Three major types of capillaries: Continuous, Fenestrated, Discontinuous (Sinusoids).
Continuous Capillaries
Most common type in the body.
Structure consists of four cells with nuclei and a red blood cell moving through.
Intercellular Cleft:
A gap between cells.
Visible in cross-section.
Tight Junctions:
Areas where cells join tightly.
Basement Membrane:
Layer beneath cells, not visible to blood except at intercellular clefts.
Acts like a foundation, largely made of protein.
Fenestrated Capillaries
Characterized by fenestrations (pores or holes).
Still contain intercellular clefts and a basement membrane.
Glycocalyx:
A slime layer inside endothelial cells, made of sugars and proteins.
Can span pores, creating diaphragms.
Diaphragms not always present.
Discontinuous Capillaries (Sinusoids)
Largest capillaries, found in liver, spleen, bone marrow.
Very leaky due to:
Large intercellular clefts.
Often incomplete basement membrane.
Glycocalyx present but less effective due to larger gaps and fewer tight junctions.
Leakiness and Transport Across Capillaries
Increasing Leakiness
Leakiness increases from continuous to discontinuous capillaries.
Modes of Transport
Diffusion:
Suitable for small, non-polar molecules (e.g., O2, CO2).
Vesicular Transport:
Vesicles transport molecules across the cell.
Intercellular Cleft:
Larger molecules can pass through gaps between cells.
Pores in Fenestrated Capillaries:
Molecules can pass through fenestrations, possibly encountering glycocalyx.
Discontinuous capillaries offer all transport opportunities, highlighting their high leakiness.
Conclusion
Understanding the structure and function of different capillaries is crucial for appreciating how substances move in and out of the circulatory system.