Tonicity refers to the solute concentration of a solution outside a cell and its effect on cellular fluid volume. The osmolarity of the solution determines the direction of water flow into or out of the cell. In normal body situations, solute concentration within and outside of the cell is usually nearly the same (isotonic).
tweetym_22 said:
Can anyone please help me understand the use of these solutions?
Isotonic:
Same osmolarity as the cells (270 - 300 mmol/l). Equal solute and water--exact same number of particles in both solutions--no net movement of water. Does not change cell volume.
Higher solute concentration surrounding cells pulls water out of the cells.
Hypertonic:
Higher osmolarity than cells (> 300 mmol/l). Greater solute, less water--water moves out of cells. The cell will shrink.
Lower solute concentration surrounding cells causes water to move into the cells.
Hypotonic:
Lower osmolarity than cells (< 270 mmol/l). Less solute, more water--water moves into cells. The cell will swell.
Isotonicity:
If the concentrations of electrolytes are the same in the cell and surrounding fluid, the situation is balanced (homeostatic). The cell fluid volume remains the same.
Hypertonicity:
The cell will shrink (crenation) by loss of its fluid to the surrounding hypertonic environment. High osmotic pressure of surrounding fluid pulls fluid out of the cell.
Hypotonicity:
In a hypotonic environment, fluid will enter a cell and cause it to swell and burst. The inside of the cell has higher osmotic pressure than the surrounding fluid, so fluid is drawn into the cell.
Both hypertonicity and hypotonicity in the extracellular fluids will destroy cells.
Need isotonicity for cell homeostasis, for balance.
NS IV is hypotonic relative to cells. Fluid moves from the vascular space into the cells. When a liter of NS is administered intravenously, it will go into the cells and very little will remain in the blood vessel (since it is hypotonic).
If you put two isotonic solutions side by side, no fluid shift occurs. A liter of normal saline or ringer's lactate is limited to the extracellular space and will expand the blood volume.
5% dextrose in NS is hypertonic compared to cells; pulls water into the vascular space from the cells or interstitium.
