Aquaporins, also known as water channels, are cell membrane proteins that facilitate the flow of water across the cell membrane. Water can diffuse slowly through the cell membrane unfacilitated, however aquaporins allow the rapid movement of water in and out of the cell. This enables cells to regulate internal osmotic pressure.

Aquaporins are present predominately in the kidneys of mammals, but are also found in plants. The most researched aquaporins are aquaporin 1, aquaporin 2, aquaporin 3, and aquaporin 4.

Aquaporin 1 (AQP1) facilitates water reabsorption through the apical and basolateral cell membranes of the cells lining the proximal convoluted tubule, proximal straight tubule, and the thin descending loop of Henle in the kidneys. Aquaporin 1 is also expressed in red blood cells, vascular endothelium, the gastrointestinal tract, sweat glands, and the lungs.

Aquaporin 2 (AQP2) facilitates water reabsorption in response to antidiuretic hormone (vasopressin) through the apical cell membrane of cells lining the initial collecting tubule, cortical collecting tubule, outer medullary collecting duct, and inner medullary collecting duct in the kidneys. Aquaporin 2 is stored in the membranes of vesicles until signalled by antidiuretic hormone to move to the cell membrane to reabsorb water from urine.

Aquaporin 3 (AQP3) facilitates water reabsorption and glycerol permeability through the basolateral cell membranes of the cells lining the medullary collecting duct in the kidneys. Aquaporin 3 is also present in the keratinocytes of the skin where it regulates skin hydration, wound healing, and tumorigenesis. It can also be found in immature dendritic cells of the immune system, and red blood cells.

Aquaporin 4 (AQP4) facilitates water reabsorption through the basolateral cell membranes of the cells lining the medullary collecting duct in the kidneys. Aquaporin 4 is also expressed in the astrocytes in the central nervous system where it controls water flow.

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