000 feet to sea level. To double the pressure in water one only has to go down 33 feet from the surface. According to the ideal gas law, doubling the pressure means halving the volume. So any breath taken at 33 feet, one is taking on twice as many gas molecules as one would take on at the surface. This is not a problem as long as the pressure is the same. To take a breath at the surface and go to 33 feet the volume of gas in your lungs would be halved. Upon returning to the surface the volume would increase to original volume. People do this safely everyday in swimming pools, take a breath go under pressure and come back. The other way is not true at all. To take a breath at a depth greater than a couple feet and travel to the surface would be most likely fatal. The human lungs can take a pressure of no more than 3 or 4 PSI. Any greater pressure causes the alveoli to burst or worse yet to completely tear the lung. This prevents the transmission of oxygen to the blood stream and allows air bubbles into the blood. What this means for a diver is holding your breath at any point in the dive is most certainly a fatal mistake.
For a basic shallow dive to 15 feet, the time that can be spent on underwater is basically unlimited. The body takes on gas but at this pressure the change of going to the surface is not great enough to cause significannot expansion of bubbles in the body's fluid and tissues. The amount of gas the body takes on at that pressure is also of negligible amount. Air for simplicity sake is basically 21% oxygen and 79 % nitrogen. With each breath according to Dalton's law of partial pressure you breathe 21% O2 and 79% N2 regardless of pressure. With each breath you body absorbs in O2 and N2 into the blood stream. The body utilizes oxygen in its functions and expels carbon dioxide as waste. When you exhale you expel unused O2, the waste gas CO2 and the unchanged N2. At any given time your body will have all three of these gasses in all tissues and blood. The nitrogen is the most dangerous of the gasses. O2 and CO2 are directly involved in the exchange in the lungs there for they are used and expelled. Nitrogen is just along for the ride. It gets into the system but once there it serves no purpose and does not get recirculated like O2 and CO2. It just builds up in the tissues. When going from a lesser pressure to a greater pressure the body starts saturated with a given amount of N2, O2 and CO2. As you go deeper the microscopic bubbles of gas get compressed allowing room for more of the gas to be diffused into the blood. With your first breath at depth at 33 feet you take on twice the amount of air you would have taken on at the...
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