Urinary System: Filtrate Formation and Processes

Overview

• Filtration: Process where water and solutes are filtered from blood plasma as it flows through the glomerulus.
• Filtrate: Not urine; material in filtrate is reclaimed through reabsorption and secretion.
• Kidney Processes:
  • Filtration
  • Reabsorption
  • Secretion

Transition from Filtrate to Urine

• Tubular Fluid: Once filtrate enters proximal convoluted tubule.
• Pathway:
  • From renal papilla to pelvis, then ureter, urinary bladder, and out through the urethra.
  • Papilla ends of medullary pyramids.
  • Pelvis collects urine before going to ureter.

Detailed Filtrate Pathway

• Capsular Space: Initial formation of filtrate.
• Tubular Fluid: Moves through several parts:
  • Parts 2 through 7 in the nephron.
  • Collects at papilla, enters minor calyces, then major calyces, renal pelvis, and eventually ureters.
• Kidney Stones: Form in the renal pelvis.

Urine Formation Processes

• Filtration: From blood to tubule, involves water and solutes.
• Reabsorption: From tubule to capillaries (peritubular capillaries or vasa recta).
  • Utilizes diffusion, osmosis, active transport.
  • Vital solutes reabsorbed include glucose, amino acids.
  • 180L of blood filtered daily; only 1.5L of urine produced, indicating massive reabsorption.
• Secretion: Movement of solutes from blood capillaries to tubule for elimination.
  • Regulates pH, hydrogen, potassium ions, drug metabolites.

Filtration Membrane Characteristics

• Similar to Respiratory Membrane: Porous, thin, negatively charged.
• Components:
  • Fenestrated capillaries: Allow plasma and dissolved substances to pass.
  • Basement Membrane: Restricts large plasma proteins.
  • Slit Diaphragm: Negatively charged, repels proteins.
  • Podocytes: Octopus-like cells with foot processes forming filtration slits.

Filtration Membrane Functionality

• Bulk Flow: Driven by hydrostatic pressure; not selective.
• Prevents Passage: Large substances like erythrocytes and proteins.
• Mesangial Cells: Between capillary loops, phagocytic, contractile.

Filtration Dynamics

• High Hydrostatic Pressure: 60 mmHg in glomerulus.
  • Larger diameter of afferent arteriole vs. smaller efferent arteriole.
• Opposing Pressures:
  • Osmotic pressure from plasma proteins.
  • Capsular Hydrostatic Pressure from filtrate.
  • Net Filtration Pressure: Promotes filtration when positive.

Glomerular Filtration Rate (GFR)

• Rate: 125 mL/min; 180L/day filtered, 1.5L/day urine.
• Regulation:
  • Intrinsic controls at nephron level.
  • Extrinsic controls affecting systemic blood pressure.
  • Affects luminal diameter of afferent arterioles and filtration membrane surface area.

Intrinsic and Extrinsic Regulation

• Intrinsic Controls: Adjust GFR at each nephron.
• Extrinsic Controls: Systemic blood pressure regulation.

Key Takeaways

• Surface Area and Diameter Regulation: Impact GFR.
• Conditions Affecting GFR: Hydration levels, systemic blood pressure, physiological conditions.
• Importance of Regulation: To prevent loss of essential substances and maintain homeostasis.