FLUID BALANCE
Fluid transport across cell membranes is largely driven by the movement of charged particles, called ions. Since ion channels govern the movement of ions across cell membranes, ion channels are integral to the maintenance of fluid balance across cells and tissues. As ions like potassium and chloride flow through their respective ion channels, the relative concentration of ions on either side of the cell membrane changes. These changes in the concentration of ions create a difference in the relative concentration of water. As this occurs, water molecules move across the cell membrane to maintain a balance in the concentration of water on either side of the cell membrane.
To maintain constant levels of water within humans (and all living organisms), water must move through tissues and across cell membranes. Water can move rapidly across cell membranes via specialized protein water channels called aquaporins, and to some degree by simple diffusion through the lipid membrane.
The flow of ions and water across cell membranes is important in the regulation of fluid balance in a variety of organ systems. This balance is key to the transport of fluids across the renal tubules in the kidney, to the secretion and absorption of water within the gastrointestinal tract, and to fluid balance in red blood cells and within the eye. Many disease states are characterized by, or influenced by, abnormalities in fluid transport. For example, in sickle cell anemia, the fluid balance across the red blood cell membrane is altered, contributing to the formation of dense and sickled cells that characterize this disease. Similarly, glaucoma is a condition characterized by excessive pressure in the fluid-filled anterior chamber of the eye.
The contribution of ion channels and water channels to the control of fluid balance therefore may offer unique therapeutic opportunities for targeted, effective treatments. Whether fluid balance is in the red blood cell, the eye, the central nervous system, the kidneys, or the gastro-intestinal tract, this activity is in large part controlled at the cellular level by the opening and closing of ion channels. Many diseases involving these organ systems, including sickle cell anemia, glaucoma, cholera and others can potentially be addressed by targeted therapeutics that modify the activity of ion or water channels in the relevant organ systems.
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