"Capillary" - what is it, definition of the term
A capillary is a microscopic conduit with an internal diameter typically measured in micrometres, whose dimensions enable fluid movement driven primarily by surface tension and adhesive forces between the liquid and the tube walls; this phenomenon, known as capillary action, permits liquids to ascend or descend within the conduit without external pressure, forming the basis for fluid transport in porous media, biological tissues, and various analytical instruments.
Detailed information
Microscopic blood vessels form the terminal network of the circulatory system, linking arterioles to venules. Their walls consist of a single layer of endothelial cells supported by a basement membrane, allowing direct contact between circulating plasma and surrounding tissues. The narrow lumen, typically 5–10 µm in diameter, creates a high surface‑to‑volume ratio that maximizes exchange efficiency.
Physiological functions include:
- Diffusion of oxygen, carbon dioxide, nutrients, and metabolic waste between blood and interstitial fluid.
- Regulation of fluid balance through Starling forces, with hydrostatic pressure driving filtration and oncotic pressure promoting reabsorption.
- Participation in thermoregulation by adjusting blood flow to the skin surface.
- Contribution to immune surveillance as a conduit for leukocyte migration during inflammatory responses.
Structural adaptations support these roles. Endothelial cells display fenestrations or continuous tight junctions depending on tissue type, optimizing permeability for specific organs. Pericytes encircle the vessel, providing contractile control over diameter and influencing blood‑brain barrier integrity. Basement membrane composition varies, affecting protein filtration and angiogenic signaling.
Clinical relevance centers on dysfunction of these tiny vessels. Increased permeability leads to edema in inflammatory conditions, while reduced flow contributes to ischemic injury in diabetic microangiopathy. Therapeutic strategies target endothelial signaling pathways to restore normal fluid dynamics and prevent pathological leakage.
Ectoparasites such as ticks, true bugs, lice, and fleas exploit this vascular network during blood feeding. Their mouthparts penetrate the skin to access the lumen of these small vessels, where rapid blood uptake is facilitated by the high pressure gradient. Salivary secretions from these arthropods contain anticoagulants and vasodilators that widen the vessel lumen, enhancing feeding efficiency. Simultaneously, the intimate contact between parasite saliva and endothelial surfaces provides a route for pathogen transmission, including bacteria, viruses, and protozoa, underscoring the importance of these vessels in vector‑borne disease cycles.