Page 97 - NCERT Science Class 10 English Medium

Our pump — the heart
The heart is a muscular organ which is
as big as our fist (Fig. 5.10). Because
both oxygen and carbon dioxide have to
be transported by the blood, the heart
has different chambers to prevent the
oxygen-rich blood from mixing with the
blood containing carbon dioxide. The
carbon dioxide-rich blood has to reach
the lungs for the carbon dioxide to be
removed, and the oxygenated blood from
the lungs has to be brought back to the
heart. This oxygen-rich blood is then
pumped to the rest of the body.
We can follow this process step by
step (Fig. 5.11). Oxygen-rich blood from
the lungs comes to the thin-walled upper
Figure 5.10
Figure
Figure 5.10
Figure 5.10
Figure 5.105.10
Schematic sectional chamber of the heart on the left, the left atrium. The left atrium relaxes
view of the human heart when it is collecting this blood. It then contracts, while the next chamber,
the left ventricle, relaxes, so that the blood is transferred to it. When the
muscular left ventricle contracts in its turn, the blood is pumped out to
the body. De-oxygenated blood comes from the body to the upper
chamber on the right, the right atrium, as it relaxes. As the right atrium
contracts, the corresponding lower chamber, the right ventricle, dilates.
This transfers blood to the right ventricle, which in turn pumps it to the
lungs for oxygenation. Since ventricles have to pump blood into various
organs, they have thicker muscular walls than the atria do. Valves ensure
that blood does not flow backwards when the atria or ventricles contract.

Oxygen enters the blood in the lungs
The separation of the right side and the left side of
the heart is useful to keep oxygenated and de-
oxygenated blood from mixing. Such separation
allows a highly efficient supply of oxygen to the
body. This is useful in animals that have high
energy needs, such as birds and mammals, which
constantly use energy to maintain their body
temperature. In animals that do not use energy
for this purpose, the body temperature depends
on the temperature in the environment. Such
animals, like amphibians or many reptiles have
three-chambered hearts, and tolerate some mixing
of the oxygenated and de-oxygenated blood
streams. Fishes, on the other hand, have only two
chambers to their hearts, and the blood is pumped
Figure 5.11
Figure
Figure 5.11 to the gills, is oxygenated there, and passes directly
Figure 5.115.11
Figure 5.11
Schematic representation of transport and exchange to the rest of the body. Thus, blood goes only once
of oxygen and carbon dioxide through the heart in the fish during one cycle of
passage through the body. On the other hand, it goes through the heart
twice during each cycle in other vertebrates. This is known as double
circulation.

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