FREEMEDSITE MRCS ACADEMY • ARTICLE 11 • PAPER 1

Cardiovascular and Respiratory Physiology Made Easy for MRCS Part A (2026)

Understand cardiac output, shock, baroreceptors, lung mechanics, V/Q mismatch, oxygen transport, ABGs and spirometry with simple clinical explanations and original MRCS-style single best answer questions.

12Teaching chapters

52Saved revision tasks

40Explained 5-option SBAs

Educational revision article • Updated October 2026 • Original teaching diagrams • No sign-in required

What this lesson is for: MRCS Part A Paper 1 tests applied basic sciences. This guide deliberately connects physiology to a surgical patient: blood loss, sepsis, anaesthesia, pulmonary embolism, oxygenation and ventilator physiology. It is a teaching resource, not an official examination question paper.

Contents

Cardiac output & calculations Preload, afterload and shock Autonomic & baroreflex Cardiac cycle & ECG Venous return & perfusion Lung volumes & mechanics V/Q, shunt and dead space Gas transport & oxygenation Chemoreceptors & ABGs Spirometry & DLCO Perioperative respiratory physiology Quick comparison Saved revision checklist 40 explained clinical SBAs FAQs References and next article

1. Cardiac output and bedside calculations

Cardiac output (CO) is the volume of blood ejected by one ventricle each minute. It depends on heart rate (HR) and stroke volume (SV). Stroke volume depends on preload, afterload and myocardial contractility. A rapid pulse can coexist with dangerous low output when circulating volume is depleted.

CO = HR × SV   |   SV = EDV − ESV   |   EF = SV / EDV × 100

Example — haemorrhage

HR 120/min and SV 35 mL → CO 4.2 L/min, which may be inadequate for the metabolic situation despite tachycardia.

Example — ejection fraction

EDV 150 mL and ESV 75 mL → stroke volume 75 mL and EF 50%. EF is a proportion, not absolute blood flow.

MAP ≈ DBP + (SBP − DBP)/3;   MAP ≈ CO × SVR (simplified)

The second expression is approximate because venous pressure is omitted. Cardiac index = cardiac output/body surface area. Fick principle calculates flow as oxygen consumption divided by arterial–mixed venous oxygen-content difference.

Clinical pearl: Oxygen delivery = CO × arterial oxygen content. Normal SpO2 does not guarantee sufficient delivery if haemoglobin is low or cardiac output is critically reduced.

2. Frank–Starling mechanism, afterload and shock

Preload roughly reflects ventricular filling/end-diastolic stretch. Within physiological limits, higher venous return increases stroke volume via Frank–Starling. Afterload is the pressure/wall stress against which the ventricle ejects, and contractility is intrinsic force generation independent of loading conditions.

Hypovolaemic shock

Low preload → low stroke volume/output; compensatory high HR and SVR. JVP often low.

Cardiogenic shock

Pump failure → low CO; raised filling pressures and often high SVR; pulmonary oedema may develop.

Distributive shock

Marked vasodilatation/low SVR; early CO can be normal or high. Tissue perfusion remains inadequate.

Obstructive shock

Mechanical limitation to cardiac filling/forward flow, as with tamponade, tension pneumothorax or massive PE.

Examiner trap: high jugular venous pressure in massive PE does not mean the left ventricle has adequate preload. The RV may be distended while left-sided forward filling is poor.

3. Baroreceptors, autonomic regulation and RAAS

Standing and blood loss rapidly reduce arterial stretch, decreasing baroreceptor afferent firing. The brainstem responds with increased sympathetic and reduced parasympathetic activity, producing tachycardia, increased contractility and vasoconstriction.

Baroreceptor vs chemoreceptor: carotid sinus senses pressure (CN IX); carotid body senses blood-gas chemistry (CN IX). RAAS responds to renal hypoperfusion and supports pressure through angiotensin II vasoconstriction and aldosterone-mediated sodium retention over a slower time course.

4. Cardiac cycle, coronary perfusion and ECG

Mitral and tricuspid closure produces S1 and begins isovolumetric contraction. When ventricular pressure exceeds outflow pressure, semilunar valves open and ejection begins. Closure of aortic and pulmonary valves produces S2 and initiates isovolumetric relaxation, followed by AV-valve opening and ventricular filling.

Left coronary perfusion

Predominantly diastolic because LV systolic compression impedes intramural flow. Tachycardia shortens diastole, reducing perfusion time.

Electrical conduction

SA node → atria → AV node → His–Purkinje → ventricles. PR interval reflects atrioventricular conduction; QRS reflects ventricular depolarisation.

Coronary perfusion pressure is approximated by aortic diastolic pressure minus LV end-diastolic pressure. Markedly low diastolic BP or high LVEDP can impair subendocardial perfusion.

Wall stress ∝ pressure × radius / wall thickness (simplified Laplace relation)

5. Venous return and perioperative cardiovascular effects

Steady-state cardiac output must match venous return. Bleeding reduces stressed circulating volume and mean systemic filling pressure; the resulting decline in venous return lowers end-diastolic filling. Positive-pressure ventilation can raise right-atrial pressure, diminish venous-return gradient and increase RV afterload in some settings.

Practical relevance: anaesthetic-induced vasodilatation plus positive-pressure ventilation may provoke hypotension in an underfilled patient. An elevated JVP from RV obstruction is not the same thing as high LV preload.

6. Lung volumes, compliance, resistance and surfactant

Minute ventilation counts all inspired volume; alveolar ventilation excludes dead-space gas. CO2 elimination depends predominantly on effective alveolar ventilation relative to CO2 production.

Minute ventilation = Vt × RR   |   Alveolar ventilation = (Vt − Vd) × RR

Functional residual capacity (FRC)

Air at end of passive expiration: residual volume + expiratory reserve volume. Reduced by supine posture and general anaesthesia.

Compliance

ΔV/ΔP. Fibrosis lowers compliance and increases recoil; emphysema tends to increase compliance and reduce recoil.

Surfactant

Type II pneumocyte product that lowers surface tension and stabilises small alveoli; deficiency promotes collapse.

Airway resistance

Markedly increases with airway narrowing; bronchospasm can raise peak airway pressure disproportionately above plateau.

7. Ventilation–perfusion mismatch, shunt and dead space

The lung needs both alveolar ventilation (V) and pulmonary capillary perfusion (Q). The whole-lung average V/Q is approximately 0.8, but local ratios differ considerably.

Oxygen response: low V/Q commonly improves with supplemental oxygen; a large true right-to-left shunt is substantially less responsive. Pulmonary embolism increases alveolar dead space but can also produce other V/Q abnormalities.

8. Alveolar oxygen, haemoglobin and diffusion

PAO2 ≈ FiO2 × (Patm − 47) − PaCO2/0.8 (sea-level approximation)

With room air, atmospheric pressure 760 mmHg and PaCO2 40 mmHg, alveolar PO2 is approximately 100 mmHg. The A–a gradient is PAO2 minus measured PaO2, and may be widened by V/Q mismatch, shunt or diffusion limitation. Pure hypoventilation and low inspired oxygen generally produce a normal A–a gradient.

Bohr effect

Increased CO2, H+, temperature and 2,3-BPG shift O2–Hb dissociation to the right, favouring unloading. P50 increases.

Haldane effect

Deoxygenated blood carries more CO2; Hb oxygenation in pulmonary capillaries facilitates CO2 release.

CaO2 ≈ 1.34 × Hb × SaO2 + 0.003 × PaO2 (mL O2/dL, with Hb in g/dL and PaO2 in mmHg). Anaemia can reduce oxygen content despite a normal saturation and PaO2. Reduced DLCO occurs with loss of alveolar-capillary area, membrane disease or pulmonary vascular disease.

Examiner trap: PaO2 is not the same as oxygen content. Pulse oximetry also cannot reliably exclude carbon monoxide poisoning.

9. Respiratory drive, CO2 and arterial blood gases

Central chemoreceptors in the medullary region sense changes in CSF H+ produced by diffusible CO2 and provide a major CO2-driven ventilatory response. Carotid bodies respond acutely to hypoxaemia, especially at PaO2 below approximately 60 mmHg, and also respond to CO2 and pH.

ABG sequence: 1) pH → 2) PaCO2 → 3) HCO3 → 4) compensation → 5) oxygenation

pH 7.27, PaCO2 62 mmHg and HCO3 27 mmol/L points toward a primary respiratory acidosis with little/acute metabolic compensation. Always assess clinical context: opioid respiratory depression, exacerbation of obstructive disease, fatigue and mechanical causes.

10. Spirometry, lung volumes and DLCO

Obstruction

FEV1/FVC ratio below its lower limit of normal; use reference equations and demographics. Bronchodilator response may support reversible airflow limitation.

