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.