Rapid diffusion of water across cell membranes depends on the presence of water channels, called aquaporins. Proximal convoluted tubule have:

Aquaporin-1 is localized to both the basolateral and apical membrane of the proximal tubules and its presence allows water to move rapidly out of the tubule along the osmotic gradients set up by active transpo of solutes, and isotonicity is maintained. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 37. Renal Function & Micturition. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.