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Chapter 3 — 9
Circulation
The blood flow from the heart is pulsatile and the
blood pressure
varies depending on the stage
of heart contraction. The higher blood pressure during the heart's contraction is called the
systolic blood pressure
and has a normal value of around 100–140mm mercury (mm Hg). The
pressure when the heart is not contracting is the
diastolic blood pressure
which has a normal
value of around 60–90mm Hg. Blood pressure is normally recorded as Systolic / Diastolic – e.g.
130
/
90.
Blood vessels can change their internal diameter under the control of the nervous system. This
allows for some variation in blood flow to parts of the body depending on specific
circumstances. For instance, during exercise the blood vessels dilate allowing more blood flow to
the muscles, while under cold conditions the blood vessels to the skin constrict, reducing the
blood flow to the skin (appearing pale) and so minimising heat loss.
The constriction or dilatation of the blood vessels also influences blood pressure. Excessively
high blood pressure
(hypertension)
can ultimately cause damage to the blood vessels and an
excessive strain on the heart. High blood pressure requires treatment, often with drugs which
dilate the blood vessels but which may interfere with safe diving.
Blood pressure is constantly maintained by a sophisticated sensing and feedback mechanism.
Variations in blood pressure caused by physical activity or standing from a reclining position are
quickly compensated for by changes in the diameter of the blood vessel walls.
When a person is in a reclining position, blood pressure is maintained easily and the effect of
gravity does not have to be opposed by the contraction of blood vessels. When standing up
quickly from this position, blood pressure in the upper part of the body may fall. Occasionally,
even in normal people, the heart and blood vessels cannot compensate rapidly enough and
fainting or light-headedness can result. This is known as
syncope
or
postural hypotension
.
The cardiovascular system is able to compensate for changes in blood volume, such as those
associated with severe bleeding (
haemorrhage)
, by constricting the blood vessels and diverting
blood from non-essential organs to essential organs such as the brain and heart.
In
pulmonary barotrauma,
air can gain access to the blood as it passes through the lungs. Air
bubbles may be carried to vital organs such as the brain and heart, obstructing their blood flow
and leading to serious consequences
(air embolism)
. In
decompression sickness,
gas bubbles
may also be transported by the blood stream.
COMPRESSED-AIR DIVING
Scuba allows the diver considerable freedom but has its own limitations. It has all the potential
problems of free diving, but adds special physiological problems of its own.
Resistance to Breathing
A major limitation to diving with scuba is resistance to breathing. During maximal exertion, a
diver can consume over 70 litres of air per minute at the surface – but the peak flow rate during