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Chapter 3 — 7
In venous blood, the haemoglobin in red blood cells returns to the heart with 75% of its O
2
load
still attached. It is then a more bluish colour.
The tissues need only 25% of the O
2
carried in arterial blood. This allows a reserve supply of O
2
which can be used during exercise or breath holding.
CO
2
is carried from the tissues by the blood in the veins, back to the lungs. Some of it is
dissolved in blood plasma and some bound to the protein of the haemoglobin molecules.
Although the CO
2
dissolved in the blood forms carbonic acid, the acidity of the blood is
prevented from rising to excessive levels by a system of buffering compounds.
It is possible to increase the O
2
carrying capacity of blood by the use of
hyperbaric oxygen
. In
recompression chambers, increased amounts of O
2
can be physically dissolved in the plasma,
even though the haemoglobin is fully saturated with O
2
.
Heart
The heart is a large muscular pump (about the size of a man's fist) located in the centre of the
chest. See fig 3.4. It is composed of two functionally separate pumps which maintain two distinct
circulations. The
right side
of the heart receives venous blood from the body and pumps this
blood through the lungs where it picks up O
2
and eliminates CO
2
. The
left side
of the heart
receives this oxygenated arterial blood from the lungs and pumps it through the body.
Each side of the heart is essentially a two-stage pump which is not unlike a two-stage
compressor. The
atrium
is the first or low pressure stage of the pump and it has a thin muscular
wall. It receives blood from the veins at low pressure. When it contracts, it propels this blood
into the second or high pressure stage – the more thickly walled and stronger ventricle.
The
ventricle
has two “one-way” valves, one valve preventing blood from flowing back into the
atrium, and the other valve preventing blood flowing back into it from the arteries. When it
contracts, it pumps blood into the arteries.
Occasionally there may be openings between left and right sides of the heart (patent foramen
ovale, septal defects). In divers this allows bubbles to pass from the venous system to the arterial,
causing serious manifestations of decompression sickness from dives that should otherwise be
safe. People with significant heart abnormalities should not undertake scuba diving.
The heart, being a muscle, requires its own blood supply. This is provided by the
coronary
arteries
which originate in the aorta, the main artery of the body. Any obstruction of these
coronary arteries will cause damage to the heart muscle – a heart attack.
Partial obstruction of the coronary arteries may produce
angina
(which is pain or discomfort
arising from insufficient O
2
in cardiac muscle), because it is receiving insufficient blood supply.
Since a heart attack can take some of the fun out of a diving expedition, it is important for divers
to have skilled medical examinations to exclude this problem or to help predict which divers will
be susceptible to such heart conditions (coronary artery disease).
The resting output of the heart is about 5 litres of blood per minute. The heart has considerable
reserve and if the tissues require it, can increase this output several fold by increasing its rate and
strength of contraction.