M E N T O R

Baby, it’s cold outside
Editor’s Prize for
Alistair Roddick
One of the greatest
achievements of the human
race has been our
ability to colonise vast
swathes of land across a full house of
continents, irrespective of climate,
from the Tuareg people of the Sahara
Desert to the Inuit of the arctic
circle. From the Steppe plain to the
thickest rainforest, humans have settled
most everywhere at some point
in history. This is in part due to our
ingenuity. Not to blow our collective
trumpet, but we invented clothes,
mastered fire and continue to build
ever more complex and efficacious
shelters. This has allowed us to survive
in climates that otherwise would
not permit a dwelling. However, being
clever can only get us so far. To
survive in the most unforgiving of
climates, the human body has been
crafted and fine tuned by millions of
years of evolution into the tall (usually),
imposing (sometimes) survival
machine that we see today.
Even the human body has its limits,
though. In the previous issue we discussed
the physiology and danger of
extreme heat. This issue, we will go
to the polar opposite: the physiology
of extreme cold.
The human body has evolved to
function as efficiently as possible,
and this can only be achieved within
a very narrow temperature range
(around 36.5-37.5 o C). As a result,
just as with an increase in temperature,
the body has a variety of mechanisms
to respond to a decrease in
environmental temperature, ranging
from the highly effective to the completely
useless to the highly effective.
I like extreme adventurers, so let’s
use an explorer on a cold day in
Antarctica as an example. Our adventurer
wakes from sleep and leaves
their tent to begin the day. Almost
instantaneously, a drop in blood
temperature is detected by cells in
the hypothalamus. Interestingly, cells
cannot determine body temperature
beyond a fairly narrow range and,
whilst we have both heat-sensitive
cells and cold-sensitive cells, no cells
can detect both. These cells trigger
activation of the autonomic nervous
system, this time driving a set of unconscious
responses that act to either
generate or conserve heat. At the
same time, the hypothalamus signals
to the cortex of the brain, activating
behavioural pathways that are executed
via conscious sensation and
complex neuromuscular programs
with environmental components.
Or, explained more practically, you
will feel cold and put on a jumper.
So, about those unconscious responses;
let’s start with the useless.
The sympathetic nervous system
(that is, the fight-or-flight part of the
autonomic nervous system) signals to
tiny muscles distributed throughout
the skin called arrector pili. Each of
these little muscles attaches to the
base of a body hair in a hair follicle
and when activated they heave
all the hairs into an upright position,
producing characteristic goosebumps.
In our furrier ancestors, this
would serve to trap a warm layer of
air against their skin, keeping them
warm in chilly weather. In humans,
whose fur is somewhat less impressive,
this response does, well, basically
nothing.
Probably the best physiological method
for preserving heat that is useful
to humans is the process of vasoconstriction
– constriction of blood
vessels around the body. In particular,
the body loses loads of heat due
to blood flow through the skin. We
saw this in our desert explorer last
time. To combat this heat loss on a
chilly morning in Antarctica, sympathetic
nervous system signals will
be sent via nerves to major arteries
and smaller arterioles supplying the
skin, causing them to constrict. This
reduces the blood flowing to the skin
and therefore minimises the amount
of heat lost into the surrounding air.
These same signals are sent to the
veins at the surface of our explorer’s
arms and legs (like the veins that
you may be able to see in your own
arms after exercising), causing them
to constrict. As a result, cold blood
coming back from the fingers and
toes towards the heart is diverted
along deep veins that run alongside
the major arteries. Because heat always
moves from hot to cold, hot
blood reaching the hands and feet
would normally conduct its warmth
out into the cold atmosphere. Instead,
the hot blood transfers its heat
into these deep veins carrying cold
blood back from the limbs, which
become warm, while the arterial
blood becomes colder as it heads
off towards the fingers and toes.
This system means that the blood in
the extremities stays cold while the
blood in the body core (where all the
important stuff is) stays warm. This
process is known as counter-current
exchange, and is one of the most
important mechanisms of holding
on to that precious heat in freezing
climates.
Everything we have mentioned so
far is a means of preserving heat, but
artwork 2016
Congratulations to Jamie Crawford who has won this year’s Editor’s Prize for an
artwork contribution. His work can be seen on page 21 of this issue.
Jamie will receive £100 in book vouchers.
Highly commended goes to Jennifer Smith, whose work can also be seen throughout
this issue. In particular, the artwork attached to “Unprepared for autopsy” on
page 40.
________________
All submissions in a calendar year are eligibile to win the Editor’s Prize. The award
goes to the piece of work deemed to be of the highest quality; taking into account
the originality and polish of the final piece, as well as the steps undertaken to achieve
it. The winning piece is chosen by the editorial team.
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