What do capillaries do? Im confused?

I understand that they exchange nutrients and wastes and theyre really small in diameter and allow for diffusion but what are they diffusing? and is the blood passing from the arteries to the veins the waste oxygen poor blood? because dont veins contain deoxygenated blood. As well, I dont t understand what... show more I understand that they exchange nutrients and wastes and theyre really small in diameter and allow for diffusion but what are they diffusing? and is the blood passing from the arteries to the veins the waste oxygen poor blood? because dont veins contain deoxygenated blood. As well, I dont t understand what hydrostatic pressure and osmotic pressure mean please help i hate bio and this concept
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