So the organism doesn't lose too much moisture and dehydrate, or absorb too much moisture from the environment and disrupt it's equilibrium/homeostasis. Neither scenario is healthy for the organism.
Hope this helps! :)
Blood delivers at the arterial end of a capillary bed and picks up at the venous end of a capillary bed. This is because the pressures in and out are close to balancing.
If hydrostatic pressure is 35 mm Hg at the arteriolar end and the colloidal osmotic pressure is 25 mm Hg. This means more pressure forcing fluid out of the capillary. At the venular end the hydrostatic pressure is 18 mm Hg and the colloidal osmotic pressure is still the same, so fluid returns to the capillary.
Need the descriptions in order to answer the question.