Physiology
Respiratory
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
Gas Transport
Physiology / Respiratory / Gas Transport in Lungs
Last Updated: 11th April 2019
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 |