Suspected restriction

Low FVC with normal or raised FEV1/FVC may suggest restriction but low TLC is needed to confirm it.

Mixed impairment

Obstruction (low ratio) plus restriction (low TLC). Spirometry alone can be misleading when air trapping lowers FVC.

DLCO

Reduced in emphysema, fibrosis and pulmonary vascular disease, but influenced by haemoglobin and technical factors.

Modern ERS/ATS interpretation: a fixed FEV1/FVC ratio of 0.70 is not a universal age-neutral diagnostic threshold. A low FVC with preserved ratio alone is insufficient to diagnose restriction; measure TLC.

11. Anaesthesia, mechanical ventilation and postoperative physiology

General anaesthesia, supine position, obesity, incisional pain and muscle relaxation can reduce FRC and favour small-airway closure and atelectasis. A collapsed but perfused lung unit acts as shunt. Apply recruitment, PEEP, analgesia and mobilisation according to clinical context rather than using fixed interventions for every patient.

Static compliance = Vt / (plateau pressure − PEEP), in a passive patient with suitable measurement

Peak pressure includes resistive and elastic components. A large peak–plateau difference suggests increased airway resistance (e.g. bronchospasm). Increased plateau pressure may reflect low compliance. Positive-pressure ventilation can also worsen venous return or RV afterload in selected patients.

Safety: these are exam-level principles, not bedside ventilator prescriptions. Actual support requires patient-specific physiology and clinical supervision.

12. Quick comparison: exam traps to remember

Carotid sinus vs carotid body

Sinus = arterial stretch. Body = blood-gas chemistry.

Bohr vs Haldane

Bohr: CO2/H+ changes O2 binding. Haldane: oxygenation changes CO2 carriage.

Shunt vs dead space

Shunt: perfusion without ventilation. Dead space: ventilation without perfusion.

CO vs EF

Output is blood volume/minute; EF is proportion of EDV ejected.

Obstruction vs restriction

Obstruction: low FEV1/FVC. Restriction: low TLC.

PaO2 vs CaO2

PaO2 reflects dissolved oxygen tension; CaO2 is mostly Hb-bound oxygen.

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Cardiovascular — calculations

Cardiac output and cardiac index Stroke volume and ejection fraction Fick cardiac-output principle Mean arterial pressure and pulse pressure Systemic vascular resistance Oxygen delivery and oxygen content

Cardiovascular — regulation

Frank–Starling mechanism Preload, afterload and contractility Baroreceptor afferent pathways (IX and X) Sympathetic and vagal effects Renin–angiotensin–aldosterone system

Cardiovascular — cardiac cycle

Cardiac cycle and heart sounds Left-coronary diastolic perfusion Coronary perfusion pressure ECG intervals and AV conduction Laplace law and wall stress

Cardiovascular — surgical applications

Hypovolaemic shock profile Cardiogenic shock profile Obstructive shock and pulmonary embolism Distributive shock and sepsis Effects of positive-pressure ventilation

Respiratory — mechanics

Tidal volume and minute ventilation Alveolar ventilation and dead space Lung compliance and elastic recoil Surfactant and Laplace law FRC, RV and closing capacity Airway resistance and bronchospasm Mechanisms of airway closure at low lung volumes

Respiratory — gas exchange

V/Q low versus high Physiological dead space in pulmonary embolism Shunt versus V/Q mismatch Alveolar gas equation A–a gradient

Respiratory — control

Central medullary chemoreceptors Carotid bodies and hypoxic drive Hypoxic pulmonary vasoconstriction Opioid-induced hypoventilation

Respiratory — O2 carriage

Oxygen content and Hb concentration Bohr effect and right shift Haldane effect Carbon monoxide and co-oximetry Diffusing capacity (DLCO)

Respiratory — tests

Interpret FEV1/FVC using lower limit of normal Identify obstruction Confirm restriction by TLC Mixed and nonspecific spirometry patterns Interpret an ABG systematically

Respiratory — perioperative

Anaesthesia-related reduced FRC Postoperative atelectasis and shunt PEEP and venous return Peak versus plateau airway pressure Driving pressure and compliance

40 original MRCS-style clinical SBAs

Each question has five options (A–E). Choose your answer, select Check answer and open the explanation. Incorrect choices are explained individually, followed by an exam pearl, an examiner trap and an answered viva extension. Correct-answer positions are balanced: eight each in A, B, C, D and E.

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Question 1 of 40

Question 1 of 40 • Cardiac output and ventricular function

Q01. After a 700 mL haemorrhage, a patient has a heart rate of 110/min and stroke volume of 45 mL. What is the estimated cardiac output?

A. 2.0 L/min B. 3.3 L/min C. 4.95 L/min D. 5.5 L/min E. 7.0 L/min

View detailed teaching explanation and answered viva

Correct answer: C. 4.95 L/min.

Why correct: Cardiac output = heart rate × stroke volume = 110 × 45 = 4950 mL/min = 4.95 L/min. A normal resting adult output is often around 4–8 L/min; clinical interpretation depends on body size and physiology.

Why the other options are incorrect:

A. 2.0 L/min: Would require a much lower stroke volume or rate.

B. 3.3 L/min: This underestimates 110 × 45 mL.

D. 5.5 L/min: This would follow a stroke volume of 50 mL at this rate.

E. 7.0 L/min: This is not consistent with the supplied measurements.

Exam pearl: Calculate in mL/min first, then divide by 1000. Cardiac index = CO/body surface area.

Examiner trap: Tachycardia does not establish adequate output: stroke volume may be severely depressed.

Viva extension — What happens to stroke volume during hypovolaemia?
Answer: Venous return and preload fall, lowering end-diastolic volume and stroke volume through the Frank–Starling relationship.

Question 2 of 40 • Cardiac output and ventricular function

Q02. An echocardiogram shows end-diastolic volume 140 mL and end-systolic volume 70 mL. What is the left-ventricular ejection fraction?

A. 25% B. 35% C. 70% D. 100% E. 50%

View detailed teaching explanation and answered viva

Correct answer: E. 50%.

Why correct: Stroke volume = 140 − 70 = 70 mL. Ejection fraction = stroke volume / EDV × 100 = 70/140 × 100 = 50%. EF reflects proportion ejected, not directly whole-body tissue perfusion.

Why the other options are incorrect:

A. 25%: This does not result from the difference in supplied volumes.

B. 35%: This underestimates stroke volume relative to EDV.

C. 70%: This confuses the absolute end-systolic volume with a fraction.

D. 100%: This incorrectly assumes complete emptying of the ventricle.

Exam pearl: EF = (EDV − ESV)/EDV × 100.

Examiner trap: Patients with preserved EF can still have impaired diastolic filling and heart failure.

Viva extension — Why may ejection fraction look preserved in a low-volume patient?
Answer: Both EDV and stroke volume can fall together, preserving a proportion despite lower absolute forward flow.

Question 3 of 40 • Cardiac output and ventricular function

Q03. A patient receives an appropriate fluid bolus and stroke volume increases as end-diastolic filling rises without a change in inotropy. Which principle explains this?

A. Frank–Starling mechanism B. Bowditch staircase C. Anrep effect D. Baroreceptor resetting E. Laplace law

View detailed teaching explanation and answered viva

Correct answer: A. Frank–Starling mechanism.

Why correct: Increased preload within physiologic limits increases myofibre stretch and calcium sensitivity, producing greater force and stroke volume. The effect is limited on a flat failing-heart curve.

Why the other options are incorrect:

B. Bowditch staircase: Frequency-dependent augmentation of contractility, not the immediate preload response.

C. Anrep effect: A slower intrinsic response to increased afterload.

D. Baroreceptor resetting: A blood-pressure regulation process, not length-dependent force.

E. Laplace law: Relates wall stress to pressure, radius and thickness.

Exam pearl: Fluid responsiveness means stroke volume increases with preload; it does not mean all hypotensive patients need fluid.

Examiner trap: Frank–Starling is a preload mechanism, not a direct increase in inotropy.

Viva extension — How is the curve altered by an inotrope?
Answer: It shifts upward: more stroke volume at the same preload.

Question 4 of 40 • Arterial pressure and resistance

Q04. A postoperative patient has systolic BP 90 mmHg and diastolic BP 60 mmHg. What is the approximate mean arterial pressure?

A. 60 mmHg B. 75 mmHg C. 90 mmHg D. 70 mmHg E. 110 mmHg

View detailed teaching explanation and answered viva

Correct answer: D. 70 mmHg.

Why correct: At an ordinary heart rate, MAP ≈ DBP + (SBP − DBP)/3 = 60 + 30/3 = 70 mmHg. The relationship is an approximation and less reliable with major tachycardia or altered waveforms.

