Physiology — Renal, Fluid & Acid-Base — USMLE Step 1 Notes
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Glomerular filtration & clearance
- GFR is set by the balance of Starling forces across the glomerulus. Afferent constriction lowers it; efferent constriction (angiotensin II) raises filtration fraction.
- Clearance = (urine conc × urine flow) / plasma conc. Inulin clearance = GFR; PAH clearance estimates renal plasma flow.
- Filtration fraction = GFR / renal plasma flow (~20%).
Tubular handling, segment by segment
| Segment | Key job |
|---|---|
| Proximal tubule | Bulk reabsorption: ~65% of Na and water, glucose, amino acids, bicarbonate (via carbonic anhydrase) |
| Thick ascending limb | Na-K-2Cl transporter (loop diuretic target); impermeable to water — dilutes filtrate, builds medullary gradient |
| Distal convoluted tubule | Na-Cl transporter (thiazide target); PTH-driven calcium reabsorption |
| Collecting duct | Aldosterone (Na reabsorption, K/H secretion) and ADH (water reabsorption via aquaporins) |
Concentrating the urine
- The countercurrent multiplier of the loop of Henle builds a hypertonic medullary interstitium; the collecting duct then reabsorbs water down that gradient only when ADH is present.
- No ADH → dilute urine (diabetes insipidus); high ADH → concentrated urine (SIADH).
Volume & electrolyte control
- RAAS: low renal perfusion → renin → angiotensin II (vasoconstriction, efferent tone, thirst) and aldosterone (Na retention, K loss).
- ANP/BNP oppose RAAS: released on atrial stretch, they promote natriuresis and vasodilation.
- Potassium shifts INTO cells with insulin, beta-agonists and alkalosis; OUT with acidosis, cell lysis and insulin lack.
Acid-base balance
- The kidney excretes fixed acid and regenerates bicarbonate; the lung handles volatile acid (CO2). Henderson-Hasselbalch: pH depends on the HCO3/CO2 ratio.
- Metabolic acidosis: low HCO3; split by anion gap (gap = added acid — MUDPILES; normal gap = HCO3 loss — diarrhoea, RTA).
- Respiratory acid-base is driven by CO2 (hypoventilation = acidosis, hyperventilation = alkalosis). Expect appropriate compensation; over-correction implies a second disorder.
- Winter's formula predicts the expected PaCO2 for a metabolic acidosis.
Body fluid compartments
- Total body water ~60% of weight: two-thirds intracellular, one-third extracellular (of which a quarter is plasma).
- Osmolality is tightly defended by ADH and thirst; sodium is the main determinant of extracellular volume.
Written by StepSherpa in our own words, grounded in Human Physiology (Wikibooks), used under CC BY-SA 3.0.
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