AS AQA Unit 1 Biology- Heart

Atrial Systole- Atria contract, puching blood into the ventricles. Ventricles remain relaxed. ( Semi-Lunar valves closed, AV valves open )

Ventricular Systole- Atria relax. Ventricles contract, pushing blood away from the heart throught pulmonary artery and the aorta. ( Semi-Lunar valves open, AV valves closed )

Diastole- Atria are relaxed and fill with blood. Ventricles are also relaxed. ( Semi-Lunar valves closed, AV valves open )

Atrioventricular Valves- Between the atrium and the ventricle, prevents the backflow of blood when ventricular pressure exceeds artial pressure. Closure of these ensures blood moves to aorta and pulmonary artery.

Semi-Lunar Valves- In aorta and pulmonary artery, prevents the backflow of blood into ventricles. Closes when pressure in the aorta and pulmonary artery are greater.

Valves prevent the backflow of blood.

Atheroma is the build up of fatty deposits in the lumen. These fatty deposits block or partially block the lumen, restricting the blood flow and therefore restricting oxygen from reaching the heart, so the heart cannot respire, so 'dies', which is a mycocardial infarction ( or a heart attack! ). 

The SAN is like a pacemaker, it sends out waves of electrical activity, which cause the atria to contract. A band of non conducting tissue prevents impulses reaching the ventricles. The AVN then sends out waves of electrical activity which is sent down the Bundle of His to the purkyre fibres. This causes the ventricles to contract. There is a delay before the AVN, to allow time for the blood to fill the ventricles from the atria.