= Capillary bed
A \b[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.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§20.2, blood flow, blood pressure and resistance]{ref}
The aggregate <cross-sectional area> helps explain why narrow individual vessels need not imply rapid transit. In an ideal model with $N$ identical open <capillaries> of <radius> $r$ sharing total <volumetric flow rate> $Q$ equally, the mean axial <velocity> is
$$
\boxed{\bar v=\frac{Q}{N\pi r^2}.}
$$
At fixed $Q$ and $r$, doubling $N$ halves $\bar v$. 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.<Oncotic pressures opposing filtration across non-fenestrated rat microvessels by R. H. Adamson and others>[abstract, matched albumin concentrations and glycocalyx interpretation]{ref}
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