Body Water Compartments
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Body Water Compartments
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
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