Overview
This lecture explains how hydrostatic and osmotic pressures regulate fluid movement in and out of capillaries, preventing tissue swelling (edema).
Capillaries and Fluid Exchange
- Capillaries have thin walls with pores, allowing nutrient, gas, waste, and fluid exchange between blood and tissues.
- Fluid is filtered out of blood at the arteriole end of the capillary into the interstitial space.
- Excess fluid in tissues can cause swelling or edema if not reabsorbed.
Hydrostatic Pressure
- Hydrostatic pressure is the force exerted by fluid inside blood vessels, pushing water out through the capillary walls.
- It is highest at the arteriole end of capillaries, promoting fluid movement out of the blood.
Osmotic (Colloid Osmotic/Oncotic) Pressure
- Osmotic pressure draws water into areas of higher solute (especially protein) concentration.
- Capillaries have a higher concentration of large proteins (like albumin) compared to the interstitial space.
- This protein-driven osmotic pressure is called colloid osmotic (or oncotic) pressure.
- Osmotic pressure pulls fluid back into the capillary, especially at the venous end.
Balance of Pressures and Fluid Reabsorption
- At the arteriole end, hydrostatic pressure > osmotic pressure, so fluid leaves the capillary.
- As blood moves to the venous end, hydrostatic pressure drops, and osmotic pressure becomes stronger, reabsorbing most fluid.
- Not all fluid is reabsorbed; the leftover is collected by the lymphatic system to prevent edema.
Key Terms & Definitions
- Hydrostatic Pressure — the force exerted by a fluid against the walls of its container, pushing fluid out of capillaries.
- Osmotic Pressure — the pressure that draws water towards higher solute concentrations; in capillaries, mainly due to proteins.
- Colloid Osmotic (Oncotic) Pressure — osmotic pressure generated by plasma proteins that cannot pass through capillary walls.
- Edema — swelling caused by excess fluid trapped in body tissues.
- Lymphatic System — a network of vessels that returns excess interstitial fluid to the bloodstream.
Action Items / Next Steps
- Review upcoming videos for abnormal cases of edema related to changes in hydrostatic or osmotic pressure.
- Prepare to learn examples, such as why ankles swell during prolonged standing.