Biomedical Engineering Reference
In-Depth Information
2.4.2 Tonicity
It refers to the effect of a solution on the shape of a cell. A solution that does not
change the shape of a cell is said to be isotonic. In an isotonic solution, the cell is in
equilibrium (i.e., the movement of water and solutes into the cell are equal to the
movement of water and solutes out of the cell). Thus the cell remains the same size.
A cell placed in a hypotonic solution will swell due to water moving into the cell.
Alternatively, if a cell is placed in a hypertonic solution, the cell will shrink due to
water osmotically moving out.
To understand the difference between tonicity and osmolarity, consider the in-
tracellular fluid of an animal cell, which has an osmolarity of 290 mOsm. Animal
cells typically contain no urea although their cell membranes are permeable to
urea. When an animal cell is placed in a 290 mOsm urea solution, urea moves into
the cell. Presence of urea inside causes the osmotic pressure to increase, which in
turn helps more water to get into the cell by osmosis. Increased water and urea
content within the cell causes the cell to swell (and burst). Thus an isosmotic urea
solution is hypotonic to animal cells. Inside cells, lysosomes contain osmotically
active material and rapidly swell and break if suspended in hypotonic solutions.
Lysosomes suspended in an isoosmotic solution of a nonpermeant solute are in
osmotic balance and are stable.
Large volumes of a solution of 5% human albumin are injected into patients
undergoing a procedure called plasmapheresis. The albumin is dissolved in physi-
ological saline (0.9% NaCl) and is therefore isotonic to human plasma. If 5%
solutions are unavailable, pharmacists may substitute a proper dilution of a 25%
albumin solution. Mixing 1 part of the 25% solution with 4 parts of a diluent re-
sults in the correct 5% solution of albumin. If the diluent used is sterile water, not
physiological saline, the resulting solution is strongly hypotonic to human plasma.
The result is a massive, life-threatening hemolysis in the patients.
Sports drinks can also be grouped based on the tonicity. Isotonic drinks (e.g.,
Gatorade) quickly replace fluids lost by sweating and supply a boost of carbo-
hydrates. This drink is typically made for the average athlete. Hypotonic drinks
quickly replace fluid lost, and are best for low-perspiration athletes such as jockeys
and gymnasts. Hypertonic drinks (e.g., Powerade) supplement daily carbohydrate
intake normally after exercise to increase glycogen stores. This drink is used pri-
marily for athletes such as long distance runners who need the extra carbohydrates
and electrolytes.
2.4.3 Osmotic Pressure
The pressure difference between regions A and B at equilibrium is the osmotic
pressure (
is derived using thermo-
dynamic principles. Consider the equilibrium state of liquid mixtures in the two re-
gions in Figure 2.9. Water diffuses from region B into region A until the free energy
per mole of water on each side of the membrane is the same. For an ideal solution
with very small solute concentration, it can be shown that
π
) of region A. The expression for calculating
π
(2.16)
π
=Δ
RT
C
