Physiology
Respiratory
Which of the following statements is INCORRECT. Fick's law tells us that the rate of diffusion of a gas across a membrane increases as:
Answer:
Fick's law tells us that the rate of diffusion of a gas increases:- the larger the surface area involved in gas exchange
- the greater the partial pressure gradient across the membrane
- the thinner the membrane
- the more soluble the gas in the membrane
- the lower the molecular weight of the gas
Alveolar Diffusion
Physiology / Respiratory / Gas Transport in Lungs
Last Updated: 11th April 2019
Gas exchange between alveolar air and blood in the pulmonary capillaries takes place by diffusion across the alveolar-capillary membrane. Diffusion occurs from an area of high partial pressure to an area of low partial pressure, thus the driving force for diffusion is the alveolar-capillary partial pressure gradient. Diffusion occurs until equilibrium is reached, but random movement of particles continues to occur and this is known as dynamic equilibrium.
Fick's Law
Diffusion occurs across a membrane and is therefore governed by Fick's law.
The rate of gas flow = permeability x surface area of gas exchange x difference in partial pressures (where permeability depends on the membrane thickness, gas molecular weight and it's solubility in the membrane).
Fick's law tells us that the rate of diffusion of a gas increases:
- the larger the surface area involved in gas exchange
- the greater the partial pressure gradient across the membrane
- the thinner the membrane
- the more soluble the gas in the membrane
- the lower the molecular weight of the gas
Although CO2 is larger than O2, it is is much more soluble and diffuses 20 times more rapidly. The average surface area of the alveolar-capillary membrane is about 50 - 100 m2, and the average thickness is 0.4 mm. This allows an enormous surface area for gas exchange and a very short diffusion distance.
Transfer Factor
For gas transfer across the lungs, the permeability and surface area are commonly combined as the diffusing capacity (or transfer factor) for that gas, a measure of the alveolar-capillary membrane function.
The diffusing capacity for oxygen (DLO2) cannot be measured directly but the rate of diffusion in the lungs can be estimated by measuring the diffusing capacity of the lungs for carbon monoxide (DLCO).
Factors affecting transfer factor:
- DLCO is reduced by reduced alveolar-capillary membrane area (as in emphysema, pulmonary embolism or lung resection).
- DLCO is reduced by increased membrane thickness (as in pulmonary oedema or pulmonary fibrosis).
- DLCO is reduced in anaemia.
- DLCO is increased in polycythaemia.
- DLCO is increased in exercise (due to increased pulmonary blood volume increasing the effective area).
- DLCO is not affected by hypoventilation.
Limitations of Gas Transfer
The rate of transfer of a gas may be diffusion or perfusion limited.
The solubility of nitrous oxide in the blood is low and it does not undergo chemical combination with any component of blood, thus the partial pressure in the blood rapidly reaches equilibrium with alveolar air, there is no alveolar-capillary partial pressure gradient and diffusion ceases along the capillary; uptake can only be increased by increased capillary blood flow and thus transfer is perfusion-limited.
Carbon monoxide is rapidly taken up and bound tightly to haemoglobin thus pulmonary capillary PCO changes little and the alveolar-capillary partial pressure gradient is maintained along the capillary. Improved ease of diffusion, with reduced thickness or increased area of the alveolar-capillary membrane would increase CO uptake, and thus transfer is diffusion-limited.
Oxygen transfer lies between these two extremes, but is normally perfusion-limited.
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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 |
