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Open and closed circulatory systems
· All vertebrates have a closed circulatory system
(blood is always enclosed in vessels)
· Insects have an open circulatory system. Their
blood is not contained within vessels and just fills
their body cavity, which is called a haemocoel.
· The blood in insects does not transport oxygen.
This is done by tubes called trachea. It carries air
directly to the tissues.…read more

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Single and double circulatory systems
· The human circulatory system is a double
circulatory system.
· One part serves the lungs and is called the
pulmonary circulation while the other serves the
rest of the body and is called the systematic
· Fish have a single circulatory system.
· The blood is pumped out of the heart to the gills,
where it picks up oxygen. But instead of going
back to the heart, it travels around the rest of the
body.…read more

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The structure of a human heart.…read more

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The cardiac cycle
· Atriole systole is when the heart is filled with blood
and the muscle in the atrial walls contract.
· Blood is forced from the atria down through the atrio-
ventricular valve into the ventricles.
· Ventricular systole is when the ventricle contracts and
pushes the blood out of the heart.
· As soon as the pressure in the ventricles becomes
greater that that in the atria the atrio-ventricular valve
shuts. This means the blood can't go backwards.
· Ventricular diastole then begins. This is when the
ventricle relaxes and the pressure drops. The blood
can't flow back though because of the semi-lunar
· Blood from the veins floods into the two atria, and the
whole cycle starts again.…read more

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The valves shut because the pressure of the
blood pushes up against the cusps of the
· Contraction of the papillary muscles, attached
to the valve by tendons, prevents the valve
from being opened the wrong way.…read more

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