Why the other options are incorrect:

A. 60 mmHg: This is the diastolic pressure, not the estimated MAP.

B. 75 mmHg: This would require another set of pressures.

C. 90 mmHg: This is systolic pressure.

E. 110 mmHg: Higher than both measured pressures.

Exam pearl: MAP supports organ perfusion assessment, but adequate MAP does not guarantee microcirculatory flow.

Examiner trap: Do not calculate MAP as a simple arithmetic average of SBP and DBP.

Viva extension — Which determinants underlie MAP?
Answer: Broadly cardiac output and systemic vascular resistance, with venous pressure often omitted in a simplified model.

Question 5 of 40 • Arterial pressure and resistance

Q05. A vasodilatory septic patient has warm extremities, a relatively high cardiac output and very low arterial pressure. Which haemodynamic change best explains the low pressure?

A. Increase in systemic vascular resistance B. Marked fall in systemic vascular resistance C. Increase in blood viscosity D. Severe bradycardia with low output E. Increase in left-ventricular afterload

View detailed teaching explanation and answered viva

Correct answer: B. Marked fall in systemic vascular resistance.

Why correct: In early distributive shock the systemic resistance can be profoundly reduced despite normal or elevated output. MAP is roughly proportional to CO × SVR.

Why the other options are incorrect:

A. Increase in systemic vascular resistance: This ordinarily raises arterial pressure for unchanged flow.

C. Increase in blood viscosity: This is not the characteristic dominant mechanism in distributive shock.

D. Severe bradycardia with low output: Contradicts the specified relatively high output.

E. Increase in left-ventricular afterload: Does not explain the distributive vasodilated state.

Exam pearl: Think of hypotension as flow (CO), resistance (SVR), or both.

Examiner trap: A normal or high CO does not rule out tissue hypoperfusion in sepsis.

Viva extension — How does vasopressor therapy improve MAP?
Answer: Predominantly by increasing vascular tone (with drug-specific effects on cardiac output).

Question 6 of 40 • Baroreceptor reflex

Q06. A patient rapidly stands after lying supine and arterial pressure transiently falls. What is the immediate normal baroreflex response?

A. Increased carotid-sinus firing and vagal activation B. Decreased sympathetic discharge C. Reduced afferent firing with increased sympathetic activity D. Increased vagal tone with sinus bradycardia E. Immediate aldosterone secretion as the primary response

View detailed teaching explanation and answered viva

Correct answer: C. Reduced afferent firing with increased sympathetic activity.

Why correct: Less stretch of carotid and aortic baroreceptors reduces input to the medullary nucleus tractus solitarius; sympathetic outflow rises and vagal outflow falls, increasing rate, contractility and peripheral resistance.

Why the other options are incorrect:

A. Increased carotid-sinus firing and vagal activation: That would occur with increased stretch/arterial pressure.

B. Decreased sympathetic discharge: Would worsen the compensatory vasodilatation and hypotension.

D. Increased vagal tone with sinus bradycardia: This is the opposite of usual orthostatic compensation.

E. Immediate aldosterone secretion as the primary response: RAAS contributes later; the neural baroreflex acts within seconds.

Exam pearl: Baroreceptors are stretch-sensitive; firing increases as pressure increases.

Examiner trap: The afferent limb differs from the efferent cardiac response.

Viva extension — What nerves carry baroreceptor afferents?
Answer: Carotid sinus via Hering nerve/CN IX; aortic arch via CN X.

Question 7 of 40 • Baroreceptor reflex

Q07. During carotid sinus stimulation, which cranial nerve carries the principal afferent impulses from the carotid sinus to the medulla?

A. Facial nerve (CN VII) B. Trigeminal nerve (CN V) C. Vagus nerve (CN X) D. Hypoglossal nerve (CN XII) E. Glossopharyngeal nerve (CN IX)

View detailed teaching explanation and answered viva

Correct answer: E. Glossopharyngeal nerve (CN IX).

Why correct: The carotid sinus nerve of Hering is a branch of CN IX; impulses end in the nucleus tractus solitarius. The aortic arch communicates via CN X.

Why the other options are incorrect:

A. Facial nerve (CN VII): Carries facial motor and selected special sensory/autonomic fibres.

B. Trigeminal nerve (CN V): Primarily face sensation and mastication.

C. Vagus nerve (CN X): Carries aortic baroreceptor afferents and cardiac parasympathetic efferents.

D. Hypoglossal nerve (CN XII): Supplies tongue motor function.

Exam pearl: Carotid sinus = IX; aortic arch = X.

Examiner trap: Do not confuse the vagal efferent pathway slowing the heart with the carotid afferent pathway.

Viva extension — Why can carotid sinus massage slow the heart?
Answer: Simulated pressure elevation increases baroreceptor signaling and vagal cardiac outflow.

Question 8 of 40 • Autonomic physiology

Q08. A patient has sinus tachycardia during an adrenaline infusion. Stimulation of which receptor increases pacemaker firing most directly?

A. Beta-1 adrenergic receptor B. Alpha-1 receptor C. Beta-2 receptor D. M2 muscarinic receptor E. Nicotinic Nm receptor

View detailed teaching explanation and answered viva

Correct answer: A. Beta-1 adrenergic receptor.

Why correct: Beta-1 receptor signaling increases cAMP-mediated pacemaker currents and calcium entry in nodal tissues, increasing chronotropy and AV conduction.

Why the other options are incorrect:

B. Alpha-1 receptor: Primarily vascular smooth-muscle constriction.

C. Beta-2 receptor: Predominantly bronchial and some vascular smooth-muscle relaxation.

D. M2 muscarinic receptor: Slows SA nodal firing and AV conduction.

E. Nicotinic Nm receptor: At the neuromuscular junction rather than cardiac pacemaker cells.

Exam pearl: Beta-1: heart rate, conduction and contractility; M2: vagal braking.

Examiner trap: A beta-blocker can lower rate without necessarily increasing peripheral vascular resistance.

Viva extension — What is the effect of vagal stimulation on AV nodal conduction?
Answer: It slows AV conduction and can prolong the PR interval.

Question 9 of 40 • Cardiac cycle and coronary flow

Q09. A patient develops marked tachycardia and subendocardial ischaemia despite no coronary thrombus. Which phase of the cardiac cycle is most shortened, impairing left-coronary perfusion?

A. Isovolumetric systole B. Ventricular ejection C. Atrial depolarisation D. Diastole E. The entire cycle length is unchanged

View detailed teaching explanation and answered viva

Correct answer: D. Diastole.

Why correct: Left-ventricular contraction compresses intramyocardial vessels, so most left coronary flow occurs during diastole. Faster heart rates disproportionately shorten diastolic time and elevate myocardial oxygen demand.

Why the other options are incorrect:

A. Isovolumetric systole: Part of systole; coronary flow to the LV is greatest in diastole.

B. Ventricular ejection: Although shortened somewhat, it is not the principal vulnerable perfusion interval.

C. Atrial depolarisation: Electrical event, not the primary perfusion phase.

E. The entire cycle length is unchanged: The cycle becomes shorter at higher rates.

Exam pearl: Coronary perfusion pressure ≈ aortic diastolic pressure − LV end-diastolic pressure.

Examiner trap: Right coronary flow is less exclusively diastolic because RV systolic pressures are lower.

Viva extension — How can severe aortic regurgitation impair coronary perfusion?
Answer: Low aortic diastolic pressure reduces the coronary perfusion gradient.

Question 10 of 40 • Cardiac cycle and valves

Q10. The aortic valve has just closed on the left-ventricular pressure trace. What begins immediately afterward?

A. Rapid ventricular ejection B. Isovolumetric relaxation C. Isovolumetric contraction D. Atrial systole E. Rapid ventricular filling

View detailed teaching explanation and answered viva

Correct answer: B. Isovolumetric relaxation.

Why correct: Aortic closure generates the aortic component of S2 and marks the onset of isovolumetric relaxation, when both aortic and mitral valves are shut and ventricular volume is stable.

Why the other options are incorrect:

A. Rapid ventricular ejection: Requires the aortic valve to be open.

C. Isovolumetric contraction: Begins after mitral closure before aortic opening.

D. Atrial systole: Occurs late in diastole before the next ventricular systole.

E. Rapid ventricular filling: Begins only after LV pressure falls below left atrial pressure and mitral valve opens.

Exam pearl: S1 marks AV valve closure; S2 marks semilunar valve closure.

Examiner trap: Valves open/close from pressure gradients, not an active contraction by the cusps.

Viva extension — Which interval follows mitral valve opening?
Answer: Rapid passive ventricular filling in early diastole.

