Our sense organs 45

The heart
– more than a high-tech pump
Did you know that the human heart beats
100,000 times a day? This amounts to 2,500 million
times over 70 years, pumping enough blood
to fill a skyscraper. Blood flows through the body
via a densely branching network consisting of
2,500 km of arteries, veins, and capillaries, equivalent
to the distance from Paris to Moscow.
The heart must supply all organs with sufficient
blood, while continually having to adjust its output
to the actual load. When more blood is
required, it responds by beating faster and by
increasing the volume of blood pumped per contraction.
The quantity of blood pumped by the
heart during one beat, is defined as the stroke
volume (approximately 70 cm 3 in the case of an
adult at rest). At 70 beats per minute the daily
volume pumped is 7,000 litres, enough to fill
40 bathtubs.
The heart is a maintenance-free pump which
functions unattended throughout life, generally
requiring no replacement parts. It drives the
blood circulation, and has an exceptional ability
to adjust its level of activity to differing loads.
The volume of blood pumped per minute by one
chamber can increase from 5 litres up to nearly
30 litres during strenuous muscular activity. This
quantity is known as the cardiac output, usually
expressed in litres per minute. It should be noted
that both sides of the heart pump equal volumes
of blood, otherwise large pressures would build
up on one side of the circulatory system while
the other side would receive too little blood.
The power output of the heart is approximately
1 Nm/s 1 . In the case of a machine the mass/power
ratio is used to measure its efficiency. This means
the mass required to produce for example 1 kW
of energy. Taking the mass of the heart to be 0.3
kg, its mass/power ratio is (300 g)/(1 W) = 300
g/W = 300 kg/kW. Much lower values obtain for
machines. For example:
Electric motor
(at 1500 rev per min; 1 kW)
Marine diesel engine (large ship)
Diesel engine (heavy truck)
Four-stroke petrol engine
(heavy vehicle)
Aeroplane engine (light construction)
15 kg/kW
60 kg/kW
6 kg/kW
1.6 kg/kW
0.6 kg/kW
But when working hard, the energy output of the
heart can increase appreciably, so that its mass/
power ratio approaches that of a mechanical
pump.
The heart, a hollow muscular organ, is located in
the cavity formed by connective tissue between
the vertebral column and the breastbone. It is
completely enclosed in a pouch called the pericardium,
which extends between the pleural
(lung lining) cavities on each side, from the
diaphragm up to the large blood vessels. Normally
the heart is one and a half times the size of
one’s fist, but it can be significantly larger in the
case of trained athletes. Its mass is between 300
and 350 g, which is about 0.5 per cent of the
body weight. In shape it resembles a rounded-off
cone, the base of which is also the cardiac base.
The septum separates the two halves of the
heart, the right half serving the pulmonary circulation,
whereas the left half independently pumps
blood throughout the whole body. Oxygendepleted
blood from the entire body is received
by the right half of the heart and passed on to
the lungs (routes 2, 6, 7, 8 in the diagram on
page 51). It is oxygenated in the lungs and subsequently
flows to the left side of the heart where
it is pumped in various directions through the
body (routes 1, 3, 4, 5 in the diagram on page 51).
The difference between arteries and veins is not
determined by the quality of the blood, but by its
flow direction to or away from the heart. Veins
carry blood to the heart, while it is pumped away
1 Power: In the SI system the Newton-metre per second
is the unit used for expressing the amount of
work divided by time. One Nm/s is equivalent to one
Watt (in the case of electricity) and also to one Joule
per second (the unit of heat transfer). We thus have:
1 Nm/s = 1 W = 1 J/s.
49