Renal Tips Flashcards
(25 cards)
USMLE Renal Physiology Tips
Thiazides increase Ca²⁺ reabsorption, loops lose Ca²⁺
PTH = ↑ serum Ca²⁺, ↓ serum phosphate, ↑ urine phosphate
ADH affects V₂ receptors → aquaporins in collecting duct
ACE inhibitors: ↓ GFR by dilating efferent arteriole
NSAIDs: ↓ GFR by constricting afferent arteriole
Diabetes insipidus: central vs. nephrogenic → water deprivation test
Fanconi = PCT defect (loss of everything); Bartter = TAL defect (like loop diuretic); Gitelman = DCT defect (like thiazide)
High creatinine with normal BUN → think rhabdomyolysis
Glomerulotubular balance keeps reabsorption proportional to filtration
PCT (proximal convoluted tubule)
Reabsorbs ~65% of filtered load (Na⁺, H₂O, glucose, AA, HCO₃⁻)
- Na⁺/glucose cotransporter → inhibited by SGLT2 inhibitors (e.g., empagliflozin)
- Carbonic anhydrase inhibitors (e.g., acetazolamide) work here
Thin descending limb (Loop of Henle)
H₂O reabsorption only (impermeable to solutes) - Concentrates urine
- Hypertonic medulla pulls water out
Thick ascending limb
Na⁺, K⁺, 2Cl⁻ reabsorption (NKCC)
Impermeable to water - Dilutes urine
- Site of action of loop diuretics (furosemide)
- Generates medullary concentration gradient
DCT (distal convoluted tubule)
NaCl reabsorption
Impermeable to water - Site of thiazide diuretics
- Also reabsorbs Ca²⁺ (stimulated by PTH)
Collecting duct
Na⁺ reabsorption/K⁺ & H⁺ secretion
Water permeability controlled by ADH - Aldosterone ↑ Na⁺ reabsorption (via ENaC), ↑ K⁺/H⁺ secretion
- ADH → aquaporins insertion = water reabsorption
Aldosterone
Adrenal cortex (zona glomerulosa) ↑ Na⁺ reabsorption, ↑ K⁺ and H⁺ secretion (in collecting duct)
ADH (vasopressin)
Posterior pituitary
↑ H₂O reabsorption via aquaporins in collecting duct
ANP/BNP
Atria/ventricles
↓ Na⁺ reabsorption, dilates afferent arteriole, constricts efferent = ↑ GFR
Renin
JG cells (afferent arteriole)
Converts angiotensinogen → angiotensin I
Erythropoietin
Peritubular capillary cells
Stimulates RBC production in response to hypoxia
PTH
Parathyroid glands - ↑ Ca²⁺ reabsorption (DCT)
- ↓ PO₄³⁻ reabsorption (PCT)
- ↑ 1,25-vitamin D production
GFR
= U_inulin × V / P_inulin
≈ Creatinine clearance ↑ GFR = ↑ filtration
Creatinine slightly overestimates GFR
RPF
= U_PAH × V / P_PAH PAH is used because it’s freely filtered & secreted
Filtration Fraction (FF)
= GFR / RPF Normal ~20%
FF ↑ with ↓ RPF and stable GFR (e.g., hypovolemia)
Volume status changes
Condition ECF Volume ECF Osmolarity
Urine Na⁺ USMLE Clue
Diarrhea, hemorrhage
↓ ↓ Same Low
Volume depletion,
↑ RAAS
SIADH
↑ ECF
↓ Osm
High urine Na⁺
Euvolemic hyponatremia
Diabetes insipidus
↓ ECF
↑ Osm
Low urine Osm
Polyuria, polydipsia
Primary hyperaldosteronism
↑ ECF
Normal/slightly ↑
Low
HTN, ↓ K⁺, metabolic alkalosis
Winter’s formula
PCO₂ = 1.5 × HCO₃⁻ + 8 ±2
Metabolic acidosis compensation
Nephrotic Syndrome – Proteinuria >3.5 g/day
Disease Key Features USMLE Buzzwords
Minimal Change Disease
Kids, post-URI, good response to steroids
Effacement of podocyte foot processes (EM), normal LM
FSGS (Focal Segmental Glomerulosclerosis) Most common in adults, HIV, heroin, Black race
Segmental sclerosis & hyalinosis