Question 11 of 40 • Conduction and ECG

Q11. An ECG shows a gradually increasing PR interval followed by a dropped QRS complex. What is the usual site of the conduction delay?

A. Sinoatrial node B. Ventricular myocardium C. Atrioventricular node D. Left bundle branch E. Purkinje fibre terminals

View detailed teaching explanation and answered viva

Correct answer: C. Atrioventricular node.

Why correct: Mobitz I (Wenckebach) reflects progressive AV nodal refractoriness until an atrial impulse fails to conduct. QRS is often narrow, although exceptions occur.

Why the other options are incorrect:

A. Sinoatrial node: Controls impulse initiation, not classic Wenckebach PR progression.

B. Ventricular myocardium: Contributes to QRS rather than progressive PR delay.

D. Left bundle branch: Causes intraventricular conduction changes, typically widened QRS.

E. Purkinje fibre terminals: Do not characteristically create Mobitz I Wenckebach behaviour.

Exam pearl: PR interval is primarily the atrioventricular conduction time.

Examiner trap: Mobitz II is often infranodal and has different risk implications.

Viva extension — Why is Mobitz II more concerning?
Answer: It is more likely to progress to complete heart block and may require pacing depending on context.

Question 12 of 40 • Myocardial oxygen demand

Q12. During severe hypertension, LV wall tension and oxygen demand rise. Which relationship best explains higher wall tension with a larger chamber radius?

A. Fick principle B. Bernoulli principle C. Poiseuille law D. Frank–Starling mechanism E. Laplace law

View detailed teaching explanation and answered viva

Correct answer: E. Laplace law.

Why correct: A simplified thin-wall Laplace relation is wall stress ∝ pressure × radius / wall thickness. Dilatation raises wall stress while compensatory hypertrophy may reduce stress.

Why the other options are incorrect:

A. Fick principle: Describes flow from oxygen uptake and arteriovenous content difference.

B. Bernoulli principle: Relates pressure and flow speed in fluid mechanics.

C. Poiseuille law: Describes resistance in laminar flow through a cylindrical tube.

D. Frank–Starling mechanism: Relates ventricular filling to stroke volume, not the radius–wall stress relation.

Exam pearl: Wall tension rises with pressure and radius; hypertrophy changes the denominator.

Examiner trap: Laplace approximations differ by chamber geometry; avoid rigid numerical application to complex ventricles.

Viva extension — How does dilated cardiomyopathy increase oxygen demand?
Answer: Larger ventricular radius increases required wall stress for a given pressure.

Question 13 of 40 • Haemodynamic shock

Q13. After myocardial infarction a shocked patient has raised jugular venous pressure, pulmonary oedema and a low cardiac output. What is the dominant shock mechanism?

A. Pump failure causing cardiogenic shock B. Pure hypovolaemic shock C. Distributive shock D. Neurogenic shock E. Isolated acute haemorrhage

View detailed teaching explanation and answered viva

Correct answer: A. Pump failure causing cardiogenic shock.

Why correct: Poor ventricular contractility reduces stroke volume and cardiac output; compensatory sympathetic vasoconstriction often increases SVR, while filling pressures and pulmonary venous congestion increase.

Why the other options are incorrect:

B. Pure hypovolaemic shock: Usually reduces filling pressures and does not cause primary pulmonary congestion.

C. Distributive shock: Causes low SVR rather than the classic high-filling-pressure pump-failure picture.

D. Neurogenic shock: Typically vasodilatation and relative bradycardia after spinal injury.

E. Isolated acute haemorrhage: Would generally lower rather than elevate preload pressures.

Exam pearl: Shock phenotypes: cardiogenic (low CO/high filling pressure) versus early distributive (low SVR).

Examiner trap: Hypotension with elevated JVP is a warning against reflex large fluid boluses.

Viva extension — What is the initial resuscitation principle?
Answer: Assess ABCs, oxygenation as indicated, ECG and bedside echo, treat the cause and titrate vasoactive support; individualise fluids.

Question 14 of 40 • Haemodynamic shock

Q14. A patient has massive pulmonary embolism with hypotension, elevated JVP and a dilated right ventricle. Which change most directly reduces LV stroke volume?

A. Increased LV contractility B. Primary fall in systemic vascular resistance C. Increased pulmonary venous return D. Reduced LV preload from impaired RV forward flow E. Increased left atrial filling

View detailed teaching explanation and answered viva

Correct answer: D. Reduced LV preload from impaired RV forward flow.

Why correct: Pulmonary vascular obstruction abruptly raises RV afterload, impairing RV output and reducing pulmonary blood flow, left-heart filling and systemic output. Septal shift can further impair LV filling.

Why the other options are incorrect:

A. Increased LV contractility: Would improve, not reduce, forward output.

B. Primary fall in systemic vascular resistance: Not the dominant early cause of obstructive PE shock.

C. Increased pulmonary venous return: Pulmonary obstruction reduces effective return to left heart.

E. Increased left atrial filling: Contrary to diminished flow across the obstructed pulmonary bed.

Exam pearl: Obstructive shock can present with high right-sided pressure but low LV preload.

Examiner trap: High JVP does not always mean the left ventricle is well filled.

Viva extension — Name two other obstructive shock causes.
Answer: Cardiac tamponade and tension pneumothorax.

Question 15 of 40 • Cardiac output assessment

Q15. A patient has oxygen consumption 250 mL/min and an arteriovenous oxygen-content difference of 50 mL O2 per litre of blood. What is the Fick-derived cardiac output?

A. 1 L/min B. 5 L/min C. 2.5 L/min D. 10 L/min E. 12.5 L/min

View detailed teaching explanation and answered viva

Correct answer: B. 5 L/min.

Why correct: Fick equation: CO = oxygen consumption / (arterial − mixed venous oxygen content) = 250/(50 mL/L) = 5 L/min.

Why the other options are incorrect:

A. 1 L/min: Would correspond to an unrealistically large content difference for these data.

C. 2.5 L/min: This misses the stated units in the arteriovenous difference.

D. 10 L/min: Would require an arteriovenous difference of 25 mL/L.

E. 12.5 L/min: Would require an even smaller extraction difference.

Exam pearl: Ensure oxygen content units match oxygen consumption units.

Examiner trap: Oxygen tension (PaO2) cannot replace oxygen content (CaO2) in the Fick equation.

Viva extension — How is oxygen content mainly determined?
Answer: By haemoglobin concentration and oxygen saturation, with only a small dissolved portion.

Question 16 of 40 • Cardiac haemodynamics

Q16. During positive-pressure inspiration in a ventilated patient, a large rise in intrathoracic pressure can initially reduce venous return. Which variable changes first?

A. Systemic venous return rises because right atrial pressure falls B. Myocardial oxygen uptake instantly doubles C. The pressure gradient for systemic venous return decreases D. Systemic vascular resistance falls to zero E. Aortic valve remains open in diastole

View detailed teaching explanation and answered viva

Correct answer: C. The pressure gradient for systemic venous return decreases.

Why correct: Raised intrathoracic pressure can increase measured right-atrial pressure and reduce the pressure difference driving venous return. Effects are complex and depend on lung volume, RV afterload and preload.

Why the other options are incorrect:

A. Systemic venous return rises because right atrial pressure falls: Positive thoracic pressure usually raises right atrial pressure relative to atmospheric reference.

B. Myocardial oxygen uptake instantly doubles: Not an obligatory immediate consequence of inspiration.

D. Systemic vascular resistance falls to zero: No such direct effect occurs.

E. Aortic valve remains open in diastole: Positive pressure does not abolish valve-cycle physiology.

Exam pearl: Positive-pressure ventilation affects haemodynamics, especially in hypovolaemia.

Examiner trap: Spontaneous negative-pressure inspiration and mechanical positive-pressure inspiration have different venous return effects.

Viva extension — Why can induction of anaesthesia precipitate hypotension?
Answer: Vasodilatation and positive-pressure ventilation may lower venous return and cardiac output.

Question 17 of 40 • Coronary circulation

Q17. An anaemic patient has a normal arterial oxygen saturation but reduced oxygen delivery. Which variable is most directly reduced?

A. Arterial partial pressure of oxygen B. Alveolar oxygen partial pressure C. Inspired oxygen fraction D. Pulmonary capillary PO2 gradient E. Arterial oxygen content

View detailed teaching explanation and answered viva

Correct answer: E. Arterial oxygen content.

Why correct: Oxygen delivery (DO2) = CO × CaO2. Since CaO2 is dominated by haemoglobin-bound oxygen, anaemia can lower delivery despite apparently normal SpO2 and PaO2.

