EO

Urinary System Review

Urinary System Review

Organs and Functions

  • Organs:

    • Kidneys (primary focus for physiology)
    • Ureters (transport urine)
    • Urinary bladder (stores urine, micturition/voiding)

  • Kidney Functions:

    • Elimination of waste.
    • Regulation of electrolytes and water.
    • Regulation of pH.
    • Endocrine Functions:
      • Renin secretion (activates the angiotensin cycle, increases blood pressure).
      • Activation of vitamin D.
      • Gluconeogenesis (during prolonged fasting).
    • Regulation of blood pressure and blood volume.

Nephrons: Functional Units

  • Microscopic functional units of the kidneys.
  • Tubular structure with distinctive parts.

Renal Corpuscle

  • Glomerulus: A network of capillaries where filtration occurs.
  • Afferent Arteriole: Brings blood into the glomerulus.
  • Efferent Arteriole: Carries blood away from the glomerulus.
  • Glomerular Capsule (Bowman's Capsule): Encloses the glomerulus, collects filtrate.
  • Filtration Process:
    • Driven by hydrostatic pressure gradient between the glomerulus and the capsule.
    • Passive movement of water and solutes from high pressure to low pressure.
    • Glomerular blood pressure: 55 \, mmHg
    • Capsule hydrostatic pressure: 15 \, mmHg
    • Colloid osmotic pressure: 30 \, mmHg
    • Net filtration pressure: 55 - (15 + 30) = 10 \, mmHg

Functional Anatomy

  • The structure of each part of the nephron is directly related to its function.
  • Important to study anatomy and physiology together.

Nephron Structure

  1. Renal Corpuscle:
    • Glomerulus + Glomerular capsule.
  2. Proximal Convoluted Tubule (PCT):
    • Convoluted tubule extending from the renal corpuscle.
  3. Loop of Henle:
    • U-shaped turn in the nephron.
    • Descending limb: Goes down into the medulla.
    • Ascending limb: Goes back up towards the cortex.
  4. Distal Convoluted Tubule (DCT):
    • Another convoluted tubule, located distal to the PCT.
    • Empties into the collecting duct.
  5. Collecting Duct:
    • Receives filtrate from multiple nephrons.
    • Delivers urine to the minor calyx.

Cortical vs. Juxtamedullary Nephrons

  • Cortical Nephrons:
    • Most of their structure is in the cortex.
    • Primary function occurs in the cortex.
  • Juxtamedullary Nephrons:
    • Long loop of Henle that extends deep into the medulla (15% of nephrons).
    • Responsible for concentrating urine (reabsorbing more water).
    • Associated with vasa recta (specialized capillary system).

Kidney Organization

  • Cortex: Outer layer of the kidney.
  • Medulla: Inner layer, forming renal pyramids.
  • Renal Lobe: Medullary pyramid with surrounding cortex.
  • Renal Columns: Tissue between the renal lobes.
  • Arcuate Artery and Vein: Located at the base of the medullary pyramids, between the cortex and medulla.
  • Cortical Radiate Artery: Branches from the arcuate artery and radiates into the cortex.
    • Afferent arterioles branch off from this artery.

Blood Vessels

  • Afferent arteriole branches off the cortical radiate artery.
  • Efferent arteriole leads away from the glomerulus.
  • Peritubular capillaries surround the tubular portions of cortical nephrons.
  • Vasa recta is associated with the loop of Henle of juxtamedullary nephrons.

Epithelial Lining of Nephron Segments

  • Different parts of the nephron have different epithelial linings.

    • Renal corpuscle: simple squamous.
    • Proximal convoluted tubule: cuboidal.

  • Epithelial lining is directly related to the function of that segment.

Juxtaglomerular Apparatus (JGA)

  • Intrinsic autoregulatory mechanism controlling glomerular filtration rate (GFR).
  • Located where the ascending limb of the loop of Henle passes between the afferent and efferent arterioles.
  • Components:
    • Macula Densa Cells:
      • Monitor filtrate flow and composition in the ascending limb.
    • Mesangial Cells:
      • Communicate signals between the macula densa and afferent arteriole.
    • Granular Cells (Juxtaglomerular Cells):
      • Smooth muscle cells in the afferent arteriole that secrete renin.

Urine Properties

  • Collecting duct delivers urine to the minor calyx (after passing the papilla).
  • Minor calyces merge into major calyces, which merge into the renal pelvis.
  • Urine is then transported to the ureters.
  • Physical and Chemical Properties:
    • pH.
    • Clarity: Normally clear with varying shades of yellow.
    • Cloudy urine indicates urinary tract infection (UTI).
    • Urine is typically sterile due to the stringent filtration membrane.
    • Filtration membrane pore size: up to 5 \, nm (nanometers, 10^{-9} m)

Ureters, Urinary Bladder and Urethra

  • Urethra:
    • Female vs. Male (longer in males).
    • Male urethra carries both urine and semen.
    • Male Urethra Segments:
      • Prostatic urethra: Passes through the prostate gland; receives secretions from prostate gland and ejaculatory ducts.
      • Membranous urethra: Passes through the urogenital diaphragm.
      • Spongy urethra: Passes through the penis.
  • Urethral Sphincters:
    • Internal urethral sphincter (smooth muscle, involuntary).
    • External urethral sphincter (skeletal muscle, voluntary).
  • Micturition:
    • Requires opening of both sphincters and contraction of the detrusor muscle in the bladder wall.
  • Bladder Structure:
    • Detrusor muscle: Smooth muscle layer responsible for bladder contraction.
    • Epithelial lining: Transitional epithelium (unique to urinary system).
  • Epithelium Type
    • Transitional: Ureters, bladder, and part of urethra.
    • Lower part of Urethra: pseudostratified.
    • Urethral orifice: stratified squamous.