Memory tricks for the kidneys, nephrons, and urine formation

Blood pressure regulation: RAAS (renin release) + fluid volume control. Acid-base balance: excrete H+ and reabsorb HCO₃⁻ — renal compensation for acid-base disorders (slower than respiratory, more powerful). Filtration and waste excretion: filter ~180 L/day → excrete ~1.8 L urine. Remove urea, creatinine, drugs, toxins. Fluid balance: adjust water excretion based on ADH (antidiuretic hormone) — concentrated or dilute urine. Erythropoiesis: produce EPO (erythropoietin) → stimulates RBC production in bone marrow. Kidney failure → anemia. Electrolyte balance: regulate Na+, K+, Ca2+, phosphate, Mg2+ — aldosterone controls Na+/K+ exchange. Vitamin D activation: kidneys convert 25-OH-D → 1,25-(OH)₂-D (calcitriol) → Ca2+ absorption.

Kidneys: retroperitoneal (behind peritoneum), at T12-L3. Right kidney lower (liver pushes it down). Renal cortex: outer region — contains glomeruli and most tubules. Renal medulla: inner region — contains renal pyramids (collecting ducts and loops of Henle). Renal pyramids → renal papillae → minor calyces → major calyces → renal pelvis → ureter. Ureter: smooth muscle propels urine by peristalsis → bladder. Bladder: detrusor muscle (smooth) — stores urine. Micturition reflex: stretch → parasympathetic → detrusor contracts + internal urethral sphincter relaxes → external sphincter voluntary. Urethra: male (~20 cm, 3 parts) vs female (~4 cm) — explains why UTIs more common in women.

~1 million nephrons per kidney. Two types: cortical (85% — short loops) and juxtamedullary (15% — long loops into medulla, concentrate urine). Renal corpuscle: glomerulus (capillary tuft) + Bowman's capsule — filtration occurs here. Proximal convoluted tubule (PCT): bulk reabsorption — 65% of Na+, water, glucose (all), amino acids, HCO₃⁻. Loop of Henle: descending (permeable to water, not solutes) + ascending (permeable to solutes, not water — impermeable!). Creates hypertonic medullary gradient. Distal convoluted tubule (DCT): fine-tuning — aldosterone acts here (Na+/K+ exchange). Collecting duct: final water reabsorption under ADH control. Passes through medulla → papilla → calyx.

Filtration occurs at the glomerulus driven by Starling forces — net filtration pressure ~10 mmHg. Glomerular hydrostatic pressure (~55 mmHg) pushes OUT. Capsular hydrostatic pressure (~15 mmHg) and blood oncotic pressure (~30 mmHg) push BACK. Net = ~10 mmHg outward = filtration. GFR = ~125 mL/min = ~180 L/day. Filtrate composition = plasma minus proteins and cells (protein-free). Normal GFR: eGFR >90 mL/min. CKD stages based on GFR. Autoregulation: myogenic reflex (↑ BP → afferent arteriole constricts → maintains GFR) + tubuloglomerular feedback (macula densa senses Na+ → signals JGA → adjust afferent tone). GFR estimated clinically by creatinine clearance or eGFR formula.

PCT (proximal convoluted tubule): 65% Na+, 65% water, 100% glucose and amino acids, 90% HCO₃⁻, 50% urea, all small proteins. Na+/K+ ATPase on basolateral membrane drives everything. Glucose reabsorbed via SGLT2 (apical) + GLUT2 (basolateral) — transport maximum: ~180 mg/dL. Exceeding Tm → glucosuria (diabetes). Loop of Henle: descending = water leaves, ascending = NaCl pumped out. NaCl gradient builds up in medulla. DCT: Na+ reabsorption (aldosterone-sensitive), Ca2+ reabsorption (PTH-sensitive). Collecting duct: water reabsorption (ADH-sensitive via aquaporin-2 insertion). ADH from posterior pituitary. No ADH → diabetes insipidus → massive dilute urine.

The loop of Henle acts as a countercurrent multiplier — flowing in opposite directions, the two limbs create and maintain a hypertonic medullary gradient. Descending limb: permeable to water, impermeable to solutes → water leaves → tubular fluid becomes concentrated. Ascending limb (thick): impermeable to water, actively pumps NaCl out → medullary interstitium becomes hypertonic (up to 1200 mOsm at papilla vs 300 mOsm in cortex). Collecting duct descends through this gradient: without ADH → water stays in duct → dilute urine. With ADH → aquaporins inserted → water drawn out by osmotic gradient → concentrated urine (up to 1200 mOsm). Vasa recta: capillaries looping alongside — countercurrent exchange preserves gradient.

RAAS: low BP/low Na+/↓ renal perfusion → juxtaglomerular cells release RENIN → cleaves angiotensinogen → angiotensin I → ACE (lung) → angiotensin II → vasoconstriction + aldosterone release → Na+ retention → ↑ blood volume → ↑ BP. ACE inhibitors block this. Aldosterone (adrenal cortex): Na+ in, K+ out in DCT/CD → water follows → ↑ BP. Spironolactone blocks. ADH (vasopressin, posterior pituitary): ↑ plasma osmolarity or ↓ volume → ADH → aquaporin-2 in collecting duct → water retained → concentrated urine. SIADH = too much ADH. ANP (atrial natriuretic peptide, heart): ↑ atrial stretch (high volume) → ANP → inhibits RAAS + aldosterone → promotes Na+ excretion → ↓ BP.

The kidneys are the ultimate regulators of pH — slower than lungs but more powerful and complete. Three mechanisms: HCO₃⁻ reabsorption: PCT reabsorbs 90% of filtered HCO₃⁻ — prevents loss of base. H+ secretion: α-intercalated cells in collecting duct secrete H+ against gradient — creates acidic urine (minimum pH 4.5). H+ buffered by phosphate (titratable acid) and ammonia. New HCO₃⁻ generation: glutamine metabolism in PCT → NH₄+ (excreted) + HCO₃⁻ (new, added to blood). Key: kidneys can generate brand new bicarbonate. Metabolic acidosis → kidneys ↑ H+ secretion + ↑ NH₄+ excretion. Metabolic alkalosis → kidneys ↓ H+ secretion + excrete HCO₃⁻.

Normal urine: pale yellow (urochrome from bilirubin breakdown), specific gravity 1.001-1.035, pH 4.5-8.0, ~95% water + urea + creatinine + electrolytes. Abnormal findings: Glucose (glucosuria): blood glucose >180 mg/dL (diabetes) or SGLT2 defect. Protein (proteinuria): kidney disease, glomerulonephritis — albumin leaks (nephrotic syndrome >3.5 g/day). Blood (hematuria): UTI, kidney stones, trauma, cancer. Ketones: diabetic ketoacidosis, starvation. Bilirubin: liver disease or hemolysis. WBCs (pyuria): UTI. Casts (cylindrical molds of tubules): RBC casts = glomerulonephritis; WBC casts = pyelonephritis; granular casts = ATN. Specific gravity: concentrated urine (dehydration) vs dilute (diabetes insipidus, overhydration).