A physiologist measures body fluid compartments in a volunteer by indicator dilution.
Which substance is most appropriate for measuring the extracellular fluid volume?
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Physiology for MRCS Part A
Applied surgical physiology is the largest single line in Paper 1: 45 of its 180 questions — general principles 15, and cardiovascular, respiratory, gastrointestinal, urinary, endocrine and neurological physiology 5 each. General principles covers the fluid compartments and electrolytes, acid–base balance and its interpretation, cell physiology and membrane transport, the metabolic response to injury, thermoregulation and starvation, and the physiology of haemostasis. The system lines follow: cardiac output and its control, the oxygen dissociation curve and ventilation–perfusion, gastric secretion and the gut hormones, glomerular filtration and the handling of sodium, potassium and water, the hormonal axes and calcium metabolism, and cerebral blood flow with intracranial pressure. Questions here are quantitative and applied — a set of blood gases, a fluid chart, a postoperative sodium — and the best answer usually comes from understanding one mechanism well enough to predict what the numbers will do.
450 questions in the bank · 7 topics Updated 6 October 2026
Topics in this domain
- General physiological principles150
- Cardiovascular physiology50
- Respiratory physiology50
- Gastrointestinal physiology50
- Renal & urinary physiology50
- Endocrine physiology50
- Neurological physiology50
What the paper tends to test
- Questions typically give arterial blood gases and ask for the disorder and its cause — the anion gap, the expected compensation, and the metabolic alkalosis of pyloric stenosis.
- Expect fluid-compartment arithmetic: where a litre of crystalloid, colloid or dextrose ends up, daily requirements, and the composition of gastrointestinal losses.
- Potassium is tested both ways — the causes and ECG changes of hyperkalaemia and the sequence of its treatment, and the hypokalaemia of vomiting, diuretics and refeeding.
- The metabolic response to injury is examined through its phases, its hormones (cortisol, catecholamines, ADH, aldosterone, glucagon) and why postoperative patients retain sodium and water and lose potassium and nitrogen.
- Cardiovascular questions test the determinants of cardiac output, the Frank–Starling relationship and the physiological response to haemorrhage.
- Respiratory questions turn on the oxygen dissociation curve and its shifts, dead space and shunt, lung volumes and the control of breathing.
- Renal and endocrine lines ask about GFR and its measurement, the renin–angiotensin–aldosterone system, ADH, calcium and parathyroid hormone, and the cortisol stress response.
Classic traps and discriminators
- Hyperkalaemia on the ECG progresses from tall tented T waves, through a flattened P wave and a prolonged PR interval, to a widened QRS and a sine wave before asystole or ventricular fibrillation; calcium gluconate stabilises the myocardium but does not lower the potassium — insulin with dextrose does.
- Pyloric stenosis and prolonged vomiting of gastric contents cause a hypochloraemic, hypokalaemic metabolic alkalosis with paradoxical aciduria; the treatment is 0.9% sodium chloride with potassium, not bicarbonate.
- Of a litre of 5% dextrose less than a tenth stays intravascular once the glucose is metabolised, because it distributes across total body water; a litre of 0.9% saline distributes across the extracellular fluid, so roughly a quarter stays in the plasma. Dextrose replaces water; it does not resuscitate.
- The oxygen dissociation curve shifts to the right — oxygen is unloaded more readily — with acidosis, a raised PaCO2, a raised temperature and raised 2,3-DPG; it shifts to the left with the opposite, with carbon monoxide and with fetal haemoglobin.
- Cardiac output falls early in haemorrhage but blood pressure is held by vasoconstriction until about 30% of the blood volume is lost, so a normal blood pressure does not exclude significant bleeding — tachycardia and a narrowed pulse pressure come first.
- Postoperative hyponatraemia is usually dilutional: ADH is secreted in response to surgery, pain and nausea regardless of osmolality, so hypotonic fluids such as 5% dextrose or 0.18% saline in dextrose given liberally after an operation cause it.
Free physiology practice questions
From the free sample — original, referenced, with the full explanation. Rate your confidence first.
An anaesthetised patient has a left ventricular pressure–volume loop displayed. An infusion of a pure α1-agonist raises systemic vascular resistance while heart rate and contractility are unchanged.
On the next beat, which change in the loop is expected?
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Free revision notes
Physiology questions, answered
How many physiology questions are in MRCS Part A?
The ICBSE blueprint lists 45 of the 180 Paper 1 questions under applied surgical physiology — general principles 15, and 5 each for the cardiovascular, respiratory, gastrointestinal, urinary, endocrine and neurological systems. It is the largest single line of Paper 1.
What are the daily maintenance fluid and electrolyte requirements?
NICE CG174 gives 25–30 ml/kg/day of water, about 1 mmol/kg/day each of sodium, potassium and chloride, and 50–100 g/day of glucose to limit starvation ketosis. Ongoing losses from drains, stomas and nasogastric tubes are replaced on top, with a fluid of matching composition.
What are the phases of the metabolic response to injury?
Cuthbertson’s ebb phase in the first hours — a reduced metabolic rate, hypothermia and hypovolaemia with catecholamine release — followed by the flow phase: a catabolic state with a raised metabolic rate, protein breakdown, hyperglycaemia and insulin resistance, and sodium and water retention, before an anabolic recovery. Its size is proportional to the injury and is reduced by good analgesia, warmth and early feeding.
How should I work through a blood gas?
Read the pH, then decide from the PaCO2 and bicarbonate whether the primary disorder is respiratory or metabolic, then check that compensation is in the expected direction and degree. For a metabolic acidosis calculate the anion gap — sodium minus chloride and bicarbonate, normally about 8–16 mmol/L — to separate lactic acidosis, ketoacidosis and renal failure from the normal-gap acidosis of diarrhoea or a fistula.
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