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RapidMRCPMRCS

Renal physiology, acid-base and electrolytes

Essential points for quick revision.

2 min synopsisUK scopeSources checked 7 Sept 2026Clinical review pending

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Synopsis

Apply filtration, tubular transport, water balance and acid-base physiology to interpret clearance, sodium-water disorders, potassium handling and mixed biochemical patterns.

Glomerular filtration depends on hydraulic conductivity, surface area and net filtration pressure; autoregulation buffers renal blood flow and GFR across a range rather than fixing them absolutely.
Clearance is the virtual plasma volume cleared of a substance per time. A freely filtered marker with no net tubular handling, renal production or metabolism approximates GFR; for such a solute, net secretion raises and net reabsorption lowers clearance relative to GFR.
The proximal tubule performs bulk iso-osmotic reabsorption; loop segments build the medullary gradient, distal nephron hormones adjust sodium, potassium, hydrogen and water handling.

Reasoning priorities

Clearance calculation

Relate urine concentration and flow to plasma concentration.

Use C equals U times V divided by P with consistent units; incomplete collection and non-steady state limit inference.

Worked reasoning

Worked exampleCalculate renal clearance

A steady-state model has urine solute concentration 60 mg/dL, urine flow 2 mL/min and plasma solute concentration 0.8 mg/dL. The solute is freely filtered and neither synthesised nor metabolised within the kidney.

Write the clearance relationship as urine concentration multiplied by urine flow, divided by plasma concentration.
Multiply 60 mg/dL by 2 mL/min to obtain the urinary excretion term before division; compatible concentration units will cancel.
Divide 120 by 0.8 to obtain a final clearance of 150 mL/min.
If true GFR is specified as 100 mL/min, infer net secretion because clearance exceeds filtration alone.
Verify dimensional cancellation and confirm that steady state and accurate urine flow were explicit assumptions.

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Sources and review status7 sources · checked 7 Sept 2026 · clinical review pending

Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 7 Sept 2026; clinical approval remains outstanding.

GMC MLA content map effective September 2026Current official assessment map includes renal, electrolyte and acid-base mechanisms and their clinical interpretation.
Intercollegiate MRCS candidate guide and syllabusOfficial applied physiology syllabus covers renal function, fluids, electrolytes and acid-base balance.
Federation MRCP Part 1 format and blueprintCurrent official blueprint includes renal medicine and clinical science.
UK Kidney Association hyperkalaemia guidelineCurrent primary UK professional guidance connecting potassium physiology, verification, ECG assessment and safe response.
NCBI Bookshelf Physiology, Glomerular Filtration RateSpecific clearance section defines virtual plasma volume and gives U times V divided by P.
NCBI Bookshelf Physiology, Acid Base BalanceSpecific interpretation section states that PaCO2 outside expected compensation indicates a mixed disorder.
ESICM, ESE and ERA–EDTA clinical practice guideline on hyponatraemia (2014)Diagnostic physiology and section 6.3 explain suppression of vasopressin and dilute urine in response to hypotonicity. Used here for water-balance interpretation, not to supply an acute treatment regimen.

Authoring stateRapid draftClinical stateAwaiting reviewJurisdictionUnited Kingdom