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system
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This is known as myogenic muscle contraction

➢ The myogenic activity needs to be controlled in order to make the timing of the
contractions unified and useful
➢ “Du - dank” sound
○

“Du” ⇒ the atrioventricular valves close

○

“Dunk” ⇒ the semilunar valves close
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It’s louder because they’re smaller and thus the pressure is higher

➢ Within the right atrium, there’s a mass of specialized tissue that has properties of
both muscle and nervous system cells within its walls
○

This tissue is called the sinoatrial node (SA node)

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The blood system

Topic 6
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8 seconds
giving you a heart rate of 72-75 beats/minute

○

It is controlled by nerves from the brain

➢ Also in the right atrium s another mass of specialized muscle tissue known as the

atrioventricular node (AV node)
○ The AV node receives the signal from the SA node, delays for approximately
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Biology SL

➢ Heart valves open and close based on blood pressure differences on either side of
any one valve
➢ Both sides have similar pressures and volumes of blood at the same time
○

All pressures are measured in mm/Hg (Hg = mercury)

➢ Diastole is the term used for a chamber of the heart that isn’t contracting
○

When both chambers are in rest, then they’re both undergoing diastole

➢ Systole is the term used for a chamber of the heart that is contracting
➢ Both chambers in diastole
○

The atrial pressure is slightly higher than the ventricular pressure keepin
the atrioventricular valve open

○

Blood returning to the atrium via the veins moves passively down to the
ventricle through the open valve

○

The pressure in the aorta is much higher than in the left ventricle
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This difference in pressure keeps the left semilunar valve closed and
prevents backflow into the ventricle

➢ Atria in systole, ventricles in diastole
○

When the SA node “fires”, the pressure in the atria increases and blood goes
into the ventricle

○

The pressure produced by this systole is low, because:
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The walls of both atria are very thin

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There’s no need for high pressure, as much of the volume of blood
has already moved passively within the ventricle through the open
atrioventricular valve

➢ Atria in diastole, ventricles in systole
○

When the AV node “fires”, the pressure in the ventricle increases a lot, so
the atrioventricular valve closes to prevent backflow of the blood to the atria
■

○

Called early ventricular systole

The pressure in the aorta is still far higher than in the ventricle, so the
semilunar valve is closed

○

The large volume of blood in the ventricle and its thick muscular walls
permit the pressure to build up as systole continues

○

The pressure in the ventricle becomes greater than that in the aorta and the
semilunar valve opens allowing the ventricle to pump blood in the aorta
■

○

Called late ventricular systole

As the ventricle finishes its contraction, the pressure inside it once again
drops below the pressure in the aorta and the semilunar valve closes
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The blood system

Topic 6

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