Gas Pressure

Aka: Gas Pressure, Partial Pressure, Gas Volume, ATPS, BTPS, STPD

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II. Definitions

  1. Ambient Temperature and Barometric Pressure (ATPS)
    1. See Atmospheric Pressure at High Altitude
    2. Gas measurements vary based on environmental conditions (altitude/barometric pressure, Temperature)
    3. Exhaled gases are assumed to be fully saturated with water vapor, and are described as ATPS
  2. Body Temperature, Ambient Barometric Pressure and Fully Saturated With Water Vapor (BTPS)
    1. All ventilated gases (inspired and expired) are converted to BTPS
    2. BTPS values normalize for Body Temperature, Ambient barometric pressure and water vapor saturated
  3. Standard Temperature and Pressure of Dry Gas (STPD)
    1. Normalized values of blood Gas Volumes
    2. Standard Temperature: 0 C
    3. Standard Pressure: 760 mmHg
    4. Dry Gas (desaturated of water vapor)
  4. Partial Pressure
    1. Pressure of each gas within a mixture of gases
    2. Total pressure of the gas mix is the sum of its contained Partial Pressures
      1. See Air Partial Pressures below for example
    3. Partial Pressure of gas in liquid phase is the same as the Partial Pressure of gas in equilibrium with the liquid
      1. Gas diffuses from higher Partial Pressure (e.g. gas) to lower Partial Pressure (e.g. liquid)
    4. Amount of gas dissolved in liquid depends on its solubility
      1. Blood Gases (oxygen and carbon dioxide) have specific transport systems their volumes in blood
      2. Oxygen, and to a lesser extent Carbon Dioxide, are otherwise poorly soluble in blood
      3. See Respiratory Physiology
  5. Gas Volume
    1. Gas Volume is constant for a given number of molecules of any gas (at the same Temperature and pressure)
    2. One Mole of any Gas (at STPD) Volume = 22.4 Liters
    3. Volume of a gas within a mixture of gases
      1. Vg = Vt * g%
      2. where Vt = Total volume of a gas mixture
      3. where g% = fractional concentration of gas
      4. For example, Oxgen (O2) in 6 liters of air (normal Minute Ventilation) = 0.2095 * 6 = 1.26 liters of oxygen

III. Physiology

  1. See Respiratory Physiology
  2. Air Partial Pressures at sea level (STPD, 760 mmHg, 0 C)
    1. Nitrogen (78%): 593 mmHg
    2. Oxygen (20.95%): 159 mmHg
    3. Carbon Dioxide (0.03%): 0.2 mmHg
    4. Argon and other inerts (0.93%): 7 mmHg
  3. Tracheal Partial Pressures (BTPS, fully water vapor saturated)
    1. Nitrogen (74.2%): 563 mmHg
    2. Oxygen (19.63%): 150 mmHg
    3. Carbon Dioxide (0.03%): 0.2 mmHg
    4. Water Vapor (6.18%): 47 mmHg
  4. Alveolar Gas (BTPS, fully water vapor saturated)
    1. Nitrogen (74.8%): 568 mmHg
    2. Oxygen (13.82%): 105 mmHg
    3. Carbon Dioxide (5.26%): 40 mmHg
    4. Water Vapor (6.18%): 47 mmHg
  5. Arterial Blood
    1. See Arterial Blood Gas (ABG)
    2. Partial Pressure Oxygen (PaO2): 100 mmHg
      1. Oxygen Content: 20.8 ml per 100 ml blood (when Hemoglobin 15 g/dl)
    3. Partial Pressure Carbon Dioxide (PaCO2): 40 mmHg
      1. Carbon Dioxide Content: 48 ml per 100 ml blood
    4. Arterial pH: 7.40
    5. Arterial Bicarbonate: 24 mEq/L
  6. Mixed Venous Blood
    1. See Venous Blood Gas (VBG)
    2. Partial Pressure Oxygen (PvO2): 40 mmHg
      1. Oxygen Content: 15.8 ml per 100 ml blood (when Hemoglobin 15 g/dl)
    3. Partial Pressure Carbon Dioxide (PvCO2): 46 mmHg
      1. Carbon Dioxide Content: 54 ml per 100 ml blood
    4. Arterial pH: 7.35
    5. Arterial Bicarbonate: 26 mEq/L

IV. References

  1. Davies (1986) Acute Respiratory Failure, Cyberlog

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