It is through that water and variety of solutes can diffuse through cell membranes. This passage may affect somewhat the biochemistry of the extracellular vs the plasma compartment. Not the osmotic pressure though.
The importance of capillaies permeability to fluid movement according to'Starling hypothesis' namely, the balance between colloid osmotic pressure (oncotic pressure ) and hydrostatic pressure between the [capillaries fluid compartment] and the [extracellular compartment] is that it controls the plasma and extracellular fluid compartments volume. The volume of the water that we are talking about is
only 2-3 liters in 24hrs. This volume is very meaningful because of its importance in controlling plasma volume.
Notably, while ions and water move freely, the permeability of albumin, a major player in determining plasma colloid osmotic pressure, is restricted and therefore its importance in plasma volume regulation. In most capillaries plasma COP is constant and the hydrostatic pressure decreases toward the venous side of the capillaries. The only vascular bed in which osmotic pressure increases toward the end of the capillary is
the kidney (in the glomerulus) and this is one of the determinant of glomerular filtration rate.
Re blood pressure: The major fall in blood pressure in the cardiovascular system is in the arterioles. The decrease in blood pressure (including the capillaries) is because of friction.
I hope this answer helps.
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