NURS 331: Hydrostatic, osmotic (oncotic) pressures, capillary filtration and perfusion Video

Introduction

  • Purpose of the video: Explain how hydrostatic and osmotic pressures drive fluid dynamics in blood vessels and relate to edema.

  • Edema: Definition as tissue swelling.

  • Context: Focus on capillaries, important for nutrient and waste exchange.

Capillary Structure and Function

  • Capillaries: Key features

    • Thin walls

    • Pores and gaps

    • Designed for nutrient, gas, waste, and fluid exchange between blood and tissues.

  • Importance in cardiovascular system: Site for fluid exchange.

  • Fluid dynamics at capillary ends:

    • Arteriole end: Fluid is filtered out into interstitial space.

    • Venous end: Reabsorption of filtered fluid to prevent swelling.

    • Edema: Occurs if excess fluid is retained in tissues.

Fluid Movement in Capillaries

Hydrostatic Pressure

  • Definition: Pressure exerted by a fluid at rest due to gravitational force.

  • Key concept illustration: Straw example

    • Finger blocking one end allows fluid to fill straw.

    • Poking a hole in the straw causes water to leak out due to gravity's hydrostatic pressure.

  • Application in capillaries:

    • Fluid is forced out due to hydrostatic pressure at the arteriole end through pores in capillary walls.

    • Example scenario: Fluid movement parallels behavior in a straw.

Osmotic Pressure

  • Definition: Pressure that drives water movement based on solute concentration.

  • Key principle of osmosis:

    • Water moves towards higher osmolarity (higher solute concentration).

  • Example scenario:

    • Vessel with pores, filled with high concentration of large particles (e.g., proteins).

    • Larger particles like protein molecules cannot pass out, creating a concentration gradient.

  • Consequence of high internal concentration:

    • Water reabsorbed back into the vessel (like capillary dynamics) due to osmotic pressure.

  • Specific type mentioned: Colloid Osmotic Pressure (or Oncotic Pressure)

    • Caused by large molecules (e.g., proteins like albumin).

  • Comparison: Other solutes like Na+ and Cl- in blood do not affect osmotic pressure.

Balancing Hydrostatic and Osmotic Pressures

  • At the arteriole end:

    • Hydrostatic pressure > Osmotic pressure leading to fluid filtration into interstitial space.

  • As blood moves through the capillaries:

    • Hydrostatic pressure decreases.

    • Osmotic pressure becomes equal to or exceeds hydrostatic pressure at the venous end:

    • Result: Most fluid reabsorbs back into capillary.

  • Remaining fluid:

    • A small fraction remains in interstitial space after reabsorption.

Role of the Lymphatic System

  • Function: Reabsorb the remaining filtered fluid that osmotic pressure did not reclaim.

  • Importance: Helps prevent edema by managing excess interstitial fluid.

Conclusion

  • Summary of fluid dynamics:

    • Hydrostatic pressure drives fluid out; osmotic pressure drives it back in.

  • Upcoming topics: Explanation of edema causes, especially what happens when standing or sitting for too long.