Diving

• diving
  • a dive is when a person enters water, a chamber, or any other environment and is subjected to pressure greater than 0.1 atm above local atmospheric pressure and who in order to survive in such an environment breathes in air or other breathing gases at a pressure greater than the local atmospheric pressure
• gases
  • physical properties of gases
    • atmospheric pressure at sea level is 1 atm (equivalent to 101.3 kPa) and is the pressure that is experienced by all parts of the human body at sea level
      • water is much denser than air (1 l of fresh water weighs 1 kg, while 1 l of air weighs 0.0012 kg)
      • a column of water 10 m high exerts a pressure of 1 atm so that for every 10 m depth that the diver descends into the sea, his body will experience an increase in the ambient pressure of 1 atm
        • thus, at a depth of 30 m for example, the diver will experience a total pressure of 4 atm (3 atm due to the pressure of the water plus 1 further atmosphere due to the pressure of the atmosphere itself)
  • the gas laws and gas solubility are important considerations when diving:
    • Boyle's Law
      • states that at constant temperature the pressure and the volume of the gas are inversely related:
        • PV=k, where P is the total pressure of the gas and V is the volume of the gas. k is a constant for a given gas or mixture of gases
        • cavities in the body that contain gas will be affected by pressure as indicated by Boyle's Law
          • for example, the air in the lungs taken in by a breath-holding diver at the surface will shrink to one-half the original volume at a depth of 10 m, to one-third at a depth of 20 m and so on
            • scuba divers breathe gas at the ambient pressure of the surrounding water and this enables the diver to breathe with ease
              • however, as the diver goes deeper, the supplied gas will become denser, and the work of breathing will become harder
            • in the condition where the volume of gas diminishes and further gas cannot be drawn in to occupy this space, it will either be filled by body tissues or by clothing - this situation is termed a "squeeze"
              
              
    • Dalton's Law of Partial Pressures
      • the pressure exerted by a mixture of non-reacting gases is the sum of the pressures that would be exerted by the individual gases if each occupied the same volume alone:
        • Ptotal=P1+P2+...Pn, where Pn is the partial pressure of gas n and Ptotal is the total pressure of the gases
          
          
    • Henry's Law of Solubility of Gases in a Liquid
      • the mass of a slightly soluble but inert gas that dissolves in a given amount of a liquid at a constant temperature is proportional to the partial pressure of that gas above the liquid
        • Mathematically: mn=cnPn
          • where mn is the mass of gas n that dissolves in the liquid, cn is the solubility constant for gas n and Pn is the partial pressure of gas n above the liquid

Whilst gases are very compressible, the tissues in the body vary in their degree of compressibility

• for example, nerve tissue will start to exhibit altered physiological function at a depth of approximately 150 m but; however bone will not be deformed until much higher pressures are reached

Physiological effects of increased pressure

• the physiological effects of changing pressure on the diver are due largely to the gases that the diver breathes. The dive may be thought of conveniently as having two stages:
  • 1. The descent and the stay at depth.
  • 2. The ascent - decompression