SCUBAPRO-UWATEC HEARTRATE MEASUREMENT User Manual page 6

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neutralizes the pressure gradient effective
outside the water.
When submerging into the water, the venous
blood volume shifts: outside the water (left
red) a large portion of the volume is found in
the leg veins. After submerging (right blue)
the volume shifts towards the chest/heart.
This affects the cardiac output and leads
to an increased urine production. Thus, the
blood volume is significantly reduced after
diving.
CHANGE OF HEART RATE THROUGH
BREATHING
Previously, we've explained how physical
exertion and body position affect the
heart rate. But there are other factors that
influence the heartbeat as well. One of
which is breathing. Of course, a person's
breathing and in particular the breathing
volume is often times impacted by physical
exertion. When muscles and tissue need a
lot of oxygen, the vessels are dilated and
the heart pumps quickly, the necessary
amount of oxygen needs to get into the
system via the lungs, i.e. through breathing.
The breathing rate and breathing volume
increase. You inhale more air into the
lungs more often. But even when you are
not puffing like a grampus, heart and
lung function are closely interconnected.
This is due to the way the human body
is constructed: namely that the heart is
embedded between the two lungs (see
figure on page 6). Therefore, any negative
or positive pressure in the lungs also affects
the activity of the heart.
During inhalation the lung develops a
negative pressure, which causes air to be
sucked in through the pharynx.
This negative pressure provides for a better
HEART RATE
However, as already explained in the
previous chapter, the position within the
water also affects the heart rate, though
no longer because the blood shifts, but
rather because the position within the water
affects the breathing (see figure on page 9).
The cardiopulmonary system shifts
according to the body position
venous return flow, but simultaneously also
for a light decline in arterial blood pressure.
HEART RATE
During exhalation it's exactly the opposite.
Especially during exhaling on exertion,
for example during pressure equalization
using the Valsalva maneuver, the venous
return flow is impeded, but the blood
pressure, at least temporarily, increased. For
these two blood pressure fluctuations, the
body has a compensation mechanism at
hand as well: the so-called autonomous
regulation. It limits the increase or decline
in blood pressure through corresponding
heart rate reactions. As a result, more or
less distinct heart rate fluctuations occur,
particularly while resting.
WHILE DIVING
The water pressure that impacts the
submerged body leads to a greater
pressure difference between the gas-filled
and the fluid-filled part of the lung, because
HEART RATE WHEN IN APNEA
So, breathing influences the heart rate. But
what happens when you are not breathing?
Particularly while diving? Of course, you
can also hold your breath above water, but
during a dive it can be absolutely necessary
– for example while buddy breathing. And
there are different «levels» of holding your
breath , the diving reflex and apnea diving,
that is to say intentionally diving without a
compressed air tank, just using the air in your
lungs.
THE DIVING REFLEX
The diving reflex is a natural reflex that allows
mammals to stay under water for extended
periods of time. It is especially pronounced
in aquatic animals, but also detectable in
humans. The trigger signal is a facial cold
stimulus, so water as well, which is designed
unlike the fluid-filled part the gas-filled part
can be compressed, i.e. squeezed. The
rule is: in an upright position the difference
is greater than in a horizontal position.
So, if you are floating under water in an
upright position, for example during a short
break in order to look at something more
closely on a steep face, the blood pressure
fluctuations of the body are more extreme
as well. But this effect can also occur while
swimming in a horizontal position, namely
while snorkeling. A snorkel can increase the
pressure difference as well. It too directly
impacts the return flow to the heart and
therefore influences the heart activity and
the heart rate.
to extend survival. This cold stimulus, which
can be triggered by an ice pack pressed to
the face for example, also triggers an easily
measurable reaction while resting: the heart
rate slows down (bradycardia).
This is due to receptors around the nose,
eyes and mouth. The slowdown of the heart
rate can reach more than 10 heartbeats per
minute in humans, in animals a slowdown of
over 50% has been shown.
This bradycardia is accompanied by other
reactions used by the body to adapt to
the new situation. The blood vessels in
the tissue, which can function without
oxygen for a short period of time, constrict
(vasoconstriction).
This way, oxygen is conserved for vital
organs (for example the central nervous
system or the heart) and simultaneously
an increase in blood pressure is prevented.
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