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Questions Answered: 93

Final Score 68%

63
30

Questions

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Physiology

Respiratory

Question 17 of 93

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

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

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