Blood is pumped from the heart and passes through arteries, then Arteroles, then Capillaries.
These blood vessels become increasingly narrower. This creates a pressure called electrostatic pressure at the arteriole end of the capillaries.
This pressure forces tissue fluid out of the blood plasma, but is opposed by the hydrostatic pressure outside the capillaries and low water potential of the blood, causing water to be drawn back into the capillaries.
This results in a lower pressure that can only force small molecules out of the blood of capillaries, leaving behind large molecules such as cells or protiens.
This process is called Ultrafiltation.
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