In addition to containing aquaporins, red cell membranes also contain urea transporters . Urea enters cells through urea transporters and water enters cells through water channels. Even though water molecules are smaller than urea molecules, water molecules do not pass through urea transporters. Urea transporters are needed in the kidneys to conserve water when blood is too concentrated. To conserve this water, the kidneys need to flush out urea without flushing water at the same time.
Water channels are equally important.
Water flows in and out of cells throughout the body except in some places in the kidneys. The AVP initiates a series of actions which results in water channel proteins being sent to cells which need more water. Once the water channel proteins are in place, the cell absorbs the water it needs. When the correct balance between ECF and ICF has been restored, the water channel proteins are removed from the cell membrane. It is the major water channel of the red blood cells .