smallcapillary

Small capillary refers to the narrow segments of the capillary network within the microcirculation. In humans, capillary lumens commonly range from about 5 to 10 micrometers in diameter, with red blood cells averaging around 7 to 8 micrometers. As a result, many capillaries are just large enough to accommodate red blood cells, often passing them with deformation or in a single-file arrangement.

Capillaries are classified by permeability into continuous, fenestrated, and discontinuous types. Small capillaries are typically continuous in tissues such as muscle and connective tissue, where they provide selective exchange of water, ions, and small solutes. Fenestrated capillaries, found in kidneys and endocrine organs, and discontinuous capillaries, seen in liver and bone marrow, support more extensive exchange and may occur in tissue areas with different metabolic demands.

Functions of small capillaries include facilitating the exchange of oxygen and carbon dioxide, nutrients, and metabolic wastes between blood and surrounding tissue. They also contribute to fluid balance within tissues and participate in immune cell trafficking during inflammation and response to injury.

Clinical relevance: microvascular function in small capillaries can be altered by conditions such as hypertension and diabetes, leading to changes in perfusion, permeability, or capillary density. Noninvasive imaging and functional tests assess microvascular health by examining capillary density, flow, and exchange efficiency.

Research and applications: in microfluidics and tissue engineering, small capillary–like microchannels model microcirculation to study diffusion, endothelial barrier properties, and pharmacologic effects on exchange processes.