Mechanical contractions of the heart per beat.

The heart as a pump in the double circulatory system.

Double circulatory system- as mammals we have a circulatory system which requires the blood to pass through the heart twice per beat. Hence, double circulatory system.

One beat allows the blood to be oxygenated (allowing oxygen to bind to the haemoglobin in our blood creating oxyhaemoglobin). Deox blood travels from our heart to our lungs.

The second beat pumps oxygenated blood around our body. Blood travels from our heart around our body.

The contractions of the cardiac cycle in order.

Initially, the heart is relaxed, artia and ventricles are relaxed causing a pressure drop in the heart.

This, allows the atria to fill with blood from both the pulmonary vein and the vena cava.

This filling with blood causes the atrial pressure to rise. Pressure in the atria is now higher than in the ventricles.

This makes the atrioventricular valves open wide. 

Then the atria contract forcing all the blood into the relaxed ventricles. Semi lunar valves are closed until ventricles contract.

Because the pressure is higher in the ventricles the AV valves shut to prevent backflow of blood. 

The final stage is the blood pumped from the ventricles into…