Physiology · Short notes

Renal Physiology: Short Notes & Cheat Sheet

Glomerular filtration, what each nephron segment does, clearance measurements, hormonal control of water and salt, and the kidney's endocrine roles.

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Short notes

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What happens along a nephron

1GlomerulusFilters ≈ 125 mL/min2Proximal tubule≈ 65% reabsorbed, all glucose3Loop of HenleConcentrates the medulla4Distal tubuleNaCl, calcium (PTH)5Collecting ductAldosterone and ADH fine-tune6Urine≈ 1–1.5 L/day
About 180 L is filtered each day, yet only about 1.5 L leaves as urine: almost everything is reabsorbed.

Filtration

  • The kidneys receive about 20–25% of cardiac output (about 1.2 L/min of blood; about 625 mL/min of plasma).
  • GFR is about 125 mL/min, or about 180 L/day, of which only 1–1.5 L becomes urine.
  • Filtration fraction = GFR ÷ renal plasma flow, about 20%.
  • GFR is kept steady by autoregulation: the myogenic response of arterioles and tubuloglomerular feedback via the macula densa.
  • Inulin clearance measures GFR exactly; creatinine clearance slightly overestimates it because of some secretion. PAH clearance measures renal plasma flow.

Along the nephron

  • Proximal tubule: reabsorbs about 65% of sodium and water and nearly all glucose, amino acids and bicarbonate.
  • Glucose appears in urine once plasma glucose exceeds the renal threshold (about 180 mg/dL).
  • Loop of Henle: the descending limb lets water out; the thick ascending limb pumps out salt (NKCC2) but is impermeable to water. This countercurrent multiplier concentrates the medulla.
  • Distal tubule: reabsorbs sodium chloride (NCC) and, under PTH, calcium.
  • Collecting duct: aldosterone increases sodium reabsorption and potassium secretion; ADH inserts aquaporin-2 channels so water is reabsorbed.

Hormones and disorders

  • The kidney makes renin (juxtaglomerular cells), erythropoietin (so CKD causes anaemia) and active vitamin D (1α-hydroxylation).
  • The kidney controls acid–base balance by reclaiming bicarbonate and excreting acid as ammonium and titratable acid.
  • Central diabetes insipidus: no ADH (treated with desmopressin). Nephrogenic diabetes insipidus: kidneys do not respond (e.g. lithium; thiazides help).
  • SIADH: too much ADH causes water retention and low sodium.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Renal numbers

QuantityValue
Renal blood flow≈ 1.2 L/min (20–25% of CO)
Renal plasma flow≈ 625 mL/min
GFR≈ 125 mL/min (180 L/day)
Filtration fraction≈ 20%
Urine output≈ 1–1.5 L/day
Glucose renal threshold≈ 180 mg/dL

Segment → key job

SegmentJob
Proximal tubuleBulk reabsorption; all glucose
Descending limbWater out
Thick ascending limbSalt out (NKCC2); water stays
Distal tubuleNaCl (NCC), calcium (PTH)
Collecting ductAldosterone (Na/K), ADH (water)

Memory aids

Inulin = Ideal GFR, PAH = Plasma flow
Inulin clearance measures GFR; PAH clearance measures renal plasma flow.

Exam traps

  • Creatinine clearance slightly overestimates GFR because creatinine is also secreted.
  • The thick ascending limb is impermeable to water, even though it pumps out salt.
  • ADH acts on the collecting duct, not the proximal tubule.
  • Glycosuria appears when the renal threshold is exceeded, even with normal kidneys.

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