NRE Step 1 Physiology High-Yield Topics 2026 — Organ-System Topics That Repeat Every Attempt
The cardiovascular, renal, respiratory, and neurophysiology concepts that appear in every NRE session — understood as clinical tools, not textbook theory.
A 70-year-old man with chronic heart failure is started on a new medication. Within one week his blood pressure has improved but his potassium has dropped to 3.0 mmol/L and his serum creatinine has risen by 30%. The physician is alarmed by the creatinine rise but the cardiologist says it is expected and acceptable. Who is correct, and what physiological mechanism explains both the creatinine rise and why it is not necessarily harmful here?
This is how physiology appears in NRE Step 1. The question is clinical. The answer requires understanding a mechanism — in this case, how ACE inhibitors alter glomerular filtration by relaxing the efferent arteriole, reducing intraglomerular pressure, and causing a modest creatinine rise that is expected and renoprotective in the long term. The graduate who memorised “ACE inhibitors can cause renal impairment — stop if creatinine rises” fails. The one who understood the physiology scores.
Physiology in NRE is the engine behind clinical decisions. Every drug mechanism, every disease complication, every investigation interpretation rests on physiological principles. This guide covers the organ-system physiology topics that appear most consistently in NRE Step 1 — taught at the level of clinical application, not academic theory.
1. Cardiovascular Physiology
Cardiovascular physiology is the most heavily tested physiological system in NRE Step 1, directly mirroring the dominance of cardiology in the clinical sciences section. The concepts below appear either as standalone physiology questions or embedded in cardiology clinical MCQs.
Cardiac Output & Its Determinants
Stroke Volume is determined by three factors:
Preload (ventricular filling — Frank-Starling mechanism) | Afterload (resistance against which the heart ejects) | Contractility (intrinsic myocardial force independent of preload/afterload)
NRE tests this by asking what happens to CO, SV, or HR when a drug or disease alters one variable. Example: aortic stenosis → increased afterload → reduced SV → reduced CO → compensatory HR rise.
| Condition / Drug | Effect on Preload | Effect on Afterload | Effect on Contractility | Net Effect on CO |
|---|---|---|---|---|
| Heart failure (systolic) | ↑ (venous congestion) | ↑ (compensatory vasoconstriction) | ↓ (primary defect) | ↓ |
| ACE inhibitors | ↓ (reduced aldosterone → less Na/water retention) | ↓ (vasodilation) | No direct effect | ↑ (in HF) |
| GTN (nitrates) | ↓↓ (venodilation — primary effect) | ↓ (mild) | No direct effect | Reduces myocardial O₂ demand → relieves angina |
| Beta-blockers | Slight ↑ | ↓ | ↓ | ↓ HR and contractility → reduced O₂ demand |
| Haemorrhage (acute) | ↓↓ | ↑ (sympathetic vasoconstriction) | ↑ (sympathetic) | ↓ initially; compensated by ↑ HR |
| Exercise | ↑ (muscle pump increases venous return) | ↓ (vasodilation in muscle) | ↑ (sympathetic) | ↑↑ (can reach 20–25 L/min) |
The Frank-Starling Mechanism
Understanding the Frank-Starling curve is essential for NRE heart failure questions. The principle: within physiological limits, increased ventricular filling (preload) increases stroke volume — the heart contracts more forcefully when it is stretched more. In heart failure, the curve shifts downward and to the right — the heart needs more filling to generate the same output, and at high preloads, it fails to increase output further.
Curve shifts RIGHT and DOWN (depressed function): heart failure, myocardial ischaemia, negative inotropes (beta-blockers in excess, calcium channel blockers), acidosis, hypoxia.
NRE presents a scenario (patient given dobutamine in cardiogenic shock) and asks which direction the Starling curve shifts and what happens to SV.
