ANATOMY

Detailed explanation-1: -The contraction begins in the upper two chambers of the heart, called the atria, and then proceeds to the lower two chambers, the ventricles. A complete contraction and relaxation of the heart is a single heartbeat, also called a cardiac cycle.

Detailed explanation-2: -The contraction of the ventricles is more efficient when starting at the apex of the heart because the vessels that carry the blood away from the heart are located in the base of the heart.

Detailed explanation-3: -A muscle contraction forces the blood through the tricuspid valve into the right ventricle. When the right ventricle contracts, blood is forced through the pulmonary semilunar valve into the pulmonary artery. Then it travels to the lungs. In the lungs, the blood receives oxygen then leaves through the pulmonary veins.

Detailed explanation-4: -The bundle of His then passes on the wave of electrical activity to the Purkyne fibres, found along the bottom of the heart around the ventricular walls. This causes the left and right ventricles to contract simultaneously, from the bottom up.

Detailed explanation-5: -There is a period called isovolumetric contraction during which the ventricles contract but the pulmonary and aortic valves are closed as the ventricles do not have enough force to open them. The atrioventricular valves also remain closed during the isovolumetric contraction period.