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Heart & Circulatory System. How are substances transported in our bodies?. The next organ system we will be considering can be known by two names…. Blood system. Circulatory system. The circulatory system is made of two parts: 1. The heart 2. The blood vessels. What we use in the air.

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how are substances transported in our bodies
How are substances transported in our bodies?

The next organ system we will be considering can be known by two names…..

Blood system

Circulatory system

The circulatory system is made of two parts:

1. The heart

2. The blood vessels

what we use in the air
What we use in the air

One of the substances that the circulatory system transports are respiratory gases.

For example, the oxygen that the body needs, must be taken to the cells and the carbon dioxidewaste gas must be taken away from the cells.

dissolved gases in blood
Dissolved gases in blood

blood high in oxygen

blood low in oxygen



As these gases are carried by the blood, it means that in terms of the presence of gas, we have two types of blood.

At all times these two types of blood cannot mix.

Why do you think this is?

a double circulatory system
A double circulatory system


Body cells

Because of this our circulatory system is in two parts.

It is in fact called a double circulatory system.

This section of the system including the left side of the heart, deals with the oxygenated blood.

This section of the system including the right side of the heart, deals with the deoxygenated blood.

the heart structure and function
The heart: structure and function

The Heart

In this unit we are going to focus on the heart..

This is the pump at the centre of the circulatory system.

Not all living organisms have hearts.

Also, the shape of the heart can change, depending on what species you look at.

one big muscle
One big muscle
  • The heart is made almost entirely of muscle. It is unlike any other muscle in your body because it never tires. And even though it is full of blood it still needs it own blood supply.
  • Oxygenated blood is carried to the heart by the coronary arteries.
  • Each side of the heart has two chambers.
        • An top chamber or atrium and
        • A bottom chamber or ventricle

Each of these chambers has its own function.

Look at the diagram on the next slide to see what that is.

the valves structure and function
The valves: structure and function

Besides the chambers there is also another very important part of the heart – the valves.

These valves are found between the top and bottom chambers on both sides of the heart.


These valves are rather like doors that only open in one direction.


valves in action
Valves in action

As the atrium fills with blood, the valves are closed.

When the atrium contracts and squeeze the blood, the valves are pushed open.

These valves are connected to the side wall of the heart by tough tendons.

These tendons allow the valves to close but not invert.

tendon holding a valve
Tendon holding a valve



wall of ventricle

These tendons can be compared to an arm holding onto the handle of a door.

analogy of a valve
Analogy of a valve

The arm bends as the door is opened.

When the door is closed the arm is fully extended.

It would be impossible for the door to open in the other direction without the person moving with it.

The tendon (represented by the arm) is held in a fixed position and therefore the valve (door) can only open in one direction.

preventing backflow
Preventing backflow

However, the valves remain firmly shut.

The blood will naturally push against the valve.

In this way, the blood can be moved from chamber to chamber quite efficiently.

The valves prevent the blood from moving in the wrong direction.

more valves
More valves

we also find valves here

...and here!

These extra valves stop the blood from re-entering the heart when it is pumped from the ventricles.

lub dub
Lub - dub




When the blood knocks against the first heart valves, it makes a ‘lub’ like sound.

When the blood knocks against the second set of heart valves, it makes a ‘dub’ like sound.

The blood ‘slaps’ against the valve and then passes along the artery.

These two sounds – lub and dub – are actually what we hear as our heartbeat. So our heartbeat is in fact the sound of the valves opening and closing.

the action in motion
The action in motion

Now that we can name all of the parts of the heart it is time to see how they work to push blood around the body.

The heart pumps blood when its muscle contracts. As the muscle contracts the chamber gets smaller and squeeze the blood out.

The two sides of the heart work together. The atria contract and relax at the same time, as do the ventricles.

The next two slides describe what occurs inside the heart during one heart cycle.

the action in motion1
The action in motion

blood from the lungs

blood from the body

1. The heart beat begins when the heart muscles relax and blood flows into the atria.

  • The atria then contract and the valves open to allow blood into the ventricles.
the action in motion2
The action in motion
  • The ventricles contract forcing the blood to leave the heart. At the same time, the atria are relaxing and once again filling with blood.

The staged cycle then repeats itself.

left and right ventricles
Left and right ventricles

Do you notice anything different about the two sides of the heart?

Well, the left ventricle wall is clearly thicker than the right ventricle wall.

Why is this?

To answer this question, think about where the blood is going when it leaves the right and left ventricles.

different destinations
Different destinations

This journey is far less demanding. The right ventricle only has to pump blood from the heart to the adjacent lungs.

This journey is enormous. The left ventricle has to pump blood all over the body!


body cells

Therefore, the left ventricle needs a thicker wall to generate a stronger ‘squeeze’ on the blood.


Drag the labels to the correct numbered boxes

on the diagram of the blood system.