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KS5 Biology
Lesson Plan 8 – Heart and Circulation
Science at Work in Healthcare
Post – 16 Science Education Pack
Resource Sheet 8.1 – The Human Heart
Your heart is a pump. Its job is to pump blood around your body so that your cells receive
vital materials such as glucose and oxygen and can get rid of waste products such as
carbon dioxide. Blood is pumped into a system of arteries which then split into smaller
capillaries inside the tissues. It is through these capillaries that exchange of materials
between the blood and cells take place.
Your heart is made up of four chambers and connects to a number of different blood
vessels. The reason that the heart is structured in this way is because it has to pump blood
around two circulatory systems.
Double Circulation
Firstly, blood is pumped through the pulmonary circuit.
This circuit takes blood from the heart to the lungs to
pick up oxygen and drop off carbon dioxide. It then
returns to the heart.
Blood is then pumped through the systemic circuit
which takes it around the body so that it can distribute
oxygen which it picked up in the lungs.
This means that any given blood cell actually passes
through the heart twice for every time it passes around
the whole body. Diagram 1 shows how this happens.
How Does Your Heart Work?
Possibly the most remarkable feature of the heart is its
ability to work continuously without fatigue. During the
course of a typical human lifespan a heart will beat
something like 2.5 billion times and will pump more than
150 million litres of blood through each ventricle.
Diagram 1. The systemic
and pulmonary circuits of
the human circulatory
system
KS5 Biology
Lesson Plan 8 – Heart and Circulation
Science at Work in Healthcare
Post – 16 Science Education Pack
The reason that it can maintain this rate of work is down to the nature of cardiac muscle.
This consists of a network of interconnected muscle fibres divided up into uninucleate cells
containing fine contractile myofibrils. The nature of the connections between the muscle
fibres ensure that there is a rapid, synchronised spread of excitation through the heart wall,
creating a uniform contraction.
Your heart works by continually contracting and relaxing. These periods are called systole
and diastole respectively. When it contracts blood is pushed out and around the pulmonary
and systemic circuits. When it relaxes, blood re-enters the heart ready to be pushed out
again on the next contraction. Take a look at diagram 2 and go through the steps laid out
below to see how it all works in a bit more detail:
1. Blood from the systemic circuit (body) fills the right atrium and blood from the
pulmonary circuit (lungs) fills the left atrium. The walls of both atria are now
stretched outwards.
2. When the pressure in the atria is high enough the cuspid valves open and the blood
flows from the atria into the ventricles. The cuspid valves are one-way valves letting
blood into the ventricles, but not out. Tendons fixed to the ventricle walls stop the
cuspid valves from being
forced backwards and
allowing blood from the
ventricles to backflow into the
atria.
3. The walls of the ventricles
then contract. The left
ventricle walls are thicker than
the right and create a higher
pressure. This is because the
left ventricle pushes the blood
around the systemic circuit,
which is much bigger than the
pulmonary circuit.
Diagram 2. The structure of the heart
KS5 Biology
Lesson Plan 8 – Heart and Circulation
Science at Work in Healthcare
Post – 16 Science Education Pack
4. As pressure in the ventricles increases, the pocket valves at the base of the
pulmonary artery and aorta are forced open, allowing blood to be forced out of the
heart. The pulmonary artery takes blood to the lungs (pulmonary circuit) and the
aorta directs blood to the body (systemic circuit).
5. Blood from the pulmonary circuit arrives in the left atrium through the pulmonary vein
and blood from the systemic circuit arrives in the right atrium and the whole process
repeats...
Keeping the Pace
But why does your heart beat? Most muscles contract as a result of impulses reaching them
through nerves.
The heart, however, is different. Cardiac muscle is myogenic, meaning that the contractions
arise from within the heart muscle itself. It will continue to beat even when removed from
the body. This is because it contains its own pacemaker, called the sino-atrial node (SAN),
which is located in the wall of the right atrium.
The SAN is actually connected to two nerves, the vagus nerve and the sympathetic nerve
but these nerves only change the rate at which the SAN causes the heart to contract. The
vagus nerve slows the heart rate and the sympathetic nerve speeds it up.