Urinary System study guide

Major Components of the Urinary System

• Kidneys: Key organs located retroperitoneally, divided into renal cortex (outer layer) and renal medulla (inner area).
• Ureters: Tubes that carry urine from the kidneys to the urinary bladder.
• Urinary Bladder: Stores urine until it is excreted from the body.

Functions of the Urinary System

• Filtration: Removal of waste and excess substances from the blood.
• Absorption: Recovery of essential substances back into the bloodstream.
• Secretion: Excretion of unnecessary substances from the blood into urine.

Nephrons: Structural and Functional Unit

• Renal Corpuscle: Composed of glomerulus (fenestrated capillaries) and glomerular capsule (containing parietal and visceral layers with podocytes).
• Renal Tubule: Contains various sections with specific epithelial structures adapted for their functions:
  • Proximal Convoluted Tubule (PCT): Cuboidal epithelium with microvilli, major site for reabsorption.
  • Nephron Loop (Loop of Henle): Mostly simple squamous and some cuboidal.
  • Distal Convoluted Tubule (DCT): Smaller cuboidal epithelium.
  • Collecting Duct: Cuboidal epithelium.
• Categories of Nephrons:
  • Cortical Nephrons: Located primarily in the renal cortex.
  • Juxtamedullary Nephrons: Extend into the renal medulla.

Urine Formation Processes

1. Filtration:

   • Occurs at the glomerulus where blood hydrostatic pressure forces fluid through the filtration membrane, forming filtrate (different from urine).
   • Components of Filtration Membrane: Fenestrated capillaries of the glomerulus, podocytes, and basement membrane.
   • Pressures Affecting Filtration:
     • Hydrostatic pressure of blood in glomerulus (HPgc, outward pressure).
     • Hydrostatic pressure in capsule space (HPcs, inward pressure).
     • Osmotic pressure of blood in glomerulus (OPgc, inward pressure).
   • Net Filtration Pressure (NFP): NFP = HPgc - (HPcs + OPgc)
   • Glomerular Filtration Rate (GFR): Rate of plasma movement into the capsule; proportional to NFP.

2. Reabsorption:

   • Process of reclaiming materials from filtrate back to blood, predominantly in the PCT:
     • Active Transport: For Na⁺, glucose, amino acids (involves transport maximum).
     • Osmosis: Passive movement of water.
     • Passive Diffusion: For Cl^- and other ions.
   • Only 30-40% of filtrate volume reaches the Loop of Henle.

3. Secretion:

   • Selected substances (H⁺, K⁺, NH₄⁺, drugs) are transferred from blood into tubule filtrate to eliminate unwanted substances like urea.

Regulating Urine Concentration and Volume

• Concentration gradient in kidney medulla established by Juxtamedullary Nephrons:
  • Descending Limb: Highly permeable to H₂O.
  • Ascending Limb: Impermeable to H₂O; reabsorbs Na⁺ and Cl^- actively.
• ADH Role: Hormonal control adjusting urine concentration:
  • Overhydrated: No ADH -> no water channels -> dilute urine.
  • Dehydrated: ADH present -> water reabsorbed -> concentrated urine.

Urination (Micturition) Process

• Components: Ureters, urinary bladder (includes trigone and detrusor muscle), urethra.
• Micturition Reflex Steps:
  • Bladder stretch receptors activated.
  • Activation of parasympathetic response.
  • Detrusor muscle contracts; internal urethral sphincter opens.
  • External urethral sphincter relaxed consciously to allow urination.

Urinalysis and Normal Properties of Urine

• Understand normal physical and chemical properties of urine and implications of substances found:
  • Protein: Indication of kidney damage.
  • Glucose: Potential diabetes or high blood sugar levels.
  • Ketone Bodies: Sign of fat metabolism (often in diabetes).
  • Erythrocytes: Hematuria, possible injury or infection.
  • Leukocytes: Sign of infection.
  • Bile Pigments: Liver issues or hemolysis.

Conditions Related to the Urinary System

• Renal Calculi (Kidney Stones): Crystal formations in urinary tract leading to obstructions.
• Urinary Incontinence: Inability to control urination, various causes.
• Renal Failure & Hemodialysis: Loss of kidney function requiring medical intervention.
• Diabetes Insipidus: Characterized by intense thirst and heavy urination.