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Physiology

Respiratory

Question 149 of 180

Regarding the partial pressure of oxygen during respiration, which of the following statements is CORRECT:

Answer:

Inspired PO2 in the airways is less than inhaled PO2 due to humidification. Alveolar PO2 is less than inhaled or inspired PO2 due to gas exchange. Exhaled PO2 is greater than alveolar O2 due to its mixing with anatomical dead space (air that has not taken part in gas exchange, therefore that has relatively higher PO2) on the way out, but is less than inhaled or inspired PO2. Typical values for a resting young healthy male (in kPa) are shown below:
  • Inhaled air: PO2 21.2, PCO2 0.0
  • Inspired air in airways (after humidification): PO2 19.9, PCO2 0.0
  • Alveolar air (after equilibrium with pulmonary capillaries): PO2 13.3, PCO2 5.3
  • Exhaled air (after mixing with anatomical dead space air): PO2 15.5, PCO2 4.3

Fractional Concentration and Partial Pressure of Gases in a Gas Mixture

Dalton's law states that when two or more gases, which do not react chemically, are present in the same container, the total pressure is the sum of the partial pressures of each gas.

In the atmosphere, the partial pressure of a gas is the contribution to barometric pressure exerted by that gas. The total pressure exerted by the atmosphere at sea level is 760 mmHg (101 kPa).

The partial pressure of each gas is determined by the fractional concentration of that gas. Dried air contains 21% oxygen, 78.1% nitrogen and 0.9% inert gases such as helium and argon. The small amount of CO2 in air (< 0.04 %) is usually ignored.

Therefore the partial pressure of oxygen in dry inhaled air = 0.21 x  760 (101) = 159 mmHg (21.2 kPa) and the partial pressure of nitrogen = 0.78 x 760 (101) = 593 mmHg (78.8 kPa).

At altitude, the oxygen fraction is unaltered but the barometric pressure and thus partial pressure of oxygen is reduced.

Oxygen and Carbon Dioxide Partial Pressures in Respiration

Typical values for a resting young healthy male (in kPa) are shown below:

  • Inhaled air: PO2 21.2, PCO2 0.0
  • Inspired air in airways (after humidification): PO2 19.9, PCO2 0.0
  • Alveolar air (after equilibrium with pulmonary capillaries): PO2 13.3, PCO2 5.3
  • Exhaled air (after mixing with anatomical dead space air): PO2 15.5, PCO2 4.3

Gases Dissolved in Body Fluids

If a gas is exposed to a liquid to which it does not react, gas particles will move into that liquid. Henry's law states that the number of molecules dissolving into the liquid is directly proportional to the partial pressure at the surface of the gas.

The constant of proportionality is the solubility of the gas in the liquid, and it is determined by the gas, the liquid and the temperature:

Content of dissolved gas X in liquid Y = (Solubility of X in Y) x (Partial pressure of X at surface)

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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