Overview
This lesson covers the structure, types, and functions of capillaries, the smallest and most numerous blood vessels in the human body.
Structure and Function of Capillaries
- Capillaries are the thinnest blood vessels, much thinner than a hair.
- They form the largest circulatory network by surface area in the body.
- Capillaries connect terminal arteries with the roots of venules.
- Their main function is to enable exchange of oxygen and carbon dioxide between blood and tissues.
- Capillary walls are only 5 to 20 microns thick, often narrower than a red blood cell.
- Red blood cells may pass through capillaries in single file due to narrow diameter.
- The capillary wall consists of an inner endothelial layer, a basal membrane, and outer contractile cells called pericytes.
- Pericytes contract or relax to regulate blood flow through the capillary bed.
Types of Capillaries
- Continuous capillaries have an unbroken endothelial lining and are found where a blood-tissue barrier is needed, like the central nervous system and gonads.
- Discontinuous capillaries have gaps in the endothelial lining, allowing greater filtration, and are found in kidneys, gastrointestinal tract, and lungs.
- Sinusoid capillaries are wider, have incomplete endothelial linings, contain specialized macrophage-derived cells called Kupffer cells, and are found only in the liver.
Key Terms & Definitions
- Capillary — a thin blood vessel connecting arteries and veins, enabling exchange between blood and tissues.
- Endothelial layer — the innermost cell layer lining capillaries.
- Basal membrane — supporting structure beneath the endothelium.
- Pericytes — contractile cells around capillaries that control blood flow.
- Continuous capillaries — capillaries with a complete endothelial lining.
- Discontinuous capillaries — capillaries with gaps in the endothelium for filtration.
- Sinusoid capillaries — wide, discontinuous capillaries found in the liver.
- Kupffer cells — liver macrophage cells found in sinusoid capillaries.
Action Items / Next Steps
- Review the structure and types of capillaries.
- Prepare for a detailed discussion of liver sinusoid capillaries in the next lesson.