Why the other options are incorrect:

A. Arterial partial pressure of oxygen: PaO2 may remain normal in isolated anaemia.

B. Alveolar oxygen partial pressure: Alveolar gas pressure need not change with haemoglobin concentration.

C. Inspired oxygen fraction: This remains set by the inhaled gas.

D. Pulmonary capillary PO2 gradient: Anaemia primarily reduces carrying capacity, not necessarily diffusion driving pressure.

Exam pearl: Normal pulse oximetry does not guarantee normal total oxygen delivery.

Examiner trap: SaO2 measures the percentage of binding sites occupied, not the amount of haemoglobin available.

Viva extension — Give the approximate oxygen-content formula.
Answer: CaO2 (mL/dL) ≈ 1.34 × Hb (g/dL) × SaO2 + 0.003 × PaO2 (mmHg).

Question 18 of 40 • Cardiovascular and renal integration

Q18. After sustained renal hypoperfusion, which hormonal effect is expected from renin–angiotensin–aldosterone activation?

A. Angiotensin II-mediated vasoconstriction with aldosterone-mediated sodium retention B. Reduced renin and immediate natriuresis C. Predominant vasodilatation and sodium loss D. Direct acetylcholine release from adrenal cortex E. Suppression of thirst and ADH release

View detailed teaching explanation and answered viva

Correct answer: A. Angiotensin II-mediated vasoconstriction with aldosterone-mediated sodium retention.

Why correct: Reduced renal perfusion, sympathetic beta-1 activity and lower distal sodium chloride delivery increase renin. Angiotensin II raises vascular tone and stimulates aldosterone; longer-term salt/water retention supports circulating volume.

Why the other options are incorrect:

B. Reduced renin and immediate natriuresis: Renal hypoperfusion stimulates rather than suppresses renin.

C. Predominant vasodilatation and sodium loss: Opposite to the pressure-preserving RAAS response.

D. Direct acetylcholine release from adrenal cortex: Not the relevant hormonal pathway.

E. Suppression of thirst and ADH release: Angiotensin II tends to promote thirst and ADH release.

Exam pearl: Baroreflex: seconds; RAAS: minutes to hours and longer.

Examiner trap: RAAS activity is not synonymous with an immediate rise in circulating volume.

Viva extension — Where does ACE predominantly act?
Answer: On vascular endothelial surfaces, classically including pulmonary vascular endothelium.

Question 19 of 40 • Venous physiology

Q19. After prolonged standing, a patient with venous valvular incompetence develops dependent ankle oedema. What Starling force change primarily favours filtration?

A. Increased plasma oncotic pressure B. Reduced interstitial hydrostatic pressure alone C. Decreased capillary permeability D. Increased capillary hydrostatic pressure E. Increased lymphatic clearance

View detailed teaching explanation and answered viva

Correct answer: D. Increased capillary hydrostatic pressure.

Why correct: Venous hypertension raises postcapillary and capillary hydrostatic pressure, increasing net fluid filtration. Lymphatic compensation may be overwhelmed, resulting in dependent oedema.

Why the other options are incorrect:

A. Increased plasma oncotic pressure: This tends to retain fluid within the circulation.

B. Reduced interstitial hydrostatic pressure alone: Not the primary stated mechanism of dependent venous hypertension.

C. Decreased capillary permeability: Would reduce protein leak and may lower filtration.

E. Increased lymphatic clearance: Would oppose interstitial fluid accumulation.

Exam pearl: Oedema mechanisms include hydrostatic pressure, oncotic pressure, permeability and lymphatic drainage.

Examiner trap: Not every oedema is due to sodium overload or low albumin.

Viva extension — Why does leg elevation help dependent venous oedema?
Answer: It reduces the hydrostatic venous column pressure and assists fluid return.

Question 20 of 40 • Cardiac cycle and pressures

Q20. A patient has aortic stenosis. Doppler shows marked systolic pressure difference between the LV and aorta. Which physiological mechanism causes the gradient?

A. Impaired mitral valve opening in diastole B. Resistance to ejection across the narrowed valve C. Right-to-left atrial shunt D. Pure systemic venous congestion E. Excess pulmonary capillary hydrostatic pressure

View detailed teaching explanation and answered viva

Correct answer: B. Resistance to ejection across the narrowed valve.

Why correct: Flow across a narrowed aortic orifice requires a larger pressure gradient, increasing LV systolic pressure and afterload. The simplified Doppler relation is ΔP≈4v² (mmHg when v is m/s).

Why the other options are incorrect:

A. Impaired mitral valve opening in diastole: Would produce a left atrium–LV filling gradient.

C. Right-to-left atrial shunt: Does not cause the specified transaortic systolic gradient.

D. Pure systemic venous congestion: A consequence of some heart diseases, not the valve-level gradient.

E. Excess pulmonary capillary hydrostatic pressure: Does not independently explain LV–aortic systolic gradient.

Exam pearl: A 4 m/s peak jet corresponds to an approximate 64 mmHg instantaneous gradient.

Examiner trap: Peak instantaneous Doppler gradient differs from peak-to-peak catheter gradient.

Viva extension — What ventricular adaptation occurs chronically?
Answer: Concentric LV hypertrophy initially reduces wall stress.

Question 21 of 40 • Ventilation and lung volumes

Q21. A mechanically ventilated adult has tidal volume 500 mL, respiratory rate 12/min and physiological dead space 150 mL. What is estimated alveolar ventilation?

A. 6 L/min B. 3.0 L/min C. 4.2 L/min D. 1.8 L/min E. 5.0 L/min

View detailed teaching explanation and answered viva

Correct answer: C. 4.2 L/min.

Why correct: Alveolar ventilation = (tidal volume − dead space) × frequency = (500 − 150) × 12 = 4200 mL/min. Only alveolar ventilation participates directly in CO2 exchange.

Why the other options are incorrect:

A. 6 L/min: This is minute ventilation, which includes dead space.

B. 3.0 L/min: Would reflect using too small an alveolar tidal volume.

D. 1.8 L/min: This equals 150 × 12 mL, the dead-space ventilation.

E. 5.0 L/min: Does not follow the stated dead-space subtraction.

Exam pearl: Minute ventilation and alveolar ventilation are not identical.

Examiner trap: Rapid shallow breathing can preserve minute volume while reducing effective alveolar ventilation.

Viva extension — What happens to PaCO2 when alveolar ventilation halves and CO2 production is stable?
Answer: PaCO2 rises approximately two-fold in the steady state.

Question 22 of 40 • Control of ventilation

Q22. A postoperative opioid-treated patient develops hypoventilation with PaCO2 65 mmHg. Which receptors normally provide the main response to the rise in CO2?

A. Carotid sinus baroreceptors B. Pulmonary stretch receptors C. Juxtacapillary (J) receptors D. Muscle spindle afferents E. Central medullary chemoreceptors sensing CSF hydrogen ions

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Correct answer: E. Central medullary chemoreceptors sensing CSF hydrogen ions.

Why correct: CO2 crosses the blood–brain barrier and changes CSF pH; central medullary chemoreceptors respond chiefly to this change. Peripheral chemoreceptors also respond to arterial PCO2 and pH.

Why the other options are incorrect:

A. Carotid sinus baroreceptors: Sense arterial stretch and pressure, not the principal hypercapnic drive.

B. Pulmonary stretch receptors: Participate in breathing reflexes, not the dominant CO2 response.

C. Juxtacapillary (J) receptors: Can trigger rapid shallow breathing in lung congestion.

D. Muscle spindle afferents: Contribute to movement control rather than primary resting CO2 drive.

Exam pearl: Central CO2 drive is indirect through CSF hydrogen-ion concentration.

Examiner trap: Central receptors do not directly sense low arterial PaO2 as their primary stimulus.

Viva extension — What is the key opioid effect?
Answer: Depression of respiratory drive and the ventilatory response to hypercapnia.

Question 23 of 40 • Control of ventilation

Q23. At sea level an otherwise healthy patient ascends to high altitude and PaO2 falls below 60 mmHg. Which receptors are especially important in the acute hypoxic ventilatory response?

A. Carotid bodies B. Carotid sinus baroreceptors C. Central medullary chemoreceptors D. Pulmonary C-fibre stretch sensors E. Renal macula densa cells

View detailed teaching explanation and answered viva

Correct answer: A. Carotid bodies.

Why correct: Peripheral arterial chemoreceptors in carotid bodies respond powerfully as PaO2 drops, especially below roughly 60 mmHg. Aortic bodies also contribute, but carotid bodies are key in humans.

