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Urinary System Summary Notes

Introduction to the Urinary System

  • Functions: Cleansing blood, waste removal, pH regulation, blood pressure regulation, solute concentration regulation, red blood cell production (EPO), vitamin D synthesis.
  • Consequences of Kidney Failure: Weakness, lethargy, shortness of breath, anemia, edema, metabolic acidosis, heart arrhythmias.
  • Key Concept: Kidneys regulate plasma makeup.

Urine Characteristics

  • Glomeruli: Filter blood based on particle size, excluding large elements.
  • Filtrate: Similar to plasma, about 200 liters produced daily.
  • Urine: Less than 2 liters excreted daily, composition varies with water intake, exercise, etc.
  • Normal Urine Characteristics:
    • Color: Pale yellow to deep amber
    • Odor: Odorless
    • Volume: 750–2000 mL/24 hour
    • pH: 4.5–8.0
    • Specific Gravity: 1.003–1.032
    • Osmolarity: 40–1350 mOsmol/kg
  • Urinalysis Clues: Protein (glomerular damage), unusual urine quantities (diabetes, tumors).
  • Urochrome: Yellow pigment from hemoglobin breakdown.
  • Urine Volume Variations:
    • Normal: 1–2 L/day
    • Polyuria: >2.5 L/day (diabetes, excess fluids)
    • Oliguria: 300–500 mL/day (dehydration, kidney disease)
    • Anuria: <50 mL/day (kidney failure, obstruction)

Gross Anatomy of Urine Transport

  • Urethra: Transports urine from bladder to outside; differs between males and females.
  • Female Urethra: Short (4 cm), higher UTI risk.
  • Male Urethra: Longer (20 cm), divided into preprostatic, prostatic, membranous, and spongy regions.
  • Bladder: Stores urine, capacity up to 500–600 mL.
  • Detrusor Muscle: Smooth muscle in bladder wall.
  • Micturition Reflex: Involuntary and voluntary control of urination.
    • Urge at 150 mL, incontinence at 300-400 mL.
  • Ureters: Transport urine from kidneys to bladder via peristalsis.
    • Oblique entry into bladder prevents reflux.

Gross Anatomy of the Kidney

  • Location: Retroperitoneal, protected by muscle, fat, and ribs.
  • Size: Roughly the size of a fist.
  • External Anatomy:
    • Left kidney: T12 to L3 vertebrae.
    • Right kidney: Lower due to liver.
  • Internal Anatomy:
    • Renal cortex (outer region)
    • Renal medulla (inner region)
    • Renal columns and renal pyramids
    • Renal papillae (bundles of collecting ducts)
  • Renal Hilum: Entry/exit for vessels, nerves, lymphatics, ureters.
  • Blood Vessels: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles.
  • Nephrons: Functional units, cleanse blood.
  • Glomerulus: High-pressure capillaries.
  • Bowman’s Capsule: Surrounds glomerulus.
  • Efferent Arteriole: Forms peritubular capillaries and vasa recta.
  • Portal System: Glomerulus drains into efferent arteriole, forming a second capillary bed.
  • Cortex: Contains renal corpuscles, PCTs, DCTs.
  • Cortical Nephrons: Short loop of Henle.
  • Juxtamedullary Nephrons: Long loop of Henle extending into medulla.

Microscopic Anatomy of the Kidney

  • Nephrons: Functional units balancing plasma and excreting toxins.
  • Renal Corpuscle: Glomerulus and Bowman’s capsule.
    • Glomerulus: High-pressure capillary bed.
    • Bowman’s Capsule: Parietal and visceral layers (podocytes with pedicels).
  • Filtration Membrane: Fenestrated capillaries, podocytes, and basement membrane.
    • Pores: 70 nm diameter.
  • Juxtaglomerular Apparatus (JGA):
    • Macula Densa: Monitors fluid composition in DCT, releases paracrine signals (ATP, adenosine).
    • Juxtaglomerular Cells: Modified smooth muscle cells regulating blood flow to glomerulus, renin release
  • Proximal Convoluted Tubule (PCT): Simple cuboidal cells with microvilli (brush border).
  • Loop of Henle: Descending (water permeable) and ascending (solute permeable) portions.
  • Distal Convoluted Tubule (DCT): Simple cuboidal epithelium, fewer microvilli.
  • Collecting Ducts: Receive filtrate from multiple nephrons, receptors for ADH, insert aquaporins.
  • Aquaporins: Water channels facilitating water movement across cell membranes.

Physiology of Urine Formation

  • Glomerular Filtration Rate (GFR): Filtrate volume formed by kidneys per minute.
    • Normal: 125 mL/min (men), 105 mL/min (women).
    • About 180 L/day in men and 150 L/day in women
    • {\approx} 1-2 liters of urine produced per day.
  • Net Filtration Pressure (NFP):
    • Determined by hydrostatic and colloid osmotic pressures.
    • NFP = Glomerular \, blood \, hydrostatic \, pressure (GBHP) - [capsular \, hydrostatic \, pressure (CHP) + blood \, colloid \, osmotic \, pressure (BCOP)]
    • NFP = GBHP - [CHP + BCOP]
    • NFP = 55 - [15 + 30] = 10 mm Hg
  • Autoregulation: Smooth muscle contracts with stretch, maintaining steady blood flow.
  • GFR Determination: Estimated by inulin or creatinine clearance.

Tubular Reabsorption

  • Reabsorption Locations: PCT, loop of Henle, DCT, collecting ducts.
  • Regulation: ADH, aldosterone, renin.
  • Mechanisms: Active transport, diffusion, facilitated diffusion, secondary active transport, osmosis.
  • Glucose Reabsorption: Secondary active transport with Na+.
  • Urea: 50% passively reabsorbed in PCT.
  • PCT: Reabsorbs 67% water, Na+, K+; nearly 100% glucose, amino acids; secretes creatinine.
  • Loop of Henle: Creates hypertonic medulla via countercurrent multiplier system.
  • DCT: Reabsorbs 10-15% water.
  • Collecting Ducts: ADH regulates water reabsorption.

Regulation of Renal Blood Flow

  • Sympathetic Nerves: Control blood flow via vasoconstriction/vasodilation.
  • Autoregulation: Myogenic mechanism (smooth muscle contraction/relaxation).
  • Tubuloglomerular Feedback: JGA, ATP, adenosine, NO regulate afferent arteriole constriction/dilation.

Endocrine Regulation of Kidney Function

  • Renin-Angiotensin-Aldosterone: Regulates blood pressure and Na+ reabsorption.
  • ADH: Promotes water recovery.
  • Natriuretic Hormones: Stimulate Na+ excretion.
  • PTH: Regulates Ca++ levels.

Regulation of Fluid Volume and Composition

  • Major Hormones: ADH, aldosterone, ANH.
  • Volume-Sensing Mechanisms: Baroreceptors, granular cells (renin release).
  • Diuretics: Increase urine volume.
  • Na+ Regulation: Renin-angiotensin-aldosterone system, ADH.
  • K+ Regulation: Aldosterone.
  • Ca++ and Phosphate: PTH.
  • pH Regulation: Buffers, lungs, kidneys.
  • Nitrogen Wastes: Ammonia (converted to urea), urea, uric acid.

The Urinary System and Homeostasis

  • Vitamin D Synthesis: Kidneys hydroxylate calcidiol to calcitriol.
  • Erythropoiesis: Kidneys produce EPO.
  • Blood Pressure Regulation: Na+ and water balance, renin-angiotensin-aldosterone system.
  • Osmolarity Regulation: Solute and water balance.
  • Electrolyte Regulation: Na+, Ca++, K+.
    pH Regulation