Urinary System: Concentration of Dissolved Particles

Introduction

• Focus on maintaining constant concentration of dissolved particles in blood (osmolarity).
• Osmolarity: Number of particles per liter. Different from molarity, which is the number of molecules in a solution.
  • Example:
    • Glucose: 1 mole = 1 osmole (doesn't dissociate in water).
    • Sodium chloride (NaCl): 1 mole = 2 osmoles (dissociates into Na+ and Cl- ions).
• Blood osmolarity: ~300 milliosmoles (mOsm).
• Control System: Maintains osmolarity despite perturbations (e.g. water intake, diarrhea).

Osmolarity and Urine

• Kidneys modulate urine concentration to maintain blood osmolarity.
• Urine color indicates osmolarity:
  • Dark urine = High osmolarity.
  • Clear urine = Low osmolarity.

Comparison with Lungs

• Lungs: Simple exchange organ using diffusion to equalize pressure of gases in blood and inspired air.
• Kidneys: More complex, dealing with different osmolarity inside the body vs. in urine.

Kidney Function Overview

1. Filtration: Removes water and waste products from the blood, creating filtrate.
2. Reabsorption: Nutrients are reabsorbed from filtrate.
3. Secretion: Further toxins are deposited into the urine.
4. Tuning: Adjusts concentration to maintain blood osmolarity.
5. Excretion: Elimination of urine.

Filtration in Kidneys

• Filtration Mechanism:
  • Semi-permeable membrane allows water movement towards higher dissolved particle concentration.
  • Osmotic Pressure: Mechanical pressure from water movement across membrane.

Hydrostatic Pressure

• Hydrostatic Pressure: Pressure exerted by weight of water column.
• Balance between hydrostatic and colloid osmotic pressure determines filtration or absorption.

Filtration and Absorption in Capillaries

• Capillary Dynamics:

  • Higher hydrostatic pressure on arterial side favors filtration.
  • Collard osmotic pressure (from proteins) resists water expulsion.
  • Downstream, lower hydrostatic pressure favors absorption.

• Lymphatic System: Manages water balance and supports roles like fat transport.

Kidney Anatomy

• Nephrons: Functional units of the kidney, responsible for filtration and reabsorption.
  • Bowman's Capsule: Captures filtrate from glomerulus.
  • Glomerulus: Capillary network facilitating filtration.

Filtration Process in Kidneys

• Glomerular Filtration:
  • Driven by high hydrostatic pressure.
  • Podocytes: Support structures for capillaries.
  • Filtration resisted by colloid osmotic pressure.

Kidney Structure

• Kidney: Factory of nephrons and capillaries.
• Cortex and Medulla: Organizational zones of the kidney.

Conclusion

• Filtration driven by hydrostatic pressure, aided by kidney anatomy.
• Filtration is a critical step in maintaining blood osmolarity.