Page 110 - Biology Form Two

Biology for Secondary Schools

The wall of an artery consists of three of 72 times a minute. However, this can
layers. The endothelium is the innermost increase or decrease due to physical
layer of the artery, which is attached activity, emotional state, or health
to a basement membrane. It has only factors.
one layer of cells. The endothelium
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surrounds the lumen (a hollow passage Activity 5.2: Examine the pulse rate
through which blood flows). The lumen
of an artery is narrow and smooth. This Materials
helps the artery to transport blood at Stopwatch or wristwatch, a notebook,
high pressure. The muscular layer is and a pen or a pencil
made up of smooth muscles and elastic
fibres. Smooth muscles are arranged in Procedure
circles around the endothelium. This 1. Get into pairs. Put the fingers of
layer makes it possible for the artery your right hand on your wrist as
to contract and relax for the efficient shown in Figure 5.4. Can you feel
movement of blood.
the pulse?
The outermost layer is the fibrous layer
made up of connective tissues such
as collagen. The fibres are arranged
parallel to the length of the vessel.
They enable the artery to withstand the
pressure caused by the blood coming
from the heart. Arteries branch to form
arterioles. An arteriole is a small artery Figure 5.4: Examining the pulse rate
that leads to a capillary. Arterioles have
the same three layers as arteries, but the 2. Use the stopwatch to count the
thickness of each is greatly reduced. number of pulses in one minute.
Record your answer.
When the ventricles contract, the 3. Repeat step 2 three times and find
arteries relax, allowing blood to flow the average. This will be your
from the heart to them. When the
ventricles relax, the arteries contract, pulse rate.
pushing the blood forward. This 4. Do physical exercise, like jumping
contraction and relaxation of arteries is for a few seconds, then repeat
felt as a pulse. The pulse or heart rate is steps 1, 2, and 3.
the number of heart beats per minute. 5. Record the pulse rate. Is it the same
The pulse rate reflects the heartbeat. An as the earlier pulse rate?

adult human’s heart beats at an average 6. Explain your results.

104 Student’s Book - Form Two

05/10/2024 15:36:38
BIOLOGY FORM 2 NEW.indd 104
BIOLOGY FORM 2 NEW.indd 104 05/10/2024 15:36:38