MICROANATOMY

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: -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-3: -Also, the top-down direction of atrial contraction maintains a unidirectional flow of blood. The ventricles, on the other hand, contract starting from the bottom going up.

Detailed explanation-4: -This structure allows the heart to beat from the apex (bottom) upwards, to ensure all the blood is pumped out of the ventricles, through the semilunar valves and into arteries (this is analogous to squeezing toothpaste out of a bottle-if you squeeze from the bottom up to the top you can get it all out).

Detailed explanation-5: -This leads to a ‘bottom-up’ contraction of the ventricles, forcing blood up and out into the pulmonary artery (right) and aorta (left). The atria then re-fill as the myocardium relaxes.