KIDNEY FUNCTION TESTS & GASTRIC FUNCTION — OmpathStudy

KIDNEY FUNCTION TESTS & GASTRIC FUNCTION KIDNEY FUNCTION OVERVIEW Key Facts Kidney safety margin : Large portion can be destroyed before symptoms appear Unilateral nephrectomy : Can survive with one kidney due to compensatory hyperactivity Cardiac output usage : Kidneys use 25% of total cardiac output Glomerular filtrate : 180 L/day (125 mL/min) Nephron count : ~1 million per kidney Normal Kidney Functions Maintain extracellular fluid constancy Excrete dietary surpluses and metabolic waste (urea, creatinine, urate, H+) Retain necessary substances (proteins, glucose, amino acids, HCO3-) Endocrine functions Erythropoietin production Renin secretion 1-alpha-hydroxylation of Vitamin D → calcitriol --- GLOMERULAR FUNCTION Structure Three barrier layers : Capillary endothelial → Basement membrane → Visceral epithelium Primary barrier : Basement membrane Charge barrier : Negative charge (sialic acid) prevents protein crossing Pressure : Hydrostatic pressure ~1 kPa Clinical Significance Oliguria in shock : When BP drops, oncotic pressure stops filtration Definitions :Polyuria: normal urine output Oliguria: <normal urine output Anuria: No urine output --- TUBULAR FUNCTION Proximal Tubule Reabsorbs 80% of sodium and water Sodium : High capacity active uptake, chloride follows passively Potassium : 95% absorbed (diet dependent) Phosphate : Active reabsorption (inhibited by PTH) HCO3- : Mostly absorbed Glucose & amino acids : Nearly complete absorption Secretes : Organic acids, urate, drugs Fanconi Syndrome : Loss of proximal tubular function Features: Glycosuria, amino aciduria, phosphaturia, acidosis, polyuria Cause: Cystinosis (classic), acquired causes Loop of Henle Unique to : Birds and mammals Function : Counter-current multiplier for concentrated/dilute urine Key features :Active NaCl pump in thick ascending limb Water impermeable ascending limb Fluid emerges hypotonic (~120-150 mosmoles/L) Distal Convoluted Tubule (DCT) Minimal volume/concentration change Aldosterone effect : Na+ exchanged for K+/H+ Related conditions : Conn's syndrome, Addison's disease, RTA type I Collecting Ducts ADH action : Increases water permeability ADH pathology : Diabetes insipidus : No ADH (pituitary) or no response (nephrogenic) SIADH : Inappropriate ADH secretion → concentrated urine + hyponatremia --- RENAL FUNCTION TESTS Glomerular Function Tests 1. Serum Creatinine Source : Creatine phosphate in muscle Characteristics : Filtered, not reabsorbed/secreted significantly Factors affecting : Muscle mass, dietary meat intake Clinical use : Increases in acute/chronic renal failure 2. Creatinine Clearance (CrCl) Formula : CrCl = (Urine Creatinine × Volume) / Plasma Creatinine Units : mL/min Normal value : ~120 mL/min Advantages : Best measure of GFR, linear with renal mass loss Limitations : Requires 24-hour urine collection, not useful in acute failure 3. Blood Urea Source : Liver from protein breakdown Reabsorption : 30-40% in tubules Factors increasing : High protein intake, catabolic states, surgery, trauma, GI bleeding 4. Estimated GFR (eGFR) Most popular : MDRD equation (adults 18 years) Variables : Serum creatinine, age, gender, ethnicity Formula : GFR = 175 × [creatinine × 0.011312]^-1.154 × [age]^-0.203 × [1.212 if black] × [0.742 if female] Children : Use Schwartz equation (includes height) 5. Abnormal Urea:Creatinine Ratio Increased : High protein intake, GI bleeding, hypercatabolic states, dehydration, urinary stasis Decreased : Low protein intake, dialysis, severe liver disease Tubular Function Tests Urinary Na+ concentration : Normally low unless high salt intake Concentration/dilution tests : After pitressin/water load Osmolality ratio : Urine/plasma osmolality Acidification tests : After NH4Cl administration Miscellaneous Tests Microscopy : Casts, cells, crystals Proteinuria : 2.5 g/day = nephrotic syndrome Bence-Jones protein : Indicates myeloma β2-microglobulin : Sensitive tubular function test --- MAJOR RENAL DISORDERS Nephrotic Syndrome Definition : Glomerular protein permeability increase Criteria : Proteinuria 2.5 g/day + edema + hypoproteinemia + hyperlipidemia Selectivity Index : (UIgG/PIgG)/(Ualb/Palb) × 100 Laboratory findings : ↑α2 globulins, hypercholesterolemia Acute Renal Failure (ARF) Definition : Urine <450 mL/day + rising urea (normally 1.7-6.7 mMol/L) Urea rise : 5 mMol/L/day (up to 15 in surgery/trauma/bleeding) Types & Causes Pre-renal : Hypovolemia, ↓cardiac output, renovascular obstruction Intra-renal : Acute tubular necrosis, glomerulonephritis, interstitial nephritis Post-renal : Bilateral ureteric obstruction, urethral obstruction Diagnostic Tests (Pre-renal vs Intra-renal) Management Post-renal : Relieve obstruction Pre-renal : Restore blood volume Acute tubular necrosis : Fluid balance, electrolyte management Dialysis indications : Urea 50 mMol/L, HCO3- <10 mMol/L, K+ 7.0 mMol/L Chronic Renal Failure (CRF) Common causes : Glomerulonephritis, diabetes, hypertension Key feature : Polyuria (opposi