Blood Vessels

25 min

Let's begin

Turn your left hand palm up. Place the tips of two fingers of your right hand gently on your wrist, just below the base of your thumb. Wait a moment. Can you feel a soft throb, again and again? That is your pulse: blood being pushed through a blood vessel every time your heart beats.

Arteries

Carry blood away from the heart.
Have thick, muscular, stretchy walls, because the blood inside is under high pressure straight from the heart.
Usually carry oxygen-rich blood. The one exception is the pulmonary artery, which carries oxygen-poor blood from the heart to the lungs.
The biggest artery is the aorta. Your pulse is felt in an artery.

Veins

Carry blood back towards the heart.
Have thinner walls, because the blood inside is under lower pressure.
Have valves to stop the blood flowing backwards, which is important in your legs, where blood must travel upwards.
Usually carry oxygen-poor blood. The exception is the pulmonary veins, which carry oxygen-rich blood from the lungs to the heart.
The biggest vein is the vena cava. The bluish lines you can see on your wrist are veins. (Your blood is not blue! Oxygen-poor blood is dark red.)

Capillaries

Are the tiniest blood vessels. Ten of them side by side would only be as thick as one hair.
Have walls only one cell thick, with tiny gaps (pores).
Join the smallest arteries to the smallest veins, making a net around every cell.
This is where the work is done: oxygen and food pass out of the blood into the cells, and carbon dioxide and wastes pass in.

Comparing blood vessels

	Artery	Vein	Capillary
Direction	Away from the heart	Towards the heart	Links arteries to veins
Wall	Thick and muscular	Thinner	One cell thick
Pressure	High	Low	Very low
Valves	No (only at the heart)	Yes	No
Usual blood	Oxygen-rich (not the pulmonary artery)	Oxygen-poor (not the pulmonary veins)	Gives oxygen, picks up carbon dioxide

Measure your heart rate

Find your pulse on your wrist or on the side of your neck with two fingertips. Do not use your thumb, which has its own pulse.
While a friend times 30 seconds, count the beats.
Multiply by 2 to get beats per minute.

Example: Keza counts 38 beats in 30 seconds. 38 × 2 = 76 beats per minute. For children of your age, a resting heart rate of roughly 70 to 100 beats per minute is normal.

Tip

A is for Artery, A is for Away. Arteries carry blood away from the heart; veins bring it back.

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Try it yourself

Which blood vessel has the thickest walls?
In which direction do veins carry blood?
Why are the walls of capillaries so thin?
Mugisha counts 40 pulse beats in 30 seconds. What is his heart rate?
Which artery carries oxygen-poor blood?

Answers

An artery.
Towards the heart.
So that oxygen, food, carbon dioxide and wastes can pass through them easily.
40 × 2 = 80 beats per minute.
The pulmonary artery.

Quick check

What you learned today

Arteries carry blood away from the heart under high pressure and have thick walls; the aorta is the largest.
Veins carry blood back to the heart and have valves; the vena cava is the largest.
Capillaries are tiny vessels where oxygen and food reach the cells; your pulse lets you measure your heart rate.

Sources and credits

Natural Sciences Grade 9, Chapter 4: Circulatory and respiratory systems (4.3 Circulation and respiration), by Siyavula Education, Siyavula, CC BY 3.0, adapted by Ganzaa.

The exercises (Try it yourself and the practice questions) are written by Ganzaa.

1Arteries

2Veins

3Capillaries

4Comparing blood vessels

5Measure your heart rate