Blood Pressure Regulation — The RAAS
The renin-angiotensin-aldosterone system is one of the most tested physiological axes in NRE — it connects renal physiology, cardiovascular physiology, and pharmacology into a single integrated question.
| Step | Stimulus | Action | Clinical Drug Target |
|---|---|---|---|
| Renin release (JGA) | Low renal perfusion, low Na delivery to macula densa, sympathetic stimulation (β1) | Converts angiotensinogen → angiotensin I | Beta-blockers ↓ renin; direct renin inhibitors (aliskiren) |
| ACE (lung) | Angiotensin I present | Converts angiotensin I → angiotensin II. Also degrades bradykinin (→ ACE inhibitor cough) | ACE inhibitors (enalapril, ramipril) — block here; also ↑ bradykinin → dry cough |
| Angiotensin II | Produced by ACE | Vasoconstriction (AT1 receptor); stimulates aldosterone release; stimulates ADH; promotes Na reabsorption in PCT | ARBs (losartan) block AT1 receptor — NO cough (bradykinin not affected) |
| Aldosterone (adrenal cortex) | Angiotensin II, hyperkalaemia | Na reabsorption + K/H excretion in collecting duct → ↑ blood volume → ↑ BP | Spironolactone (aldosterone antagonist) → ↑ K, ↓ BP; used in HF and Conn’s |
ECG Physiology — What Each Wave Represents
| ECG Component | Physiological Event | Abnormality & Cause |
|---|---|---|
| P wave | Atrial depolarisation | Absent P waves → AF; peaked P waves → right atrial enlargement (P pulmonale); bifid P waves → left atrial enlargement (P mitrale) |
| PR interval (0.12–0.20s) | AV node conduction delay | Prolonged → 1st degree heart block; progressively prolonged then dropped → Wenckebach; fixed prolonged with dropped beats → Mobitz II; no relationship P to QRS → complete heart block |
| QRS complex (<0.12s) | Ventricular depolarisation | Wide QRS → bundle branch block, hyperkalaemia, VT, accessory pathway (WPW); absent Q waves in leads where expected → posterior MI |
| ST segment | Plateau phase of ventricular AP | Elevated → STEMI, pericarditis (saddle-shaped, diffuse), Prinzmetal angina; Depressed → NSTEMI, digoxin effect (reverse tick), subendocardial ischaemia |
| T wave | Ventricular repolarisation | Peaked T waves → hyperkalaemia (early sign); inverted T waves → ischaemia, PE (right heart strain), LBBB, Wellens syndrome (LAD critical stenosis) |
| QT interval (corrected) | Total ventricular electrical activity | Prolonged QTc → risk of Torsades de Pointes. Causes: hypokalaemia, hypomagnesaemia, hypocalcaemia, drugs (amiodarone, haloperidol, erythromycin, methadone), congenital LQTS |
2. Renal Physiology
Renal physiology appears in NRE both as direct questions and embedded within medicine questions on AKI, CKD, electrolyte abnormalities, and acid-base disturbances. The two highest-yield areas are tubular function and electrolyte regulation.
Tubular Reabsorption — Where and What
| Nephron Segment | What Is Reabsorbed | Hormone / Drug Effect | Clinical Correlation |
|---|---|---|---|
| Proximal Convoluted Tubule (PCT) | 67% Na, water, glucose (100%), amino acids, HCO₃⁻, phosphate, uric acid | Angiotensin II → ↑ Na reabsorption. PTH → ↓ phosphate reabsorption. Acetazolamide → ↓ HCO₃⁻ reabsorption | Fanconi syndrome: generalised PCT dysfunction → glucosuria (normal glucose), aminoaciduria, phosphaturia, bicarbonaturia. Causes: Wilson’s disease, multiple myeloma, tenofovir. |
| Loop of Henle (thick ascending) | Na, K, Cl (NKCC2 cotransporter); water impermeable here → concentrating gradient created | Loop diuretics (furosemide) block NKCC2 → Na, K, Cl lost → dilute urine; hypokalaemia, hyponatraemia, metabolic alkalosis | Loss of concentrating gradient in loop diuretics → can’t concentrate urine → treats oedema and hypertension. |
| Distal Convoluted Tubule (DCT) | Na, Cl (NCC cotransporter); Ca reabsorption (PTH-stimulated) | Thiazide diuretics block NCC → Na loss, but Ca reabsorption ↑ (→ useful in hypercalciuria/kidney stones). PTH → ↑ Ca reabsorption. | Thiazides: hypokalaemia, hyponatraemia, hyperglycaemia, hyperuricaemia (gout), hypercalcaemia. Gitelman syndrome: NCC mutation → mimics thiazide effect (hypomagnesaemia + hypocalciuria). |
| Collecting Duct | Na (principal cells — ENaC); K and H excretion. Water reabsorption via AQP2 (ADH-dependent) | Aldosterone → ↑ Na reabsorption + ↑ K/H excretion. ADH → inserts AQP2 → ↑ water reabsorption → concentrated urine. Spironolactone blocks aldosterone receptor. | SIADH: excessive ADH → concentrated urine + dilute plasma → hyponatraemia. Diabetes insipidus: ADH absent (central) or unresponsive (nephrogenic) → dilute urine + hypernatraemia. |
Electrolyte Disturbances — Clinical Physiology
Hypokalaemia (K⁺ <3.5)
ECG: U waves, flattened T waves, prolonged QT → Torsades. Causes: loop/thiazide diuretics, vomiting (alkalosis drives K into cells + renal loss), diarrhoea, Conn’s, Cushing’s. Treatment: oral/IV KCl; treat underlying cause. Never give IV K rapidly — cardiac arrest risk.
