Question: How does the capillary membrane differ from
the cell membrane in terms of movement of glucose, ions and water across
it?
Answer: Since glucose, ions and water readily pass
through the capillary membrane, the osmolality of extravascular and
extracellular fluid is the same as it is for plasma. However, the cell
membrane is impermeable to certain ions, sodium in particular. Sodium
concentration is the major determinant of fluid distribution across
membranes. Glucose can enter the cell only by facilitated diffusion, in
exchange for sodium. Once in the cell, glucose is metabolized to two
molecules of 3-carbon sugars. This increases the number of
osmotically-active particles and water is drawn into the cell.
Question: Which fluids allow water to move from the
ECF into the ICF, and so distribute through the entire volume of body
water?
Answer: Of the isotonic fluids, 0.18% NaCl + 4%
glucose and 5% glucose, and all hypotonic fluids, i.e. 0.18% and 0.45%
NaCl, allow water to move from the ECF to the ICF.
0.9% NaCl allows water to distribute only in the ECF. The lactate in
Hartmann's solution enters the cell taking a small proportion of water
with it, but not as much as for the glucose-containing fluids.