Why the other options are incorrect:

B. Carotid sinus baroreceptors: Detect pressure/stretch rather than low PaO2.

C. Central medullary chemoreceptors: Mainly respond to CO2-associated CSF acidity.

D. Pulmonary C-fibre stretch sensors: Not the principal systemic hypoxic sensor.

E. Renal macula densa cells: Sense tubular sodium chloride and regulate renin.

Exam pearl: Carotid body = chemoreceptor; carotid sinus = baroreceptor.

Examiner trap: Arterial oxygen content and arterial oxygen tension are different physiological quantities.

Viva extension — Why can hyperventilation produce alkalosis at altitude?
Answer: Reduced atmospheric oxygen stimulates ventilation, lowering arterial CO2.

Question 24 of 40 • Ventilation–perfusion

Q24. A postoperative patient has an acute pulmonary embolus that blocks perfusion to ventilated alveoli. Which V/Q state best describes the affected units?

A. V/Q = 0 B. Uniform V/Q = 0.8 in all units C. Reduced anatomical dead space D. Very high V/Q approaching infinity E. Increased perfusion without ventilation

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Correct answer: D. Very high V/Q approaching infinity.

Why correct: Perfusion approaches zero while ventilation persists, so V/Q becomes very high: wasted ventilation or alveolar dead space. Resulting hypoxaemia also reflects redistribution and V/Q mismatch elsewhere.

Why the other options are incorrect:

A. V/Q = 0: Represents perfused but unventilated units, i.e. shunt.

B. Uniform V/Q = 0.8 in all units: Describes approximate whole-lung average, not an occluded vascular segment.

C. Reduced anatomical dead space: Pulmonary embolism raises alveolar dead space.

E. Increased perfusion without ventilation: Opposite to the vascular obstruction described.

Exam pearl: PE increases dead space; complete airway obstruction creates shunt-like units.

Examiner trap: Pulmonary embolism does not automatically cause a measured PaCO2 rise: compensatory hyperventilation often lowers it.

Viva extension — Define anatomical versus alveolar dead space.
Answer: Anatomical is conducting airway gas; alveolar is ventilated alveoli receiving inadequate perfusion.

Question 25 of 40 • Ventilation–perfusion

Q25. Following complete obstruction of a segmental bronchus, perfusion persists but no fresh gas reaches the affected alveoli. What is the limiting V/Q ratio?

A. Infinity B. Zero C. One D. Two E. Negative one

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Correct answer: B. Zero.

Why correct: When ventilation = 0 but perfusion continues, V/Q = 0, representing an intrapulmonary shunt-like state. A true right-to-left shunt has limited response to high inspired oxygen.

Why the other options are incorrect:

A. Infinity: Describes ventilated but unperfused units.

C. One: Indicates matched local ventilation and perfusion.

D. Two: An elevated ratio, opposite to absent ventilation.

E. Negative one: Ventilation and perfusion ratios cannot be negative.

Exam pearl: V/Q = 0 is shunt; V/Q approaching infinity is dead space.

Examiner trap: Low V/Q and true shunt are different; low V/Q usually improves more with oxygen.

Viva extension — What happens to alveolar oxygen in a nonventilated perfused alveolus?
Answer: It approaches mixed venous values as blood continues to equilibrate.

Question 26 of 40 • Alveolar gas equation

Q26. At sea level on room air, PaCO2 is 40 mmHg and respiratory quotient is 0.8. Approximate alveolar PO2 is closest to which value?

A. 40 mmHg B. 60 mmHg C. 100 mmHg D. 150 mmHg E. 210 mmHg

View detailed teaching explanation and answered viva

Correct answer: C. 100 mmHg.

Why correct: PAO2≈FiO2(Patm − 47) − PaCO2/R = 0.21(760−47)−40/0.8≈150−50≈100 mmHg.

Why the other options are incorrect:

A. 40 mmHg: Confuses alveolar oxygen with arterial carbon dioxide.

B. 60 mmHg: Would underestimate inspired oxygen under the stated conditions.

D. 150 mmHg: Approximates humidified inspired PO2 before subtracting CO2/R.

E. 210 mmHg: Uses the FiO2 percentage numerically rather than as a fraction.

Exam pearl: At sea level on room air, PAO2 is around 100 mmHg when PaCO2 is 40.

Examiner trap: Use water vapour pressure 47 mmHg at 37 °C; distinguish it from partial pressure of dry inspired gas.

Viva extension — What is the A–a oxygen gradient?
Answer: Calculated alveolar PO2 minus measured arterial PaO2.

Question 27 of 40 • A–a oxygen gradient

Q27. A young adult is hypoxaemic after opioid overdose with increased PaCO2. Imaging and lungs are otherwise normal. Which mechanism is most likely to produce a near-normal A–a oxygen gradient?

A. Pulmonary embolism B. Right-to-left intracardiac shunt C. Extensive pneumonia D. Interstitial diffusion limitation E. Alveolar hypoventilation

View detailed teaching explanation and answered viva

Correct answer: E. Alveolar hypoventilation.

Why correct: Isolated hypoventilation lowers alveolar and arterial PO2 together because CO2 displaces alveolar oxygen. Therefore the A–a gradient may remain normal (age-dependent).

Why the other options are incorrect:

A. Pulmonary embolism: Creates V/Q disturbance and usually broadens A–a gradient.

B. Right-to-left intracardiac shunt: Widens A–a gradient.

C. Extensive pneumonia: Creates low V/Q and shunt physiology, widening gradient.

D. Interstitial diffusion limitation: Often widens A–a gradient, especially with exercise.

Exam pearl: Hypoventilation and low inspired PO2 classically cause low PaO2 with normal A–a gradient.

Examiner trap: Never interpret A–a gradient without the FiO2 and appropriate alveolar-gas calculation.

Viva extension — Why is the gradient elevated in pulmonary embolism?
Answer: V/Q inequality and associated impaired arterial oxygenation.

Question 28 of 40 • Oxygen dissociation

Q28. A febrile, acidotic septic patient shows lower haemoglobin oxygen affinity. Which combination shifts the oxygen dissociation curve to the right?

A. Increased temperature, increased PCO2 and reduced pH B. Reduced temperature and reduced PCO2 C. Increased pH and reduced 2,3-BPG D. Foetal haemoglobin predominance E. Carbon monoxide binding

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Correct answer: A. Increased temperature, increased PCO2 and reduced pH.

Why correct: The Bohr effect and increased temperature or 2,3-BPG reduce haemoglobin affinity, aiding oxygen unloading in active tissues. A right shift increases P50.

Why the other options are incorrect:

B. Reduced temperature and reduced PCO2: Both generally shift the curve left.

C. Increased pH and reduced 2,3-BPG: Increase affinity and shift left.

D. Foetal haemoglobin predominance: Increases affinity, giving a left-shifted curve.

E. Carbon monoxide binding: Left-shifts remaining Hb binding sites and reduces capacity.

Exam pearl: Right = release; left = load (higher affinity), recognising site-specific effects.

Examiner trap: Do not mistake reduced saturation at a fixed PO2 for reduced total Hb oxygen content in all circumstances.

Viva extension — What happens to P50 after a right shift?
Answer: P50 increases: a higher PO2 is needed for 50% saturation.

Question 29 of 40 • Oxygen carriage

Q29. A poisoned patient has a normal PaO2 but impaired tissue oxygen delivery because haemoglobin binding sites are occupied. What mechanism best matches carbon monoxide exposure?

A. Reduced inspired oxygen fraction as the sole cause B. Increased total haemoglobin concentration C. Primary alveolar diffusion obstruction D. Reduced oxygen-carrying capacity with increased affinity of remaining sites E. Increased physiologic O2 unloading from a right shift

View detailed teaching explanation and answered viva

Correct answer: D. Reduced oxygen-carrying capacity with increased affinity of remaining sites.

Why correct: CO binds Hb with high affinity, reducing sites available for O2 and left-shifting the residual oxygen dissociation curve, impairing unloading. PaO2 reflects dissolved oxygen and may be normal; standard pulse oximetry can mislead.

Why the other options are incorrect:

A. Reduced inspired oxygen fraction as the sole cause: CO exposure may occur at ordinary ambient oxygen tension.

B. Increased total haemoglobin concentration: CO does not generate additional functional Hb.

C. Primary alveolar diffusion obstruction: The principal abnormality is Hb binding and delivery, not alveolar transfer.

E. Increased physiologic O2 unloading from a right shift: CO produces a left shift of remaining functional Hb sites.

Exam pearl: Normal PaO2 does not exclude CO poisoning; use co-oximetry for carboxyhaemoglobin.

Examiner trap: A normal-looking pulse oximeter reading is not a rule-out test for CO poisoning.

Viva extension — Why is high-flow oxygen beneficial?
Answer: It accelerates carboxyhaemoglobin dissociation and enhances dissolved oxygen.

