form and break. But as the temperature
approaches 4 °C, hydrogen bonds form more
often than they break. This forces the molecules into an arrangement that would crowd
them, as the temperature drops further. So,
to create more space, the cooling water then
expands (Fig. 1) until it reaches the freezing
point (0 °C). When it freezes, it forms crystallized ice and then expands further.
In the presence of red blood cells, newly
forming ice crystals push on the membranes
that surround the red blood cells, causing
damage to the cells.
When the cells thaw, the damaged membranes are leaky, so the precious hemoglobin
escapes. As long as people keep donating,
blood banks prefer to keep packed red blood
cells at just above freezing temperature and
stay within the 42-day limit by using older
supplies before new ones. But when necessary, blood banks can cryopreserve the cells.
Prior to dropping the temperature to the
freezing point, lab technicians treat the cells
and the surrounding liquid with either glycerol or another chemical called hydroxyethyl
starch. Glycerol and hydroxyethyl starch are
called cryoprotectants, because they get into
the cytoplasm, the liquid inside the cells,
which is mostly water, and act as antifreeze.
Just like the antifreeze used to winterize cars,
cryoprotectants prevent ice-crystal formation
inside the cells by lowering the freezing point.
In the liquid outside the cells, both the
ions and the cryoprotectants also lower the
sometimes up to 8 hours. Limits are a little
better for other organs, but in all cases, the
time limit results from both temperature and
the solution itself.
With the lungs, the longest preservation
time has been 9 to 10 hours and for the liver
and pancreas, the time limit is 12 to 18 hours.
When it comes to the kidneys, the time limit
is 36 to 48 hours, so in this case donors and
recipients can be on opposite sides of the
globe. But compare that with donated blood
products, such as fresh frozen plasma, which
can be kept between –25 °C and –30 °C for
one year, or packed red blood cells, which are
usually refrigerated just above freezing tem-
perature ( 6 °C). This makes packed red blood
be cryopreserved, maintained below
freezing temperature, for 10 years.
Why organs cannot
Why are plasma and certain other
components of blood routinely
frozen, while packed red blood cells
are usually kept at 6 °C? Shouldn’t
colder mean better? The answer is
yes, until ice crystals start forming.
Because one side of each water
molecule has a slightly positive
charge and the other has a slightly negative
charge, water molecules are attracted toward
each other through hydrogen bonds. At higher
temperatures, hydrogen bonds constantly
a machine that acts as a temporary heart
and lungs. His body is cooled, but not to the
extent that the donor heart was cooled. Then
Jim’s failing heart is removed. The donor
heart, partly re-warmed, is fitted into place
and attached. Jim and the heart are warmed
slowly, the new heart is restarted, Jim is
removed from bypass, and his chest is closed.
He is put on multiple medications to keep his
blood pressure up as the donor heart regains
its normal strength. Jim is also given medicine to keep his immune system from rejecting the new heart, and after two weeks he is
out of the hospital and ready to begin life with
his new heart.
No time to waste
A teenager getting a new heart is a rather
extreme example of organ transplantation,
but it happens. Not all cases are as successful
as Jim’s. In the United States, 79 organs,
on average, are transplanted every day.
These organs include not just hearts but also
kidneys, lungs, livers, and other organs.
However, each day, 22 people, on average, die
waiting for an organ that they never received,
because there were not enough and because
donated organs must be used immediately;
they cannot be stored in ice coolers the way
donated blood is.
Jim’s need for a new heart was an emergency, and transplant teams had just four
hours to transfer the heart from the donor to
Jim. The solution that was pumped into the
donor heart contained high concentrations of
potassium ions. Because it was also cold
( 4 °C), the organ would survive 10 to 20 times
as long as it would at body temperature, but
the solution also helps with preservation.
New, advanced solutions allow preserva-
tion of a donor heart for 6 hours at 4 °C, and
Figure 1. Water molecules in (left) liquid water and (right) ice. Hydrogen bonds between water
molecules are represented with green dotted lines. I L U
10 ChemMatters | DECEMBER 2016/JANUAR Y 2017 www.acs.org/chemmatters
Hydrogen bonds between
continuosly moving H2O molecules
at 10 °C
Hydrogen bonds between
H2O molecules at 0 °C,
forming a rigid hexagonal crystal
Large gaps between
molecules held rigidly
apart Lots of open space
white blood cells
red blood cells