A capillary bed is a network of capillaries connecting arterioles to venules. It spreads blood flow across many small vessels, providing a large exchange surface between blood plasma and surrounding interstitial fluid. Upstream vascular resistance and the arrangement of the network affect how much blood reaches that surface.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. §20.2, blood flow, blood pressure and resistance
The aggregate cross-sectional area helps explain why narrow individual vessels need not imply rapid transit. In an ideal model with identical open capillaries of radius sharing total volumetric flow rate equally, the mean axial velocity isAt fixed and , doubling halves . This is a conservation of mass calculation; actual exchange also depends on the wall, the exchanged substance and uneven flow through real networks.
The endothelial glycocalyx is one such wall feature. R. H. Adamson and colleagues found that effective oncotic pressure still opposed filtration when bulk albumin concentrations on both sides of rat microvessels were matched. Their imaging and modelling implicated the local environment near the endothelial glycocalyx, showing why bulk interstitial fluid measurements alone do not specify the filtration barrier.[ref]
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