Page 98 - NCERT Science Class 10 English Medium

Blood pressure
The force that blood exerts against the wall of a vessel is called blood pressure. This
pressure is much greater in arteries than in veins. The pressure of blood inside the
artery during ventricular systole (contraction) is called systolic pressure and pressure
in artery during ventricular diastole (relaxation) is called diastolic pressure. The normal
systolic pressure is about 120 mm of Hg and diastolic pressure is 80 mm of Hg.

More to Know!

Blood pressure is measured with an instrument called sphygmomanometer. High
blood pressure is also called hypertension and is caused by the constriction of
arterioles, which results in increased resistance to blood flow. It can lead to the rupture
of an artery and internal bleeding.

The tubes – blood vessels
Arteries are the vessels which carry blood away from the heart to various
organs of the body. Since the blood emerges from the heart under high
pressure, the arteries have thick, elastic walls. Veins collect the blood
from different organs and bring it back to the heart. They do not need
thick walls because the blood is no longer under pressure, instead they
have valves that ensure that the blood flows only in one direction.
On reaching an organ or tissue, the artery divides into smaller and
smaller vessels to bring the blood in contact with all the individual cells.
The smallest vessels have walls which are one-cell thick and are called
capillaries. Exchange of material between the blood and surrounding
cells takes place across this thin wall. The capillaries then join together
to form veins that convey the blood away from the organ or tissue.

Maintenance by platelets
What happens if this system of tubes develops a leak? Think about
situations when we are injured and start bleeding. Naturally the loss of
blood from the system has to be minimised. In addition, leakage would
lead to a loss of pressure which would reduce the efficiency of the

Life Processes 93

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