VPB 121: Veterinary Physiology - II (2+1)

GAS LAWS

• Avogadro's hypothesis
  • Equal volumes of different gases at equal temperature contain the same number of molecules.
  • Equal numbers of molecules in the same volumes at the same temperature will exert the same pressure. (One mole of any gas will contain 6.02x10²³ molecules and will occupy a volume of 22.4 L at a temperature of 0°C and a pressure of 760 mmHg.)
• Dalton's law
  • In a gas mixture, the pressure exerted by each individual gas in a space is independent of the pressures of other gases in the mixture.
  • e.g. PAl_v = PH₂O + PO₂ + PCO₂ + PN₂
    Partial Pressure of O₂= %O₂ content  X Total partial pressure
• Boyle's Law: P₁V₁ = P₂V₂ (at constant temperature)ie at constant temperature pressure of the gas varies inversly with its volume. 
• Charles' law or Gay-Lussac's Law
  • V₁/V₂=T₁/T₂
  • P₁/P₂=T₁/T₂
  • ie at constant pressure the volume of given mass of the gas increases by 1/273 of its volume at zero degree rise in temperature.
• Henry's Law
  • The weight of a gas absorbed by a liquid with which it does not combine chemically is directly proportional to the pressure of the gas to which the liquid is exposed (and its solubility in the liquid).
• Graham's Law
  • The rate of diffusion of a gas (through the gas phase) is inversely proportional to the square root of its molecular weight.
• Diffusion
  • All gas moves across the blood-gas interface by passive diffusion; that is, net movement is from an area of high partial pressure to low.
• Diffusion of O₂ and CO₂ obey Fick's Law
  • Rate of diffusion or flow of a gas is directly propotional to
    A X D X (P₁-P₂)/T
  • where: A = area , T = thickness, D = diffusivity, P₁-P₂ = partial pressure gradient
  • The volume of gas per unit of time moving across a tissue sheet is directly proportional to the surface area of the sheet, the diffusibility, and the difference in gas concentration between the two sides, but is inversely proportional to the tissue thickness.
  • The surface area is in several square meters depends upon size of the lung. The thickness is generally ½ micron. This large surface area and small thickness are excellent for diffusion.
• Diffusivity or Diffusion constant
  • D = Solubility / Squreroot of Mol.Wt. 
  • CO₂ diffuses about 20% slower because of its molecular weight but 24 times faster due to its greater solubility. Therefore the diffusivity of CO₂ is about 20 times that of O₂.
• Dalton's law
  • Dalton's law states that each gas within a mixture exerts its own pressure independent of the other gases present.,
  • Atmospheric air is a combination of nitrogen (N₂),oxygen (O₂), carbon dioxide (CO₂), and water vapor. At sea level, atmospheric pressure is 760 mmHg.
  • Atmospheric air is 78.6% nitrogen, 20.9% oxygen, 0.04% carbon dioxide, 0.06% other gases, and varying water vapor, depending on the humidity. PH₂O presence would cause a dilution of the other gases, and thus their partial pressures would be lowered to maintain the total pressure at 760 mm Hg. The ventilation process does not evacuate the alveoli completely with each breath, but rather it is a gradual replenishment and evacuation
• The approximate composition of alveolar air can be
  • PO₂    = 104 mm Hg 
  • PCO₂    = 40 mm Hg
  • PN₂    = 569 mm Hg 
  • PH₂O    = 47 mm Hg
  • Total   = 760 mmHg
• All the components are diluted by water vapor, which is equal to 47 mm Hg in the alveolar air. PH₂O = 47 mm Hg represents 100% humidification of alveolar air at body temperature.
• The PO₂ is lower and the PCO₂ is higher than their respective atmospheric pressures because oxygen is continually diffusing from alveolar air to the tissues (where it is used) and CO₂ is continually diffusing from the tissues (where it is produced) to the alveolar air (where it is expelled).
• The PN₂ of alveolar air is lower than its value in atmospheric air primarily because of its dilution by water vapor.
• Altitude - air pressure at 10,000 ft = 563 mm Hg
• Scuba diving - air pressure at 100 ft = 3000 mm Hg 
• Henry's law
  • Henry's law states that the quantity of a gas that will dissolve in a liquid is proportional to its partial pressure and its solubility coefficient.
  • Gases will dissolve in body fluids more readily if they have a greater partial pressure and solubility coefficient.
  • The solubility coefficient of CO₂ is 24 times higher than that of O₂. Therefore, CO₂ dissolves in blood more readily than O₂.
  • In contrast, the solubility of nitrogen is very low, so even though atmospheric air has 79% N2, it has no effect on body functions.