Understanding Hydrostatic and Osmotic Pressure in Capillaries

Introduction

• Focus: Role of hydrostatic and osmotic pressure in fluid movement in blood vessels.
• Application: Helps explain edema (tissue swelling).

Capillary Function

• Capillaries allow exchange of nutrients, gases, wastes, and fluids between blood and tissues.
• At arteriole end, fluid is filtered out of the blood vessel.
• Fluid needs to be reabsorbed at the venous end to prevent swelling (edema).

Causes of Fluid Movement

Hydrostatic Pressure

• Drives fluid out of capillaries.
• Example: Water in a straw with a hole at the bottom shows how gravity generates hydrostatic pressure.
• In capillaries, fluid is forced out by hydrostatic pressure through pores in capillary walls.

Osmotic Pressure (Colloid Osmotic Pressure)

• Water flows towards high solute concentration (high osmolarity) by osmosis.
• Large protein molecules (e.g., albumin) in blood create a high concentration inside the vessel.
• Water is reabsorbed into vessels due to osmotic pressure.
• Osmotic pressure primarily due to proteins, not smaller solutes like Na+ or Cl-.

Interplay of Pressures in Capillaries

• On arteriole end: Hydrostatic pressure > Osmotic pressure, leading to fluid being forced out.
• Through capillary: Hydrostatic pressure decreases, osmotic pressure becomes greater.
• At venous end: Fluid reabsorbed due to higher osmotic pressure.
• Some fluid remains in tissues; lymphatic system reabsorbs this to prevent edema.

Conclusion

• Hydrostatic pressure drives fluid out, osmotic pressure drives it back in.
• Lymphatic system assists in reabsorbing remaining fluid.
• Future videos will explore cases of fluid accumulation due to changes in these pressures, like ankle swelling from standing too long.