Capillaries are the smallest of a body's blood vessels. They
connect arteries and veins. Capillaries have walls composed of a
single layer of cells, the endothelium. This layer is so thin
that molecules such as oxygen, water and lipids can pass through
them by diffusion and enter tissues. Waste products such as
carbon dioxide and urea can diffuse back into the blood to be
carried away for removal from the body. Capillary permeability
can be increased by the release of certain cytokines. Very large
molecules may be too big to diffuse across endothelial cells. In
some cases, vesicles contained in the capillary membrane use
endocytosis and exocytosis to transport material between blood
and the tissues. Some nutrients are also carried out by 'bulk
flow', the flow of 'water' (plasma) caused by a high pressure
inside the capillary. Nutrients are carried out through
capillary clefts. In an immune response, the endothelial cells
of the capillary will upregulate receptor molecules, thus
"catching" immune cells as they