Expansion
Diffusion for gases, bulk flow for water, vesicular transport for large molecules
Expansion
By mechanism
- Diffusion - the dominant route. Lipid-soluble substances such as oxygen and carbon dioxide cross the whole endothelial surface; water-soluble substances pass through intercellular clefts and pores
- Bulk flow (filtration and reabsorption) - governed by the Starling forces; important for fluid balance rather than nutrition
- Transcytosis - vesicular transport of large molecules such as proteins
By capillary type
- Continuous - muscle, skin, lung, and with tight junctions in the brain, forming the blood-brain barrier
- Fenestrated - gut, endocrine glands, renal glomerulus; permeable to water and small solutes
- Sinusoidal (discontinuous) - liver, spleen, bone marrow; large gaps permitting cells and proteins to pass
Exchange is favoured by the enormous total surface area (around 500 to 700 square metres) and by the very low velocity of capillary flow, giving red cells about one second of transit time.