Question 30 of 40 • Spirometry and airflow obstruction

Q30. A smoker has FEV1 1.4 L and FVC 3.0 L, with both manoeuvres technically acceptable. What ventilatory pattern does this ratio suggest?

A. Confirmed restriction B. Obstructive ventilatory impairment C. Normal spirometry D. Pure respiratory muscle weakness E. Increased vital capacity with normal flow

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Correct answer: B. Obstructive ventilatory impairment.

Why correct: FEV1/FVC≈0.47 indicates airflow obstruction. Modern interpretation compares the ratio with the age-adjusted lower limit of normal; a fixed 0.70 cut-off is used in some COPD guidelines but should not be applied indiscriminately to all ages.

Why the other options are incorrect:

A. Confirmed restriction: Requires a low total lung capacity; the ratio here is low.

C. Normal spirometry: A ratio of 1.4/3.0≈0.47 is markedly reduced.

D. Pure respiratory muscle weakness: Often gives proportional falls with relatively preserved ratio.

E. Increased vital capacity with normal flow: Not consistent with measured FEV1 reduction.

Exam pearl: A low FEV1/FVC ratio establishes airflow obstruction when spirometry is reliable.

Examiner trap: FEV1/FVC above 0.70 does not automatically rule out obstruction in every younger patient.

Viva extension — What confirms reversibility?
Answer: Assess bronchodilator responsiveness according to current spirometry standards; clinical interpretation requires history and repeatable measures.

Question 31 of 40 • Spirometry and restriction

Q31. A patient has FEV1 1.8 L and FVC 2.1 L. FEV1/FVC is 0.86 and the FVC is below its lower limit of normal. Which test confirms true restrictive ventilatory impairment?

A. Peak expiratory flow B. Pulse oximetry C. Measurement of total lung capacity D. Arterial blood gas only E. Repeat FEV1 alone

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Correct answer: C. Measurement of total lung capacity.

Why correct: A preserved ratio with low FVC suggests possible restriction, but ATS/ERS interpretation requires low TLC for confirmation. Air trapping, suboptimal effort and non-specific patterns can mimic restriction on spirometry.

Why the other options are incorrect:

A. Peak expiratory flow: Effort-dependent and cannot define reduced TLC.

B. Pulse oximetry: Measures oxygen saturation, not lung volume restriction.

D. Arterial blood gas only: Gives gas exchange and acid-base status, not TLC.

E. Repeat FEV1 alone: Does not distinguish restriction from non-specific patterns or effort limitations.

Exam pearl: Low FVC + normal/high ratio is not synonymous with confirmed restriction.

Examiner trap: A normal ratio does not exclude mixed or complex ventilatory disorders; assess lung volumes when indicated.

Viva extension — Name a true restrictive cause.
Answer: Interstitial fibrosis, chest-wall disease or neuromuscular weakness may lower TLC.

Question 32 of 40 • Compliance and mechanics

Q32. An anaesthetised patient with pulmonary fibrosis is difficult to ventilate and has low static compliance. What change characterises these lungs?

A. Greater volume change per unit transpulmonary pressure B. Complete loss of elastic recoil C. Increased residual volume as the defining finding D. Absent pressure requirement during inspiration E. Greater pressure needed for a given change in lung volume

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Correct answer: E. Greater pressure needed for a given change in lung volume.

Why correct: Compliance = Δvolume/Δpressure. Stiff fibrotic lungs have low compliance and high elastic recoil, so a given tidal volume requires a greater driving pressure.

Why the other options are incorrect:

A. Greater volume change per unit transpulmonary pressure: That describes high compliance.

B. Complete loss of elastic recoil: Fibrotic lungs have increased recoil.

C. Increased residual volume as the defining finding: Fibrosis generally reduces total lung volumes.

D. Absent pressure requirement during inspiration: Opposite to reduced compliance.

Exam pearl: Fibrosis: low compliance, high recoil; emphysema: high compliance, low recoil.

Examiner trap: Airway resistance and lung compliance are distinct components of ventilatory pressure.

Viva extension — How does emphysema differ?
Answer: Loss of alveolar elastic tissue increases compliance but impairs recoil and expiratory airflow.

Question 33 of 40 • Surfactant

Q33. A premature infant develops diffuse atelectasis due to surfactant deficiency. What is the direct mechanical consequence?

A. Increased alveolar surface tension and reduced lung compliance B. Reduced surface tension and greater stability C. Increased pulmonary compliance D. A selective increase in haemoglobin affinity E. Reduced closing pressure in small alveoli

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Correct answer: A. Increased alveolar surface tension and reduced lung compliance.

Why correct: Type II pneumocytes secrete surfactant, reducing surface tension especially in smaller alveoli, increasing compliance and preventing end-expiratory collapse. Laplace relation for a sphere is P≈2T/r.

Why the other options are incorrect:

B. Reduced surface tension and greater stability: Opposite to loss of surfactant.

C. Increased pulmonary compliance: Surfactant deficiency stiffens lungs.

D. A selective increase in haemoglobin affinity: Unrelated to surfactant.

E. Reduced closing pressure in small alveoli: Surface tension tends to increase collapse pressure.

Exam pearl: Surfactant deficiency causes high work of breathing and atelectasis.

Examiner trap: Alveolar pressure–radius arguments must incorporate surfactant and interdependence, not just simple bubbles.

Viva extension — What adult syndrome also has surfactant dysfunction?
Answer: ARDS often involves surfactant impairment and alveolar flooding.

Question 34 of 40 • Lung volumes

Q34. At the end of an ordinary passive expiration, which lung volume remains in the respiratory system?

A. Residual volume B. Vital capacity C. Inspiratory reserve volume D. Functional residual capacity E. Total lung capacity

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Correct answer: D. Functional residual capacity.

Why correct: FRC = residual volume + expiratory reserve volume, the resting volume at end-expiration where inward lung recoil balances outward chest-wall recoil. General anaesthesia and supine positioning reduce FRC.

Why the other options are incorrect:

A. Residual volume: This is air remaining after maximal expiration, not after a normal breath.

B. Vital capacity: Maximum exhalable volume from full inspiration.

C. Inspiratory reserve volume: Additional air inspired above ordinary tidal inspiration.

E. Total lung capacity: Volume at maximal inspiration.

Exam pearl: FRC is the oxygen reservoir during periods of apnoea.

Examiner trap: Normal spirometry cannot directly measure RV or FRC because they contain non-exhalable residual gas.

Viva extension — Why does obesity increase desaturation risk during induction?
Answer: Reduced FRC and higher oxygen consumption shorten the safe apnoea interval.

Question 35 of 40 • Respiratory pressure and airway resistance

Q35. A patient develops bronchospasm during general anaesthesia. Which parameter primarily increases?

A. Static lung compliance must always increase B. Airway resistance C. Alveolar surface area increases D. Pulmonary capillary permeability necessarily falls E. Residual volume becomes zero

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Correct answer: B. Airway resistance.

Why correct: Narrowed airway calibre increases resistance sharply. Under laminar conditions Poiseuille resistance varies approximately inversely with radius to the fourth power; real airways often have turbulent flow.

Why the other options are incorrect:

A. Static lung compliance must always increase: Bronchospasm is primarily a resistance problem.

C. Alveolar surface area increases: Not caused by acute airway narrowing.

D. Pulmonary capillary permeability necessarily falls: No direct relationship.

E. Residual volume becomes zero: Air trapping often increases rather than abolishes residual gas.

Exam pearl: Peak airway pressure can increase from high resistance or low compliance; plateau pressure helps distinguish them.

Examiner trap: A high peak pressure alone does not prove pneumothorax or poor compliance.

Viva extension — How does bronchospasm affect peak–plateau pressure difference?
Answer: Peak rises relatively more than plateau as resistive pressure increases.

Question 36 of 40 • Respiratory acid-base

Q36. Postoperative blood gas: pH 7.27, PaCO2 62 mmHg and bicarbonate 27 mmol/L. What is the primary acid-base disturbance?

A. Metabolic acidosis B. Respiratory alkalosis C. Respiratory acidosis D. Metabolic alkalosis E. Normal acid-base status

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Correct answer: C. Respiratory acidosis.

Why correct: Low pH with elevated PaCO2 identifies primary respiratory acidosis. HCO3 of 27 suggests minimal/acute compensation; consider inadequate ventilation, airway obstruction, opioid effects and respiratory muscle weakness.

