Total body water:

• Water comprises 50-70% of total body weight; the rest comprises solids.
• Precise volume largely depends on proportion of muscle tissue (which have more water) to adipose tissue (which has less).
• Body water is distributed between two major compartments:
  – Intracellular compartment = 2/3; this is the water contained within cells, and bound by cell membranes.
  – Extracellular compartment = 1/3; this is the fluid that bathes cells, and is outside of the cell membrane.
  The extracellular fluid is further subdivided:
  – Eighty percent is in the interstitial fluid, which is the fluid that “bathes” the non-blood cells of the body.
  – The remaining twenty percent is in the plasma, which is the fluid that suspends the blood cells; it is bound by capillary walls.

Water shifts compartments in response to osmotic conditions

• We can think of the body compartments as containers of solution:
  – The solvent is water.
  – Solutes include electrolytes, which are charged particles, and nonelectrolytes, which include mostly organic molecules (such as glucose and lipids).
• Osmolarity is the concentration of solute particles within a solution (be aware of intertextual variation regarding osmolarity vs. osmolality).
• In homeostasis, the intracellular osmolarity and extracellular osmolarity are equal.

Key solutes of intracellular fluid:

• Potassium and magnesium ions.
• Proteins and organic phosphates (for example, ATP).

Key solutes of extracellular fluid:

• Sodium, chloride, and bicarbonate ions.
• Plasma proteins.
• Because the interstitial fluid is an ultrafiltrate of plasma, it contains no proteins (this is discussed in detail, elsewhere).

Osmotic gradients:

• Solutes create osmotic gradients, which drive shifts in water between compartments.
  – Shifts between compartments occur in response to changes in the amount of solute and/or water.
  – This can be because of changes in solute amount or water amount.
• Direction of water shifts:
  – Between the plasma and interstitial fluid of the extracellular fluid.
  – Between interstitial fluid and the intracellular fluid.