Describe the cardiac cycle (7 marks)

During diastole the atria and ventricles are relaxed. The atrioventricular valves are open and the semilunar valves are closed. Deoxygenated blood enters the right atrium from the venae cavae and oxygenated blood enters the left atrium from the pulmonary veins. This is followed by atrial systole. The left and right atria contract. The volumes of the atria reduce, causing the pressure of the atria to increase. This results in the atrioventricular valves opening and blood moving from the atria into the ventricles. Blood in the right atrium moves past the tricuspid valve to the right ventricle. Blood from the left atrium moves past the mitral or bicuspid valve into the left ventricle. 

Ventricular systole occurs next. The right and left ventricles contract, reducing the volume in each ventricle. The resulting increase in pressure causes the atrioventricular valves to close. The semi-lunar valves open. Oxygenated blood from the left ventricle flows through the aorta and is carried to the rest of the body. Deoxygenated blood from the right ventricle flows through the pulmonary vein to the lungs, where it gains oxygen.