Hyperkalaemia (K⁺ >5.5)
ECG: peaked T waves → wide QRS → sine wave → VF. Causes: AKI, CKD, ACE inhibitors/ARBs, spironolactone, DKA (K shifts out of cells), Addison’s. Treatment: calcium gluconate (membrane stabilisation — FIRST), then insulin + dextrose (shifts K in), salbutamol nebuliser, sodium bicarbonate (if acidotic), dialysis (definitive).
Hyponatraemia (Na⁺ <135)
Most common electrolyte disorder. Classify by volume status: hypovolaemic (vomiting, diarrhoea), euvolaemic (SIADH, hypothyroidism, Addison’s), hypervolaemic (heart failure, cirrhosis, nephrotic syndrome). SIADH: urine osmolality >100, urine Na >20 (inappropriately concentrated). Treatment: fluid restrict for SIADH; correct slowly — <10 mmol/L per 24 hrs to avoid osmotic demyelination.
Hypercalcaemia (Ca²⁺ >2.6)
“Bones, stones, groans, psychic moans.” Causes: malignancy (#1 in hospitalised), hyperparathyroidism (#1 outpatient), sarcoidosis, Vit D toxicity, thiazides, milk-alkali syndrome. ECG: shortened QT interval. Treatment: IV normal saline (rehydration first), then IV bisphosphonates (for malignancy-related); treat primary cause.
GFR & Renal Function — Clinically Applied
- GFR normal: 90–120 mL/min/1.73m². CKD defined as GFR <60 for >3 months.
- Creatinine vs GFR: Creatinine is inversely related to GFR — a doubling of creatinine means approximately halving of GFR. But creatinine rises only after ~50% of nephron function is lost.
- ACE inhibitors in CKD: Dilate efferent arteriole → reduce intraglomerular pressure → modest creatinine rise (up to 30% acceptable and expected) → renoprotective long-term. Stop if creatinine rises >30% or K⁺ >6.0.
- Prerenal vs intrinsic AKI: Prerenal — urine Na <20, FeNa <1%, concentrated urine (SG >1.020), urine:plasma creatinine >40:1. Intrinsic (ATN) — urine Na >40, FeNa >2%, isosthenuric urine, muddy brown casts.
3. Respiratory Physiology
Spirometry — Pattern Recognition
| Parameter | Obstructive Pattern (Asthma, COPD) | Restrictive Pattern (Fibrosis, Neuromuscular) |
|---|---|---|
| FEV₁ | ↓ | ↓ |
| FVC | Normal or ↓ | ↓↓ |
| FEV₁/FVC ratio | <0.7 (defining criterion) | Normal or >0.7 (both reduced proportionally) |
| TLC | ↑ (air trapping) | ↓ |
| DLCO (gas transfer) | ↓ in emphysema (alveolar destruction); Normal in asthma/chronic bronchitis | ↓ in fibrosis (thickened membrane); Normal in neuromuscular causes |
| Bronchodilator response | Significant improvement (≥12%, ≥200mL FEV₁) → asthma; Partial → COPD | No change |
Oxygen-Haemoglobin Dissociation Curve
The O₂-Hb dissociation curve is tested both directly (what shifts it) and clinically (why a COPD patient with SpO₂ 88% may still have adequate tissue oxygen delivery).
