Lecture Notes: Urinary System Functions

Key Functions of the Urinary System

• Filter Plasma and Eliminate Waste: Removes waste products from the blood.
• Regulate Blood Volume and Blood Pressure: Maintains homeostasis through volume and pressure control.
• Regulate Blood pH, Osmolarity, and Electrolyte Balance: Ensures proper chemical balance.
• Red Blood Cell Production: Aids in the production of red blood cells.

Waste and Excretion

• Metabolic Waste: Produced by bodily processes.
• Nitrogenous Waste: Most toxic form of waste.
• Blood Urea Nitrogen: Measures nitrogen waste in the blood.
• Excretion Methods:
  • Respiration (excretes CO2)
  • Digestive System (excretes cholesterol)
  • Urinary System (excretes metabolic waste, toxins, drugs, and excess salts)

Kidney Structure and Function

• Nephrons: Functional units of the kidney (~1.2 million per kidney).
  • Cortical Nephrons: 85%, located in cortex with short loops.
  • Juxtamedullary Nephrons: 15%, located in medulla with long loops for urine concentration.
• Blood Supply:
  • Two sets of arteries and capillaries.

Urine Formation Process

1. Glomerular Filtration: Creates a plasma-like filtrate of blood.
2. Tubular Reabsorption and Secretion:
   • Reabsorbs useful solutes back into the blood.
   • Secretes waste from blood into filtrate.
3. Water Conservation: Removes water from urine, concentrating waste.

Filtration and Reabsorption

• Filtration Pressure: Driven by high to low pressure gradient.
• Autoregulation:
  • Myogenic Mechanism: Muscle contraction in response to stretching.
  • Tubuloglomerular Feedback: Fluid flow feedback to the arterioles.
• Sympathetic Nervous System: Affects renal blood flow and filtration.

Regulation of Reabsorption and Secretion

• Proximal Convoluted Tubule: Major site for reabsorption (65% of filtrate).
• Regulated by Hormones:
  • Aldosterone and ANP: Opposite effects on sodium reabsorption.

Hormonal Regulation

• Renin-Angiotensin-Aldosterone System:
  • Activated by low blood volume/BP.
  • Increases sodium and water reabsorption.
  • Leads to vasoconstriction and increased BP.
• Atrial Natriuretic Peptide (ANP):
  • Activated by high BP/volume.
  • Decreases sodium and water reabsorption.

Osmolarity and Urine Concentration

• Osmolarity Gradient: Created in renal medulla by nephron loop.
• Countercurrent Multiplier and Exchanger: Essential for urine concentration.
• ADH: Regulates water channels and water reabsorption.

Urine Characteristics

• Normal Urine: Colorless to amber, clear, distinctive odor.
• Abnormal Content: No protein, glucose, WBCs, or blood.

Diuretics and Diabetes

• Diuretics: Increase urine volume (e.g., caffeine, alcohol).
• Diabetes Mellitus: Glucose not fully reabsorbed, leading to high water excretion.
• Diabetes Insipidus: Lack of ADH response leads to high water excretion.