Our sense organs 45

The heart beats 70 times per minute, and pumps
about 70 cm 3 of blood into the major circuit at
each contraction. This means that the volume of
blood pumped each minute is about 5 litres (70 x
70 = 4,900 cm 3 ). This is equal to the total volume
of blood in the body. When physically active the
muscles require more blood, so that the pressure
and the flow volume of the blood increases. The
volume per minute can then be raised to 25 litres,
meaning that the entire blood volume is pumped
through the body five times per minute. This is
achieved by a doubling of the stroke volume
from 70 to 140 cm 3 and the rate increases to as
many as 180 beats per minute (140 x 180 =
25,200 cm 3 = 25 litres).
The human heart is morphologically and functionally
a masterpiece of its Creator. The focal
point of circulation, it responds to every demand,
even from the most distant corners of the body.
The larger blood vessels, arteries and veins, are
the main roads carrying the necessary volumes of
blood, but the capillaries provide the actual nourishment.
In this cleverly designed network, the
arteries branch repeatedly and supply the entire
capillary network with blood. These capillaries in
turn combine to form larger and larger veins.
The following table shows that 1,200 million capillaries,
with a total length of 1,200 kilometres,
must be supplied with blood.
Cross-
Total sectional
Type of vessel Number length area Diameter
(m) (cm 2 ) (mm)
Aorta 1 0.4 0.8 10.0
Large arteries 40 8 3 3.0
Arterial branches 600 60 5 1.0
Small art branches 1,800 18 5 0.6
Arterioles 40,000,000 80,000 125 0.02
Capillaries 1,200,000,000 1,200,000 600 0.008
Venules 80,000,000 160,000 570 0.03
Small vein branches 1,800 18 30 1.5
Venous tributaries 600 60 27 2.4
Large veins 40 8 11 6.0
Vena cava (superior and inferior) 2 0.4 1.2 12.5
Table 1: Numerical aspects of human blood vessels.
54
Blood pressure: The pressure against which the
left ventricle must push the blood, is called the
arterial blood pressure. The resulting pressure
waves can be felt as pulses when one touches
the skin over a superficial artery. Arterial pressure
changes continually, varying between a maximum
systolic pressure (full contraction of the
left ventricle) and a diastolic minimum (when the
aortic valve is open). The systolic pressure is normally
about 120 mm (in the old mercury pressure,
which equals 16 kPa 2 ), and the diastolic
about 80 mm (= 10.7 kPa).
The heart of a human embryo already starts
beating at 25 days after conception. Its size is
then only 2.5 mm, while the embryo as such
measures 6 mm. The heart of an adult man
weighs about 320 g and that of a woman 270 g.
It is noteworthy that the strict separation of the
left (arterial) and the right (veinous) sides of the
heart only commences at birth. In the foetus
both atria are connected by the foramen ovale 3 .
The Latin word foetus means begotten; in medical
terms it refers to the embryo at four months
and later.
Did you know that a near-instantaneous “change
of heart” takes place directly after birth? The two
halves of the heart of a foetus operate in parallel.
Both atria and ventricles function together as a
single hollow muscle, and the blood is oxygenated
in the placenta. The lungs require very little
blood, since they do not function, and they are
2
Pressure: The SI unit for pressure is the Pascal (Pa),
named after the French mathematician and physicist
Blaise Pascal (1623 – 1662). 1 Pa = 1 N/m 2 = 1 kg/(ms 2 ).
The previously used unit for air pressure, millimetres
of mercury (mm Hg), was defined as follows in 1954
by the World Organisation for Metrology: “One mm
Hg is the pressure exerted by a 1 mm column of mercury
at a temperature of 0 °C and at a place where
the gravitational acceleration is normal (9.80665
m/s 2 ).” The conversion factors are: 1 mm Hg =
133.332 Pa, 1 kPa = 1,000 Pa. In medicine, the old
unit is used, for historical and practical reasons.
3
Foramen ovale (Latin foramen = hole): The orifice in
the septum between the two atria of a foetus.