| Curve Shifts RIGHT (↓ Hb-O₂ affinity — releases O₂ to tissues) | Curve Shifts LEFT (↑ Hb-O₂ affinity — holds O₂) |
|---|---|
| ↑ Temperature | ↑ CO₂ (Bohr effect) | ↑ H⁺ (acidosis) | ↑ 2,3-DPG (exercise, high altitude, anaemia) | Sickle haemoglobin | ↓ Temperature | ↓ CO₂ | ↓ H⁺ (alkalosis) | ↓ 2,3-DPG | Fetal haemoglobin (HbF — higher O₂ affinity than HbA) | CO poisoning | Methaemoglobinaemia |
Ventilation-Perfusion (V/Q) Relationships
Dead Space (V/Q = ∞)
Ventilated but not perfused. Physiological dead space increased in PE (alveoli ventilated but pulmonary vessels occluded). Patient hyperventilates to compensate → respiratory alkalosis. PaO₂ may be low, PaCO₂ low initially.
Shunt (V/Q = 0)
Perfused but not ventilated. Examples: pneumonia, atelectasis, ARDS, intracardiac shunt (VSD, ASD). Key feature: hypoxia that does NOT correct with 100% O₂ (shunted blood bypasses alveoli entirely). Differentiates shunt from simple V/Q mismatch.
Lung Zones (West Zones)
In upright position: Zone 1 (apex) — dead space dominates; Zone 3 (base) — perfusion dominates. TB reactivation at apex (high O₂ tension). Pulmonary oedema at bases (gravity-dependent). NRE tests why TB reactivation prefers the upper lobes.
Four Types of Hypoxia
Hypoxic (↓ PaO₂ — altitude, V/Q mismatch, hypoventilation). Anaemic (normal PaO₂, ↓ Hb — anaemia, CO poisoning). Stagnant/ischaemic (↓ blood flow — shock, HF). Histotoxic (normal O₂ delivery, cells can’t use O₂ — cyanide poisoning). NRE tests this classification to distinguish management.
4. Neurophysiology — High-Yield Concepts
Action Potential — Clinical Drug Targets
NRE does not test the action potential as a graph-drawing exercise. It tests which drugs act on which phase and what clinical effect results.
| Channel / Phase | Drug Acting Here | Clinical Effect |
|---|---|---|
| Na⁺ channel (Phase 0 depolarisation) | Local anaesthetics (lidocaine), Class I antiarrhythmics (quinidine, flecainide), phenytoin | Block Na⁺ entry → slow conduction → antiarrhythmic effect or anaesthesia |
| Ca²⁺ channel (Phase 2 plateau) | CCBs (amlodipine, verapamil, diltiazem) | Verapamil/diltiazem → slow HR (nodal) + vasodilation. Amlodipine → vasodilation predominantly (peripheral) |
| K⁺ channel (Phase 3 repolarisation) | Class III antiarrhythmics (amiodarone, sotalol) | Prolong repolarisation → prolong QT → risk of Torsades (sotalol > amiodarone) |
| Na⁺/K⁺ ATPase pump | Digoxin (inhibits) | ↑ intracellular Ca²⁺ → positive inotropy. Also prolongs AV node refractory period → rate control in AF |
Autonomic Nervous System — Receptor Physiology for NRE
| Receptor | Location | Stimulation Effect | Drug Examples |
|---|---|---|---|
| α1 | Vascular smooth muscle, iris dilator, bladder sphincter | Vasoconstriction, mydriasis, urinary retention | Phenylephrine (α1 agonist); prazosin (α1 blocker → used in BPH and hypertension) |
| α2 | Presynaptic (inhibitory), platelets, fat cells | Inhibits noradrenaline release; platelet aggregation | Clonidine (α2 agonist → ↓ sympathetic outflow → ↓ BP); methyldopa (α2 agonist — safe in pregnancy) |
| β1 | Heart (SA node, AV node, myocardium), kidney (JGA) | ↑ HR, ↑ contractility, ↑ renin release | Beta-blockers reduce all three; dobutamine (β1 agonist → used in cardiogenic shock) |
