THE CARDIAC
CYCLE,
CIRCULATION AND
HEART RATE…read more

THE CARDIAC CYCLE
The heart pumps blood around the body by contracting and relaxing the
atria and ventricles in sequence
Blood is prevented from flowing backwards by semilunar and atrio-
ventricular valves
The sequence of events in one complete heartbeat is called the cardiac cycle
Blood flows into the The atria contract, The ventricles
atria. Atrio-ventricular pushing the blood into contract, forcing the
valves open the ventricles blood into the aorta
and pulmonary artery…read more

SCIENTISTS
Claudius Galen (Ancient Greece)
o Cut up animals to study them, so knew about the chambers in the
heart
o He thought arterial blood was made by the heart, while blood in
veins was made by the liver, sucked through veins by the heart
and consumed by the organs
William Harvey (1578-1657)
o Showed heart valves stop the backflow of blood
o The heart is a pump, rather than something that sucks
o The pulse is caused by the heart pumping blood into the arteries
o The same blood is circulated around the body over and over again,
not manufactured and consumed…read more

CHANGING HEART
RATE
When you exercise, your muscles need more oxygen to work
harder, so you breathe faster.
Your heart also pumps faster to deliver more oxygenated
blood to your muscles
When you stop exercising, your heart rate gradually returns
to resting heart rate
Hormones also affect the heart rate e.g. adrenaline is
released when you get a shock or are in danger (triggers
"fight or flight" response)
This makes the heart rate increase so that muscles have
more oxygen…read more

THE HEART AS A
PACEMAKER
The heart is `told' how to beat by a group of cells called the
pacemakers
These cells produce a small electric current which spreads to
the surrounding muscle cells, causing them to contract
The 2 clusters of these in the heart are:
o The sino-atrial node (SAN)
o The atrio-ventricular node (AVN)…read more

HOW THE PACEMAKER
CELLS CAUSE THE
HEART TO BEAT
1. The electrical impulse is produced in the SAN
2. The signal causes the atria to contract, and then goes to
the AVN
3. The signal is delayed at the AVN to allow time fro the
blood to flow into the ventricles
4. The electrical signals then travel along the purkinje
fibres and along the bundle of His
5. This causes the atria to contract
6. The blood is forced out of the heart along the aorta and
the pulmonary artery…read more