Microcirculation and Lymphatics Flashcards
What are the mechanisms by which substances move through the capillary endothelium?
Diffusion
Bulk Flow
Vesicles
Active Transport
What types of molecules move by diffusion?
Gases and lipid-soluble substances
Movement is described by Fick’s First Law of diffusion
Describe bulk flow.
Occurs through intercellular clefts between endothelial cells
Water, Lipid-insoluble substances, electrolytes
Occurs in fenestrated and discontinuous capillaries
Describe vesicular transport
Macromolecules
Relatively minor role in transcapillary exchange
What are three ways to increase the rate of oxygen diffusion can be enhanced?
Increasing capillary blood pO2
Decreasing tissue pO2
Increasing the surface area available for oxygen diffusion
What does the diffusion of carbon dioxide depend on?
Primarily on the blood flow
Carbon dioxide readily diffuses from cells into the blood
Describe transcapillary fluid exchange
Crucial that a steady state is achieved so that volume is maintained in the extravascular compartment
Most filtered fluid returns to the blood at the venular end
Terminal lymphatics take up the excess fluid and proteins and transport them into larger lymphatics that leave the tissue
What are the physical mechanisms governing fluid exchange?
Hydrostatic pressure and oncotic pressure in each the capillary and interstitial fluid
How does arterial and venous pressure affect capillary hydrostatic pressure?
Increase in arterial or venous pressure elevates capillary hydrostatic pressure
How does arteriolar and venous resistance affect capillary pressure?
Arteriolar resistance - reduces capillary pressure
Venous resistance - Increases capillary pressure
What does Interstitial hydrostatic pressure depend on?
Tissue fluid volume/dehydration
The effect of intersitial fluid volume on interstitial pressure is determined by interstitial compliance
P = V/C
Describe terminal lymphatic capillary flow.
About 10% of filtered fluid enters the lymphatic system
Substances of high molecular weight can enter the lymphatic capillaries relatively unimpeded due to their special structure
What is the effect of intersitial fluid on lymph flow?
Any factor that increases interstitial fluid pressure also increases lymph flow
However there is a maximum lymph flow rate due to fluid compressing on the lymphatic capillaries
What is the major force for propelling lymph?
The intrinsic lymph pump
Lymph vessels beyond the terminal lymphatic capillaries are surrounded by smooth muscle that automatically contracts when when the vessels are stretched with fluid
External compression will also propel lymph (e.g. muscle contraction during exercise)
How does edema form?
Fluid volume within the interstitial compartment increases because filtration exceeds the rate of reabsorption plus lymphatic flow
The compartment increases in volume
