Blood vessels and aquaporins, illustration. Aquaporins are integral membrane proteins that aid the transport of water between cells. They are also referred to as water channels. They help regulate water levels in the bloodstream, as they are smaller in size (0.5 nanometres) than the small pores (5 nanometres) and large pores (25 nanometres) shown here. This process of water transport is particularly important in the kidneys, where water is reabsorbed into the bloodstream.