CC CJM | GU

Urinary System: 80/20 Summary

1. Anatomy & Blood Supply The upper urinary tract has two retroperitoneal, bean-shaped kidneys (T12–L3) and 2 ureters[1]. The lower tract comprises the bladder and urethra[1]. Each kidney is shielded by a fibrous capsule and cushioned by fat[1]. The functional unit is the nephron (~1 million), consisting of a glomerulus, Bowman's capsule, and tubule system (PCT, loop of Henle, DCT, and collecting ducts)[2]. Blood flow is ~1200 mL/min (20-25% of cardiac output) via the renal artery, which divides into afferent arterioles[2].

2. Physiology of Urine Formation Urine is formed through filtration, reabsorption, secretion, and excretion[2].

• Glomerular Filtration: Hydrostatic pressure filters blood across a porous, semipermeable membrane into Bowman’s capsule, yielding a filtrate lacking cells and large proteins[3]. Normal Glomerular Filtration Rate (GFR) is ~125 mL/min; only ~1 mL/min is excreted as urine[3].
• Tubular Function:
  • PCT: Reabsorbs 80% of electrolytes and water, all glucose, amino acids, and bicarbonate; secretes H+ and creatinine[4].
  • Loop of Henle: Conserves water and concentrates filtrate. Descending loop is water-permeable; ascending limb actively reabsorbs Cl- and Na+ (25% of sodium load) and is water-impermeable[4].
  • DCT & Collecting Ducts: Finalize regulation of water (controlled by ADH, which increases permeability) and acid-base balance (reabsorbing HCO3- and secreting H+)[4][5]. Aldosterone promotes Na+ and water reabsorption in exchange for K+ excretion[5]. Atrial Natriuretic Peptide (ANP) opposes this by increasing Na+ excretion and GFR[5].

3. Hormonal & Regulatory Functions

• Erythropoietin: Released during hypoxia or hypoperfusion, stimulating RBC production (deficient in renal failure, causing anemia)[5].
• RAAS: Juxtaglomerular cells release renin during hypoperfusion, low BP, or hyponatremia. Renin converts angiotensinogen to angiotensin I, which ACE converts to angiotensin II—causing vasoconstriction and aldosterone release[6].
• Prostaglandins (PGE2, PGI2): Medullary vasodilators that maintain renal blood flow and counteract vasoconstrictors[7].
• Vitamin D: Kidneys perform the final activation step of vitamin D, essential for calcium absorption[5].

4. Diagnostics & Assessment

• Key Labs: Serum creatinine is the most reliable renal function index (released at a constant rate from muscle)[8][9]. Creatinine clearance (24-hr urine) approximates GFR[9][10]. BUN rises with dysfunction but is altered by protein intake, tissue breakdown, and hydration[8].
• Physical Exam: Involves palpation of the right kidney and indirect fist percussion of the costovertebral angle (CVA) to elicit pain in infection or obstruction[11][12].
• Nephrotoxic Risks: Drugs like aminoglycosides, NSAIDs, and contrast media are highly nephrotoxic[13]. Dehydration increases the risk of contrast-induced nephropathy[9].