1.1 how does the circulation work? (3)

1.1 bio.

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cardiac cycle phases

The cardiac cycle can be simplified into three phases:

1. atrial systole- The atria contract, forcing blood into the ventricles.

2. Ventricular systole- contraction of blood up into the arteries

3. Diastole- Elastic recoil as the heart relaxes causes low pressure in the heart , helping to refill the the chambers with blood from the veins.

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1st phase: Artial systole

blood returns to the heart due to skeletal and gaseous exchange muscles as you move and breath.

blood under low pressure flows into the left and right atria from the pulmonary veins adn vena cava.

As the atria fill , the pressure of blood against the atrioventricular valves pushes them open and blood begins to leak into the ventricles.

Then the atria walls contract forcing more blood into the ventricles.

This is known as the atrial systole.

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2nd phase: ventricular systole

The artial systole is immediately followed by ventricular systole.

The vetricles contract from the base of the heart upwards, increasing pressure in the ventricles.

This pushes blood up and out through the arteries.

The pressure of blood against the atrioventricular valves closes them and prevents backward flow of blood into the atria.

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3rd phase: diastole

The atria and ventricles then relax during diastole. elastic recoil of the relaxing heart walls lowers pressure in the artia and ventricles.

Blood under higher pressure in the arteries is drawn back towards the ventricles, closing the semilunar valves and preventing further backflow.

The coronary arteries fill during diastole. low pressure in the atria helps draw blood into the heart from the veins.

closing of the atrioventricular valves and then the semilunar vales creates the characteristic sounds of the heart.

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