Why the other options are incorrect:

A. Metabolic acidosis: Would principally lower bicarbonate, not elevate CO2.

B. Respiratory alkalosis: Would produce low PaCO2 and usually higher pH.

D. Metabolic alkalosis: High bicarbonate should produce alkalemia unless another dominant disorder exists.

E. Normal acid-base status: pH 7.27 is acidemia.

Exam pearl: Read ABGs in order: pH, CO2, bicarbonate, oxygenation, then compensation.

Examiner trap: A mildly high bicarbonate does not automatically make metabolic alkalosis the primary disorder.

Viva extension — What is initial management principle?
Answer: Assess airway, ventilation and the underlying cause; reverse respiratory depressants when indicated and support ventilation appropriately.

Question 37 of 40 • Oxygen–carbon dioxide interaction

Q37. Blood returning from peripheral tissues carries more CO2 after haemoglobin loses oxygen. Which named phenomenon contributes to this?

A. Bohr effect B. Fick diffusion law C. Frank–Starling mechanism D. Boyle law E. Haldane effect

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Correct answer: E. Haldane effect.

Why correct: Deoxygenated haemoglobin buffers H+ and carries more CO2; oxygenation in the lungs facilitates CO2 unloading. This is the Haldane effect.

Why the other options are incorrect:

A. Bohr effect: Describes how H+ and CO2 influence haemoglobin O2 affinity/unloading.

B. Fick diffusion law: Describes diffusion driven by concentration gradient.

C. Frank–Starling mechanism: Concerns myocardial stroke volume response to preload.

D. Boyle law: Relates gas volume inversely to pressure at constant temperature.

Exam pearl: Bohr: CO2/H+ affects O2 carriage; Haldane: O2 loading affects CO2 carriage.

Examiner trap: Bohr and Haldane effects are related but not interchangeable.

Viva extension — Where does the Haldane effect facilitate CO2 release?
Answer: At the pulmonary capillaries as haemoglobin becomes oxygenated.

Question 38 of 40 • Diffusing capacity

Q38. A patient with emphysema has reduced carbon monoxide diffusing capacity (DLCO). What explains this most directly?

A. Loss of functional alveolar–capillary surface area B. Increased alveolar surface area C. Increased total haemoglobin as the sole mechanism D. Isolated bronchospasm as a definition of low DLCO E. Increase in inspired CO2 concentration

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Correct answer: A. Loss of functional alveolar–capillary surface area.

Why correct: Emphysema destroys alveolar septa and capillary beds, reducing effective exchange area and DLCO. Interstitial fibrosis reduces DLCO through membrane thickening and loss of area; pulmonary vascular disease can also reduce it.

Why the other options are incorrect:

B. Increased alveolar surface area: Would favour rather than reduce diffusion.

C. Increased total haemoglobin as the sole mechanism: Higher Hb may increase measured DLCO after appropriate correction.

D. Isolated bronchospasm as a definition of low DLCO: Simple asthma may have normal or high DLCO.

E. Increase in inspired CO2 concentration: DLCO uses tracer CO and reflects diffusion/capillary blood volume, not simply inspired CO2.

Exam pearl: DLCO is often low in emphysema and interstitial lung disease, but may be normal/high in asthma.

Examiner trap: Always consider haemoglobin correction when interpreting DLCO.

Viva extension — What other disease lowers DLCO despite preserved lung volumes?
Answer: Pulmonary vascular disease can reduce diffusing capacity.

Question 39 of 40 • Perioperative respiration

Q39. A patient becomes hypoxaemic after abdominal surgery from dependent atelectasis. Which normal perioperative change promotes this?

A. Large increase in FRC from supine positioning B. Elimination of physiological shunt C. Marked increase in surfactant delivery to all collapsed units D. Reduced functional residual capacity after anaesthesia E. Complete loss of lung perfusion

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Correct answer: D. Reduced functional residual capacity after anaesthesia.

Why correct: Induction, supine posture, muscle relaxation and obesity can reduce FRC below closing capacity, encouraging airway closure and absorption atelectasis. Strategies depend on clinical context and include lung recruitment, appropriate PEEP, analgesia and mobilisation.

Why the other options are incorrect:

A. Large increase in FRC from supine positioning: Supine position and anaesthesia commonly lower FRC.

B. Elimination of physiological shunt: Atelectasis creates shunt-like units.

C. Marked increase in surfactant delivery to all collapsed units: Not an assured response to anaesthesia.

E. Complete loss of lung perfusion: Atelectatic regions may remain perfused.

Exam pearl: Low FRC predisposes to airway closure and postoperative hypoxaemia.

Examiner trap: Excessive PEEP can worsen haemodynamics; individualise rather than maximising indiscriminately.

Viva extension — Why does deep breathing help after surgery?
Answer: It improves alveolar recruitment and helps restore end-expiratory lung volume.

Question 40 of 40 • Pulmonary vascular physiology

Q40. In an alveolar region with low oxygen tension, local pulmonary arterioles constrict. What is the physiological purpose?

A. Reduce systemic arterial pressure directly B. Redirect blood flow toward better ventilated lung regions C. Increase blood flow to nonventilated alveoli D. Increase surfactant secretion directly E. Abolish pulmonary vascular resistance

View detailed teaching explanation and answered viva

Correct answer: B. Redirect blood flow toward better ventilated lung regions.

Why correct: Hypoxic pulmonary vasoconstriction diverts perfusion away from poorly ventilated alveoli and can improve matching. If hypoxia is diffuse, widespread vasoconstriction raises pulmonary vascular resistance.

Why the other options are incorrect:

A. Reduce systemic arterial pressure directly: This is a local pulmonary response, not a systemic BP reflex.

C. Increase blood flow to nonventilated alveoli: Would worsen V/Q mismatch.

D. Increase surfactant secretion directly: Not the main vascular mechanism.

E. Abolish pulmonary vascular resistance: Local constriction increases resistance in the hypoxic area.

Exam pearl: Pulmonary vasculature constricts in hypoxia, unlike many systemic vascular beds.

Examiner trap: Local HPV is useful, but global HPV contributes to pulmonary hypertension.

Viva extension — What can abolish useful HPV during anaesthesia?
Answer: Some anaesthetic agents and vasodilators can attenuate the response.

Frequently asked questions

Is this a complete MRCS physiology syllabus?

It is a high-yield cardiovascular and respiratory foundation. Renal, endocrine, gastrointestinal and neurophysiology require additional dedicated lessons.

Are these 40 questions official MRCS exam recalls?

No. They are original educational SBAs informed by published physiology principles and recurring recall topics; they should not be treated as authentic exam questions.

How many physiology questions appear in MRCS Part A?

A historical official content guide gave an indicative allocation for physiology within Paper 1; actual examination allocations may vary. Always consult current Intercollegiate MRCS candidate guidance.

Does a low FVC prove restrictive lung disease?

No. Formal restriction requires low total lung capacity, interpreted relative to appropriate reference limits.

Will my answers remain when I return?

Yes, if this Blogger page retains its JavaScript and your browser permits localStorage on the same site. It will not automatically sync to other devices.

What should I study after this article?

Continue with renal, acid-base, gastrointestinal and endocrine physiology, while revising wrong answers from this cardiovascular and respiratory quiz.

Sources, previous lessons and next steps

Teaching references and exam guidance. The recall inventory was used only to identify areas learners find testable; source-marked recall answers were not assumed to be verified.

1. Intercollegiate MRCS examinations — candidate guidance — Official examination guidance and syllabus entry point
2. Royal College of Surgeons of England — MRCS Part A — Part A examination structure
3. Control of Cardiac Output — NCBI Bookshelf — Integrated cardiac output, preload and venous return
4. Cardiovascular Physiology — NCBI Bookshelf — Cardiac haemodynamics and autonomic reflexes
5. Arterial Pressure Regulation — NCBI Bookshelf — Baroreceptor reflex
6. Lung Dead Space — NCBI Bookshelf — Dead space and alveolar ventilation
7. Alveolar Gas Equation — NCBI Bookshelf — Alveolar oxygen calculations and interpretation
8. ERS/ATS Interpretive Strategies for Lung Function Tests — Current spirometry and TLC-defined restriction

Continue through MRCS Academy

Article 02
Complete MRCS Syllabus Article 04
12-Week Study Plan Article 05
How to Solve SBAs Article 07
Surgical Anatomy Overview Article 08
Upper Limb Anatomy Article 09
Inguinal Canal Anatomy Article 10
Head & Neck Anatomy

Next planned article: Article 12 — Renal, Acid–Base and Electrolyte Physiology for MRCS Part A.

Publish Article 11, share its exact permalink, then add it to the Academy index and Article 10 next-lesson navigation. Do not guess a Blogger permalink before it exists.

Editorial note: reviewed against the cited educational references in October 2026. The article is for examination revision and does not replace local clinical protocols or supervised medical decisions.