The intracellular and extracellular compartments are separated from one another by the plasma
membrane of the cells. The extracellular
compartments (interstitial/
plasma/lymph) are separated by
a layer of endothelial cells
surrounded by a basement
membrane; the capillaries. To
cross from the plasma to the cells
or vice versa, substances must
either cross both membranes of
the endothelial cells or travel
between the cells and then cross
the basement membrane.

Capillaries come in three main types distinguished largely on the permeability of their walls.

1) Continuous capillaries have a close connection between adjacent cells and will permit only small molecules < 10nm in diameter to cross. Continuous capillaries surround muscle, skin lungs, adipose tissue CNS, retina and mammary glands.

2) Fenestrated capillaries contain
'windows' that offer easy passage to larger molecules(10-100nm) and are found around the kidneys, pancreas,
gallbladder and intestine.

3) Discontinuous have wide gaps between the cells and will allow practically anything (even cells)
across. Discontinuous capillaries surround the liver, spleen,
ovaries and some endocrine glands.

Capillaries act rather like a leaky hosepipe; although the bulk of
the fluid continues along the pipe, the pressure forces some
out of the walls. The fluid and soluble contents of plasma small enough to cross the capillary wall
circulate into the interstitial fluid at the high pressure arterial end of the capillary bed and returns
to the capillary, bringing with it
small soluble waste products from the cells, at the low
pressure venous end of the capillary bed (Oxygen and CO2
being lipid soluble, diffuse from the plasma across the capillaries and to and from the cells as necessary).

Hydrostatic (blood)
pressure is not the only force acting to cause fluid movement
in and out of the capillaries. The plasma proteins that cannot
cross the capillary walls exert an osmotic pressure to draw water back into the capillaries which
outweighs the hydrostatic pressure at the venous end of
the capillaries. The balance of hydrostatic and osmotic forces causing movement out and into the capillaries are known as Starling forces.