Membranous Nephropathy
SLE, solid tumors, HBV/HCV
Subepithelial spikes & dome (EM), thick GBM
Amyloidosis Chronic diseases (RA, MM, TB) Apple-green birefringence with Congo red
Diabetic Nephropathy Long-standing diabetes
Kimmelstiel-Wilson nodules, hyaline arteriolosclerosis
🧠 USMLE Tip: Nephrotic syndrome → edema, ↓ albumin, ↑ lipids, ↑ thrombotic risk (loss of ATIII)
Nephritic Syndrome – Hematuria, HTN, RBC casts
Disease Key Features USMLE Buzzwords
Post-Strep GN Kids, 1–2 wks after skin/throat infection
“Lumpy-bumpy” on IF (IgG, C3), hypercellular glomeruli
IgA Nephropathy (Berger’s) Days after URI
Mesangial IgA deposits, most common worldwide
Henoch-Schönlein Purpura (HSP) Palpable purpura + joint pain + abdominal pain + hematuria
IgA deposition
RPGN (Crescentic) Rapid renal failure Crescent moon (fibrin + macrophages)
Causes:
Goodpasture’s (anti-GBM, hemoptysis)
Granulomatosis with polyangiitis (Wegener) (c-ANCA, ENT)
Microscopic polyangiitis (p-ANCA) | | Alport Syndrome | Young males, hematuria + hearing loss + eye issues | Type IV collagen defect, basket-weave GBM (EM) |
🧠 USMLE Tip: Nephritic = immune complexes + inflammation → hematuria, RBC casts
Tubulointerstitial Diseases
Disease Features USMLE Clues
Acute Interstitial Nephritis (AIN) Drug-induced (NSAIDs, penicillin, PPIs, rifampin), fever, rash, eosinophilia
WBC casts (no infection), eosinophils in urine
Acute Tubular Necrosis (ATN) Ischemia or toxins (aminoglycosides, contrast) Granular “muddy brown” casts, PCT vulnerable
Renal Papillary Necrosis
Sickle cell, analgesic use, DM, acute pyelonephritis
Gross hematuria, sloughed papillae
🧠 USMLE Tip: AIN = eosinophils + rash; ATN = muddy brown casts
Renal Vascular Pathologies
Condition Features USMLE Clues
Renal Artery Stenosis Atherosclerosis (old men) or FMD (young women)
Refractory HTN, ↓ kidney size, ↑ renin
Thrombotic Microangiopathy (TMA) HUS, TTP
Schistocytes, thrombocytopenia, AKI
Malignant HTN BP >180/120 + end-organ damage
Fibrinoid necrosis, “onion-skinning” arterioles
Renal Infections & Pyelonephritis
Disease Key Features USMLE Buzzwords
Cystitis Dysuria, frequency
Suprapubic pain, no fever
Pyelonephritis Fever, flank pain, CVA tenderness
WBC casts, often E. coli
Chronic Pyelonephritis Recurrent infections → scarring
Thyroidization of tubules (eosinophilic casts)
🧠 USMLE Tip: WBC casts = pyelonephritis or AIN
Urinary nitrites = gram-negative rods (esp. E. coli)
Renal Congenital & Hereditary Disorders
Disease Key Features Buzzwords
Polycystic Kidney Disease (ADPKD) Hematuria, HTN, flank pain, liver cysts Berry aneurysms, PKD1 mutation (Chr 16)
ARPKD Infants, Potter sequence
Hepatic fibrosis
Medullary Cystic Disease Shrunken kidneys, salt wasting
Poor prognosis
Nephronophthisis Childhood CKD Corticomedullary cysts
Renal tumors
Tumor Features USMLE Buzzwords
Renal Cell Carcinoma Painless hematuria, flank mass, smoking
Clear cells, originates from PCT, VHL gene (Chr 3)
Wilms Tumor (nephroblastoma) Kids, abdominal mass
WT1 mutation, part of WAGR syndrome
Urothelial carcinoma Painless hematuria Smoking, cyclophosphamide, phenacetin
Angiomyolipoma Benign, Tuberous sclerosis association
Vessels + fat + smooth muscle
