2025 First Sitting — Prototype Sample

. (a) The definition of closing capacity was well answered and is relatively straight forward. (b) (i) The main physiological factor is age (decreases from birth to 16 years and then increases to become higher than FRC in older adults). A discussion of the age at which it typically exceeds FRC in the supine and upright positions attracted marks. A mention of other factors that alter it like high expiratory flow rates and increased expiratory effort made for a comprehensive answer. (ii) This part of the question required candidates to cover how increases in closing capacity effect VQ mismatch and PaO2 by air-trapping, reduced compliance, cyclical atelectasis (resulting in lung injury), slower de-nitrogenation during pre-oxygenation and increased work of breathing. These changes are pronounced in situations where FRC is also reduced. This was the section most candidates struggled with given the breadth of effects required. (iii) Techniques used to measure closing capacity are well described in both Nunn’s and West’s respiratory physiology textbooks. One technique involves taking a vital capacity breath of 100% oxygen and then measuring exhaled nitrogen as the subject exhaled back to the residual volume. The closing capacity is the point at which the nitrogen concentration begins to rise above the alveolar plateau in phase III.

. (a) A definition of systemic blood pressure is found in most physiology textbooks and describes this as the pressure exerted on the arterial walls in the systemic circulation. It is a product of vascular resistance and blood flow. An expected formula would be: Pa-Pv (arterial venous pressure difference) = QR where Q is blood flow and R is peripheral arterial resistance. An alternative formula of MAP = CO X TPR was also accepted and provided a structure for the second part of the question. (b) This section required a more detailed discussion of the factors in the equation including determinants of cardiac output, arterial blood volume, arterial elastance and peripheral arterial resistance (based on the Poiseuille equation). Your discussion should include naming the factor and then providing a description of how that factor increases/decreases or contributes to systemic blood pressure relating this to the formula provided in the first part of the question.

. The most effective way to approach this question was to structure information into the headings provided and consider the mark allocation to determine the level of detail for each section. High scoring answers described the unique physical and biochemical features that make up the blood barrier (such as the capillary endothelial cells with tight junctions and numerous mitochondria), and relate these to its barrier and regulation functions. Part (c) required detailed information about the factors affecting permeability across the blood brain barrier and includes transporters and efflux pumps. Answers that provided examples of specific substances and how they move across the blood brain barrier scored best. When responding to the areas outside the blood brain barrier, answers that included not just the area but the rationale similarly scored higher.

. High scoring answers had a good structure utilising the following headings: location; femoral triangle boundaries; relations and contents including the femoral sheath; origin and tributaries; and the important surface anatomy for insertion of a femoral line. Commonly omitted were descriptions of the femoral sheath; the surface anatomy including the ideal puncture site; and anatomical pitfalls. A description of the procedure of central line insertion was not required for this question.