Overview
This lecture explains the forces driving capillary exchange, including diffusion, filtration, and reabsorption, and their importance in nutrient delivery, waste removal, and fluid balance.
Blood Pressure in Circulatory System
- Blood pressure is highest in elastic arteries near the heart and decreases continuously throughout the circulatory system.
- Capillaries have low pressure but high total cross-sectional area, resulting in high resistance and slow blood flow.
- Slow capillary blood flow allows time for exchange of materials between blood and tissues.
Structure of Capillaries
- Capillary walls consist of a single layer of simple squamous endothelial cells.
- Thin walls and occasional junctions or pores facilitate exchange between blood and interstitial fluid.
Capillary Exchange Mechanisms
- Exchange between blood and interstitial fluid involves diffusion, filtration, and reabsorption.
Diffusion
- Diffusion is the passive movement of substances from high to low concentration, requiring no energy.
- Small hydrophilic (water-loving) molecules like water, ions, and glucose diffuse via clefts or channels.
- Fenestrated capillaries, found in intestines, kidneys, and endocrine glands, have larger pores for rapid diffusion.
- Large water-soluble molecules (e.g., polypeptides) require fenestrated capillaries to cross.
- Lipid-soluble molecules (e.g., O2, CO2) diffuse directly through endothelial cells.
- Large molecules (e.g., plasma proteins, blood cells) can only cross in sinusoidal capillaries (liver, bone marrow).
Filtration
- Filtration is driven by hydrostatic pressure, forcing water and small solutes from blood into interstitial fluid.
- Capillary wall acts as a filter, trapping large solutes like proteins and blood cells inside the bloodstream.
Reabsorption
- Reabsorption returns fluid from interstitial fluid to blood, driven by osmotic (colloid) pressure.
- Osmosis is the diffusion of water toward the side with higher solute concentration.
- Blood colloid osmotic pressure results mainly from plasma proteins that cannot cross capillary walls.
Importance of Filtration and Reabsorption
- The balance between filtration and reabsorption creates a flushing action, keeping plasma and interstitial fluid in communication.
- This process accelerates nutrient and waste exchange and supports immune surveillance by moving fluid into the lymphatic system.
Key Terms & Definitions
- Diffusion — passive movement of molecules from high to low concentration.
- Filtration — movement of water and small solutes out of blood due to hydrostatic pressure.
- Reabsorption — movement of water back into blood due to osmotic pressure.
- Hydrostatic Pressure — physical force exerted by fluid pressing against vessel walls.
- Osmotic Pressure — pressure exerted by solutes drawing water into a solution.
- Blood Colloid Osmotic Pressure — osmotic pull created by plasma proteins in blood.
- Fenestrated Capillaries — capillaries with large pores for rapid exchange.
- Sinusoidal Capillaries — specialized capillaries with large gaps for big molecules.
- Interstitial Fluid — fluid surrounding tissue cells outside blood vessels.
Action Items / Next Steps
- Review the mechanisms of diffusion, filtration, and reabsorption.
- Study the structure and function differences between continuous, fenestrated, and sinusoidal capillaries.
- Prepare to discuss the role of capillary exchange in immune function in the next class.