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Urinary System - 2

Mar 10, 2025

Human Anatomy and Physiology: Urinary System and Fluid Balance

Key Topics

  • Fluid Movement
  • Hydrostatic and Osmotic Pressure
  • Capillary Exchange
  • Facilitated Diffusion
  • Net Filtration Pressure

Fluid Movement

  • The body consists of various compartments where water and elements move.
  • Hydrostatic pressure is the force exerted by fluid against a wall.
  • Blood exerts pressure on blood vessel walls through the heart's pumping action, especially in capillaries.

Hydrostatic and Osmotic Pressure

  • Hydrostatic Pressure:

    • Higher at the arterial end of capillaries, pushing plasma and nutrients out into tissues.
    • Decreases at the venous end, allowing cellular waste to enter capillaries.
    • Important in kidneys for filtering blood to form urine.
    • If too low (e.g., dehydration), kidney function is impaired.
  • Colloid Osmotic Pressure:

    • Opposes hydrostatic pressure, retaining fluid within containers like sponges.
    • Albumin is a key protein that cannot leave the container, providing osmotic pressure.

Capillary Exchange

  • At the arterial end, higher hydrostatic pressure results in filtration into tissues.
  • Venous end sees reabsorption due to lower hydrostatic pressure and higher osmotic pressure.
  • Surplus interstitial fluid drained by the lymphatic system.

Facilitated Diffusion

  • Movement of molecules, such as glucose, down a concentration gradient through carrier proteins.
  • Passive transport relies on the ability to pass through membranes and concentration gradients.
  • Water, gases, and lipids pass easily, while glucose requires facilitated diffusion.

Net Filtration Pressure

  • Combination of osmotic and hydrostatic pressures.
  • Positive net filtration pressure at the arterial end leads to filtration.
  • Negative net filtration pressure at the venous end leads to reabsorption.
  • Critical for kidney function in blood filtration and urine production.

Analogies

  • Bucket Analogy:
    • Bucket with no sponges leaks water easily.
    • More sponges retain more water, analogous to albumin in the body.