Physiology
Respiratory
Regarding the alveolar air-fluid interface, which of the following statements is INCORRECT:
Answer:
Pulmonary surfactant, secreted by TYPE II pneumocytes, acts to reduce surface tension.Alveolar Interface
Physiology / Respiratory / Lung Mechanics
Last Updated: 15th October 2022
Alveolar Air-Fluid Interface
The surface tension of the fluid lining the alveoli contributes to lung stiffness, as the attraction of water molecules for each other at the air-fluid interface creates a collapsing pressure that is directly proportional to surface tension and inversely proportional to alveolar radius.
This is a manifestation of Laplace's law which states that the pressure (P) in a bubble (or alveolus) is proportional to the surface tension (T)/radius (r). A small bubble will therefore have a higher collapsing pressure than a larger one, and be more difficult to keep open. The inward force created by this surface tension also tends to suck fluid into the alveoli (transudation).
In the lungs, pulmonary surfactant secreted by type II pneumocytes aims to minimise these problems.
Pulmonary Surfactant
Pulmonary surfactant is a mixture of phospholipids that floats on the alveolar fluid surface and reduces surface tension. As the alveoli shrink during expiration, the effective concentration of surfactant increases, further lowering the surface tension (which more than balances the increased tendency for alveoli to collapse as they shrink). Alveolar stability is also aided by the connection and mutual pull of neighbouring alveoli, a phenomenon known as alveolar interdependence.
Surfactant prevents alveolar collapse, increases lung compliance and prevents transudation of fluid into alveoli. Infant respiratory distress syndrome, with stiff lungs, lung collapse and transudation, occurs in premature babies (< 34 weeks gestation) due to a deficiency of surfactant production.
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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 |
