Physiology
Core concepts
Focus on renal physiology as an integrative system: glomerular filtration (Starling forces, filtration fraction), tubular handling of sodium and water, countercurrent multiplication and the loop of Henle, and hormonal control via renin-angiotensin-aldosterone, ADH, and ANP. Understand acid-base handling (bicarbonate reabsorption, titratable acid, ammonia) and potassium distribution between intracellular and extracellular compartments. Bladder physiology-detrusor compliance, the micturition reflex, and sphincter coordination-bridges to voiding dysfunction.
Traps
Questions often test the difference between clearance and excretion, or ask which nephron segment handles a solute (e.g., the thick ascending limb's Na-K-2Cl cotransporter and its role in the diluting segment). Watch for confusion between inulin clearance (GFR) and PAH clearance (effective renal plasma flow). Acid-base stems frequently hinge on the anion gap and the expected respiratory compensation. Another trap: confusing the filtration fraction change in efferent vs afferent arteriolar constriction.
Rule of thumb
For any acid-base disturbance, calculate the anion gap and the expected compensation before choosing an answer; if compensation is off, a mixed disorder is present. For renal hemodynamics, remember: efferent constriction raises filtration fraction; afferent constriction lowers it.
Worked example
A patient with vomiting has a pH of 7.50, PCO2 48, HCO3 36. The elevated bicarbonate with compensatory hypoventilation fits metabolic alkalosis; check chloride (vomiting causes chloride-responsive alkalosis).