Urinary System Study GuideeeE

Urinary System Study Guide

Organs of the Urinary System

• Main Organ: Kidneys
• Other Organs: Ureters, Urinary Bladder, Urethra

Functions of the Urinary System

1. Regulate blood volume and blood pressure
2. Regulate osmolarity of body fluids
3. Regulate Renin (Renin-Angiotensin-Aldosterone System - RAAS) for blood pressure and electrolyte balance
4. Secrete Erythropoietin (EPO) for red blood cell production
5. Coordinate with lungs for regulation of P(CO₂) and acid-base balance
6. Produce and secrete active form of Vitamin D (Calcitriol)
7. Perform gluconeogenesis in extreme cases (starvation)

Characteristics of Urine

• Production: 1.0–1.8 liters/day
• Composition: 95% water, 5% solutes
• Normal Appearance: Clear, pale yellow due to urochrome (from hemoglobin breakdown)

Abnormal Appearance of Urine

• Cloudy: Possible urinary tract infection
• Pink, brown, smoky color: Could indicate food, bile pigments, blood, drugs

Normal Components of Urine

• Nitrogenous Wastes:
  • Urea: from amino acid breakdown
  • Uric Acid: from nucleic acid metabolism
  • Creatinine: from creatine phosphate metabolism
• Electrolytes: Cl⁻, Na⁺, K⁺, PO₄³⁻, SO₄²⁻, Ca²⁺, Mg²⁺, HCO₃⁻

Components That Should NOT Be Present

• Amino acids
• Glucose
• Cells or large plasma proteins

Properties of Urine

• Odor: Slightly aromatic when fresh; ammonia smell after standing
• pH: Slightly acidic (approximately 6.0; range 4.5–8.0)
  • Acidic diet (protein, whole wheat) lowers pH
  • Alkaline diet (vegetarian), prolonged vomiting, infection raises pH

Urinary Bladder

• Structure:
  • Detrusor muscle: Three layers of smooth muscle
  • Mucosa: Transitional epithelium
• Function: Temporary urine storage
• Capacity: Full bladder holds ~500 mL
• Special Features:
  • Trigone: Triangular area with openings for ureters and urethra
  • Folds and thick walls allow expansion without pressure increase

Urethral Sphincters

• Internal Sphincter: Involuntary (smooth muscle at bladder-urethra junction)
• External Sphincter: Voluntary (skeletal muscle surrounding urethra)

Kidneys: Internal Anatomy

• Regions:
  • Renal cortex
  • Renal medulla (contains renal pyramids, papillae, renal columns)
  • Renal pelvis (funnel-shaped, continuous with ureter)
• Hilum: Entry/exit for ureter, blood vessels, and nerves
• Adrenal Gland: Located on top of each kidney
• Right Kidney: Slightly lower due to the liver's position

Renal Blood Flow (RBF)

• 20% of cardiac output (~1L/min or 1440L/day) passes through kidneys
• Process: Filtration → Reabsorption → Secretion → Excretion
• Blood Flow Pathway: Renal artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries/vasa recta → renal vein

Nephrons

• Structural and Functional Units (~1 million/kidney)
• Parts:
  • Renal corpuscle: Glomerulus + Bowman's capsule
  • Renal tubule: PCT → nephron loop (Loop of Henle) → DCT → collecting duct

Types of Nephrons

• Cortical Nephrons (85%): Short loops, mostly in cortex
• Juxtamedullary Nephrons (15%): Long loops deep into medulla; important for maintaining concentration gradient

Processes in Nephron

1. Glomerular Filtration:
   • Plasma pushed from glomerulus into Bowman's capsule
   • Filters water, electrolytes, glucose, urea, amino acids (substances <4nm)
2. Tubular Reabsorption:
   • Substances reclaimed into blood (mainly at PCT)
   • 67% of filtrate reabsorbed at PCT
3. Tubular Secretion:
   • Selected substances moved from blood into tubules (e.g., H⁺, K⁺, creatinine, drugs)
4. Excretion:
   • Final urine passes to ureter

Glomerular Filtration Rate (GFR)

• Normal GFR: 125 mL/min = 180 L/day
• Factors Influencing GFR:
  • Hydrostatic pressure in glomerular capillaries (HPgc)
  • Oncotic pressure in glomerular capillaries (OPgc)
  • Hydrostatic pressure in Bowman's space (HPbs)
  • Oncotic pressure in Bowman's space (typically negligible)
• Net Filtration Pressure (NFP): Always favors filtration

GFR Regulation

• Autoregulation:
  • Myogenic mechanism: Stretch of afferent arteriole triggers constriction
  • Tubuloglomerular feedback: Macula densa cells sense NaCl levels and adjust arteriole resistance
• Juxtaglomerular Apparatus (JGA):
  • Macula densa cells: Sense electrolyte levels
  • Juxtaglomerular cells: Sense blood pressure, secrete renin if needed

Hormonal Regulation

• Renin-Angiotensin-Aldosterone System (RAAS):
  • Triggered by low BP or low Na⁺
  • Increases aldosterone → Na⁺ and water reabsorption → raises blood volume and pressure
• Vasopressin (ADH):
  • Produced by hypothalamus, stored in posterior pituitary
  • Released in dehydration/high osmolarity
  • Increases water reabsorption by inserting aquaporins in DCT and CD
• Atrial Natriuretic Peptide (ANP):
  • Secreted by heart atria when stretched
  • Decreases Na⁺ reabsorption, opposes RAAS effects

