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Urinary System - Anatomy & Physiology

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  • Major functions of the urinary system

    Excretion of metabolic wastes, maintenance of water, salt, and electrolyte balance, acid-base balance, hormone secretion, activation of vitamin D, and gluconeogenesis during prolonged fasting.

  • Location and size of the kidneys

    Retroperitoneal, between T12 and L3 vertebrae, about 4 inches long, 3 inches wide, 1 inch thick, and weighing approximately 5 ounces each.

  • Three tissue layers covering the kidney

    Renal capsule, adipose capsule, and renal fascia.

  • Two types of nephrons

    Cortical nephrons with short loops of Henle and juxtamedullary nephrons with long loops of Henle near the cortex-medulla junction.

  • Components of a nephron

    Renal corpuscle (glomerulus and glomerular capsule) and renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule).

  • Function of the juxtaglomerular apparatus

    Regulates filtrate formation and blood pressure by releasing renin and monitoring sodium concentration.

  • Three major processes of urine formation

    Glomerular filtration, tubular reabsorption, and tubular secretion.

  • Glomerular filtration

    Passive process where hydrostatic pressure forces fluids and solutes through the filtration membrane into the glomerular capsule.

  • Filtration membrane layers

    Fenestrated endothelium of glomerular capillaries, basement membrane, and podocyte foot processes with filtration slits.

  • Net filtration pressure (NFP)

    Sum of glomerular hydrostatic pressure (55 mm Hg), capsular hydrostatic pressure (15 mm Hg), and blood colloid osmotic pressure (30 mm Hg) determining filtrate formation.

  • Glomerular filtration rate (GFR)

    Volume of filtrate formed per minute by both kidneys; normally 125 ml/min in men and 105 ml/min in women.

  • Reabsorption routes in the nephron

    Transcellular route (through tubular cells) and paracellular route (between tubular cells).

  • Role of Na+-K+ ATPase pump in reabsorption

    Creates electrochemical gradient for sodium movement, driving reabsorption of water and other solutes by secondary active transport.

  • Transport maximum (Tm)

    Maximum rate at which a substance can be reabsorbed due to limited carrier proteins; excess is excreted in urine.

  • Countercurrent multiplier mechanism

    Loop of Henle creates an osmotic gradient from 300 to 1200 mOsm to regulate urine concentration.

  • Countercurrent exchanger function

    Vasa recta preserves the osmotic gradient by removing reabsorbed water and preventing washout of salts.

  • Effect of ADH on urine concentration

    ADH increases water reabsorption in collecting ducts, producing concentrated urine via facultative water movement.

  • Diuretics

    Chemicals that increase urine output by inhibiting ADH, sodium reabsorption, or medullary gradient formation.

  • Renal clearance

    Volume of plasma cleared of a substance per minute; used to measure GFR with substances like inulin.

  • Urine composition

    Approximately 95% water and 5% solutes including Na+, K+, phosphate, sulfate, calcium, magnesium, bicarbonate, and nitrogenous wastes like urea, uric acid, and creatinine.

  • Pathway of urine from kidney to outside

    Collecting duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra.

  • Differences between male and female urethra

    Male urethra is about 8 inches, transports urine and semen, and is encircled by the prostate; female urethra is about 1.5 inches and transports only urine.

  • Micturition reflex

    Involuntary emptying of the urinary bladder controlled by parasympathetic nervous system and voluntary control of external urethral sphincter.