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54 X Class Transportation - The circulatory system
He checked that by tying off and blocking different veins in animals he
experimented on the veins always bulged on the side of the block away from
the heart. As though the blood as trying to flow toward the heart and to
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accumulate just below the block because it simply couldn’t flow away from
the heart. This was true of all veins.
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In the arteries, the blood bulged on the heart side of any block he put in,
as though it were trying to flow away from the heart and couldn’t move in the
other direction.
Harvey now saw what was happening. The heart pushed blood into the
arteries, and the blood returned by way of the veins. It did this for both
ventricles. The blood had a double circulation. If one started from the right
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ventricle, it left by way of the arteries to the lungs, and returned by way of the
veins to the left atrium is known as pulmonary circulation and from there
into the left Ventricle.From the left ventricle, it left by way of the arteries to
the rest of the body and returned (in a “greater circulation”) by way of the
veins to the right atrium and from there into the right ventricle. It is known as
systemic circulation. As the blood reaches the heart two times, once through
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pulmonary cycle and again through systemic cycle, it is called double
circulation.
Harvey also showed that it was impossible, suppose that the blood was
used up in the body and that new blood was formed. He measured how
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much blood the heart pumped in one contraction and also counted the
number of contractions.
He found that in one hour, the heart pumped out a quantity of blood
that was three times the weight of a man. The body couldn’t use up
blood and form new blood at such a rate. The same blood had to circulate
and be used over and over again.
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Harvey still had some problem. The smallest arteries and veins that
could be seen had to be connected by vessels too small to see. Were fig-7: Marcello
they really there?
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Malpighi
In the 1650s, scientists had learned to put lenses together in such a
way that objects too small to see with the naked eye could be magnified and
made visible. Marcello Malpighi (1628-1694), with the microscope, he could
see tiny blood vessels that were invisible with naked eye.
In 1661, four years after Harvey’s death, Malpighi studied the wings of
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bats. He could see blood vessels in their thin membranes and, under the
microscope; he could see that the smallest arteries and veins were connected
by very fine blood vessels.
He called these blood vessels “capillaries” from the Latin word for “hair”,
because they were as thin as the finest of hairs.
With the discovery of capillaries, the idea of the circulation of the blood
was complete, and it has been accepted ever since.
Free distribution by T.S. Government 2019-20 55
Now, we know that blood circulates in the blood vessels.
But how did the scientists find out that blood moves in blood
vessels? Is it possible to demonstrate the movement of blood
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in vessels without damaging the vessels?
Let us repeat the classical experiment to demonstrate
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the movement of blood in veins conducted by William Harvey
fig-8(a): Try like this in early 17 th century, when there was no compound
microscope or any other modern equipment.
1. Tie a tornquit just above the elbow of a person as shown
in fig-8(b) whose blood vessels are prominent in the hand.
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