Osmolality and water Movement

• Water distributes between different fluid compartments based on the concentration of solutes, or osmolality, of each compartment
• Osmolality of a fluid is proportional to the total concentration of all dissolved molecules, including ions, organic metabolites, and proteins, and is usually expressed as milliosmoles (mOsm) per kilogram water
• The semipermeable cellular membrane separating extracellular and intracellular compartments contains ion channels through which water can move freely, but other molecules cannot
• Water can move freely through the capillaries separating the interstitial fluid and the plasma, moving from a compartment with lower solute concentration to one with higher concentration to achieve equal osmolality on both sides of the membrane
• The force needed to keep the same amount of water on both sides of the membrane is called osmotic pressure
• As water is lost from one fluid compartment, it is replaced with water from another compartment to maintain a nearly constant osmolality
• Blood contains a high content of dissolved negatively charged proteins and electrolytes needed to balance these charges
• When the osmolality of the blood and interstitial fluid is too high, water moves out of the cells
• Loss of cellular water can occur in hyperglycemia due to the high concentration of glucose increasing the osmolality of the blood