Intracellular fluid

How does water get in there?

This compartment contains 23 liters, which is 33% of the total body mass, and 55% of total body water.

The Intracellular Fluid Compartment

Water Movement

Water moves through the lipid membrane. This may seem odd. The issue is that water does mix with lipid, just not very well.

The solution is 1 part per million. Because the water concentration of the body is so high, and the cell membrane surface is so large, this is enough to have a decent flux of water in and out of the compartments.

Water can move freely between the cells and the extracellular fluid.
Most membranes are like this. Most cells don't possess water pump proteins.

Thus, cells are defenseless against changes in extracellular fluid tonicity

The contents of intracellular fluid, and its very weird properties.

About 70% of it is water. Weirdly, its viscosity is very near to that of water.
However, diffusion though this compartment takes 4 times longer than though pure water.

Given the massive number of various other molecules in this solution, any single one trying to diffuse through will be frustrated by multiple collisions. Because of this molasses-like delay in diffusion, the intracellular compartment is a heterogenous mosaic of regions with differing local microenvironments.

The intracellular fluid compartment is further separated into numerous other compartments. For instance, there are marked differences between the fluid composition and electrolyte contents of the mitochondria, proteasomes, lysosomes, peroxisomes et cetera.

References, as always; my source for most of this information has been the Kerry Brandis Anaesthetic Viva book.  Everyone should have one.

Schoeller DA, van Santen E, Peterson DW, Dietz W, Jaspan J, Klein PD: Total body water measurement in humans with 18O and 2H labeled water. Am J Clin Nutr 1980, 33(12):2686-2693.