| β2 | Bronchial smooth muscle, uterus, vascular smooth muscle, liver | Bronchodilation, uterine relaxation, vasodilation, glycogenolysis | Salbutamol (β2 agonist → asthma, tocolysis); non-selective beta-blockers block β2 → bronchospasm in asthma |
| M2 (muscarinic) | Heart | ↓ HR (vagal slowing), ↓ AV conduction | Atropine blocks M2 → ↑ HR (used in bradycardia/asystole) |
| M3 (muscarinic) | Smooth muscle (gut, bladder, bronchi), glands | ↑ secretions, ↑ GI motility, bronchoconstriction, bladder contraction, miosis | Ipratropium blocks M3 in airway → bronchodilation; organophosphate poisoning overstimulates M3 → SLUDGE |
5. Endocrine Physiology — Hormone Axes
The Pituitary Axes — Feedback Logic
NRE endocrine questions follow feedback logic: is the problem primary (gland itself), secondary (pituitary), or tertiary (hypothalamus)? Knowing which hormone goes up and which goes down in each scenario is the key skill.
| Axis | Primary Failure (Gland) | Secondary Failure (Pituitary) | NRE-Tested Drug/Condition |
|---|---|---|---|
| Thyroid | Primary hypothyroidism: ↑ TSH, ↓ FT4 (TSH high because negative feedback lost) | Secondary hypothyroidism: ↓ TSH, ↓ FT4 (pituitary not driving thyroid) | Amiodarone: contains iodine → causes hypo- OR hyperthyroidism. Lithium → hypothyroidism. Carbimazole → agranulocytosis (check FBC). |
| Adrenal | Primary (Addison’s): ↑ ACTH, ↓ cortisol. Hyperpigmentation (↑ ACTH → ↑ MSH → melanin). Electrolytes: ↓ Na, ↑ K. | Secondary: ↓ ACTH, ↓ cortisol. No hyperpigmentation. Normal electrolytes (mineralocorticoids intact). | Exogenous steroid withdrawal → secondary adrenal insufficiency (pituitary suppressed). Always taper steroids slowly. |
| Gonads | Primary hypogonadism (Klinefelter, Turner): ↑ LH/FSH, ↓ testosterone/oestrogen | Secondary (hypopituitarism, Kallmann): ↓ LH/FSH, ↓ sex hormones. Kallmann: anosmia + hypogonadism (GnRH neuron migration failure). | OCP suppresses LH/FSH → prevents ovulation (secondary mechanism). Post-pill amenorrhoea: hypothalamic axis suppression. |
| ADH (Vasopressin) | SIADH: excess ADH from ectopic source (small cell lung cancer, head injury, meningitis, drugs). ↑ urine osmolality, ↓ serum osmolality, ↓ Na. Urine Na >20. | Central DI: ↓ ADH from posterior pituitary (head trauma, surgery). ↓ urine osmolality, ↑ serum osmolality, ↑ Na. Treat: desmopressin. | Nephrogenic DI: ADH present but collecting duct unresponsive (lithium toxicity, hypercalcaemia, hypokalaemia). Treat: thiazide + low-Na diet (paradoxical — reduces urine output). |
6. Normal Physiological Values — What NRE Uses
These are the reference values that NRE MCQs use. Knowing the normal range allows you to interpret a given value as abnormal and reason toward the diagnosis.
Frequently Asked Questions
Is physiology tested separately from clinical medicine in NRE Step 1?
Do I need to know ECG interpretation for NRE Step 1 or only Step 2?
The RAAS is very complex — how deeply does NRE test it?
How are physiology concepts from Chinese MBBS different from what NRE tests?
Want Physiology Taught the Way NRE Actually Tests It?
Our live NRE Step 1 sessions teach every physiological concept through clinical scenarios — RAAS, Starling curves, spirometry, and ECG interpretation all covered in the exam format. Message us for the next session schedule.
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