Special Topics

• Kidney Stones (Renal Calculi):
  • Crystallized calcium, magnesium, or uric acid salts
  • Large stones can block ureters causing pain
  • Treatment: Shock wave lithotripsy (noninvasive)
• Acid-Base Balance (pH Buffering):
  • CO₂ + H₂O
    ightleftharpoons H₂CO₃
    ightleftharpoons H^+ + HCO₃^-
  • Tubular cells regulate H⁺ secretion and HCO₃⁻ reabsorption
• Potassium Balance:
  • High plasma K⁺ → aldosterone release → increased K⁺ secretion at DCT and CD

Summary of Main Transport Mechanisms

Osmolarity and Water Balance

• Osmolarity: Number of solute particles in 1 kg of water; reflects ability to cause osmosis.
• Body Fluid Osmolarity: Maintained at ~280–300 mOsm/kg.
• Dilute Urine: Produced when water intake is excessive (large volume, low osmolarity).
• Concentrated Urine: Produced when water loss is significant (small volume, high osmolarity).

Countercurrent Mechanisms

• Countercurrent Multiplier (Nephron Loop):
  • Descending Limb: Permeable to water, impermeable to ions.
  • Ascending Limb: Impermeable to water, actively transports ions (uses ATP).
• Countercurrent Exchanger (Vasa Recta):
  • Blood flows in opposite directions in limbs, maintaining medullary osmotic gradient.

Water Reabsorption

• 60% of water reabsorbed at PCT.
• 15% reabsorbed at thin descending limb of Loop of Henle.
• 20% available for reabsorption at DCT and CD (under hormonal control, e.g., ADH).
• Aquaporins:
  • Always present in PCT.
  • Inserted in collecting ducts only if ADH is present.

Electrolyte and Acid-Base Balance

• Blood composition depends on:
  • Diet
  • Cellular metabolism
  • Urine output
• Kidneys maintain blood composition by:
  • Excreting wastes
  • Maintaining water balance
  • Maintaining electrolyte balance
  • Regulating blood pH

Hormonal Regulation of Urine Output

ADH (Antidiuretic Hormone/Vasopressin)

• Inhibits urine formation by increasing water reabsorption.
• Triggered by:
  • High plasma osmolarity (osmoreceptors)
  • Low blood pressure (baroreceptors)
  • Low blood volume
• Produced by hypothalamus, released by posterior pituitary.

Aldosterone

• Secreted from adrenal cortex.
• Triggered by:
  • Low blood volume or pressure (via RAAS)
  • High K⁺ in ECF
• Effects:
  • Increases Na⁺ reabsorption (and water follows).
  • Increases K⁺ secretion.

Atrial Natriuretic Peptide (ANP)

• Secreted by atrial cells when stretched by high blood volume.
• Effects:
  • Inhibits Na⁺ and water reabsorption.
  • Promotes Na⁺ excretion.
  • Antagonizes Aldosterone.

Actions of Angiotensin II

• Stimulates Aldosterone release.
• Acts as a vasoconstrictor → increases total peripheral resistance → increases blood pressure.
• Stimulates ADH release and thirst.

Acid-Base Homeostasis

• Acid Sources:
  • Phosphoric Acid, Lactic Acid, Fatty Acids
  • Carbonic Acid (from CO₂)
  • Ammonia (NH₄⁺ is a base)

Three Lines of Defense Against pH Imbalances:

1. Buffer Systems (Immediate):
   • Bicarbonate Buffer System (ECF): HCO₃^- + H^+ ⇌ H₂CO₃
   • Phosphate Buffer System (ICF): HPO₄^{2-} + H^+ ⇌ H₂PO₄^-
   • Protein Buffer System: Proteins bind H⁺ ions.
   • Hemoglobin in RBCs (Bohr Effect)
2. Respiratory Compensation (Minutes):
   • Controls blood pH by altering breathing rate.
   • Hyperventilation → decreases CO₂ → increases pH (respiratory alkalosis).
   • Hypoventilation → increases CO₂ → decreases pH (respiratory acidosis).
3. Renal Compensation (Hours to Days):
   • Adjusts H⁺ secretion and HCO₃⁻ reabsorption.
   • Corrects for persistent acidosis or alkalosis.

Respiratory System Control of Acid-Base Balance

• ↑ Ventilation:
  • Decreases CO₂ → decreases H⁺ → pH rises (more alkaline).
• ↓ Ventilation:
  • Increases CO₂ → increases H⁺ → pH drops (more acidic).

Types of Acid-Base Disorders

Respiratory Acidosis

• Cause: Hypoventilation → ↑ CO₂ → ↑ H⁺ → ↓ pH
• Compensation: Kidneys secrete H⁺ and reabsorb HCO₃⁻.

Respiratory Alkalosis

• Cause: Hyperventilation → ↓ CO₂ → ↓ H⁺ → ↑ pH
• Compensation: Kidneys decrease H⁺ secretion and HCO₃⁻ reabsorption.

Metabolic Acidosis

• Cause: Loss of bicarbonate (e.g., diarrhea), or acid overproduction (e.g., lactic acidosis, ketoacidosis).
• Compensation:
  • Respiratory: Hyperventilation.
  • Renal: Increase H⁺ secretion, increase HCO₃⁻ reabsorption.

Metabolic Alkalosis

• Cause: Excessive vomiting, excessive intake of alkaline substances (e.g., baking soda).
• Compensation:
  • Respiratory: Hypoventilation.
  • Renal: Decrease H⁺ secretion, decrease HCO₃⁻ reabsorption.

Key Equations

• Carbonic Anhydrase Reaction:
  • CO₂ + H₂O ⇌ H₂CO₃ ⇌ H^+ + HCO₃^-
  • Important for both buffering blood and respiratory regulation of pH.

Summary Tables

Hormone Action

Compensation for Conditions