As a reminder of the heart's A & P, blood returns to right atrium of the heart from the inferior and superior vena cavae. [Remember that there are no valves between the vena cavae and the right atrium]. Cardiac veins also empty blood into the right atrium. From there, the blood passes through the tricuspid valve into the right ventricle, then it passes during systole through the pulmonic valve into the pulmonary artery. Blood circulates through the pulmonary circulation becoming oxygenated in the capillaries around the lungs and returning to the left atrium via the pulmonary veins. [Remember that there are no valves between the pulmonary veins and the left atrium]. The blood travels from the left atrium through the mitral (bicuspid) valve into the left ventricle. It is pumped out of the ventricle through the aortic valve into the aortic arch and throughout the systemic circulation.
The Cardiac Cycle
Systole occurs during depolarization and contraction of the cardiac ventricles. This is when blood is ejected out of the heart into either the pulmonary or systemic circulations. The duration of systole is approximately 1/3 of the total cardiac cycle. During systole, the semilunar valves are open.
Diastole, the longer period, is when the ventricles fill with blood, initially passively following opening of the AV valves and more completely near the end when the atria contract. The cell membranes of the myocardial cells of the ventricles are repolarizing early during diastole and maintaining the resting potential later in diastole.
Two main coronary arteries branch off the trunk of the aorta close to the aortic valve (but outside of the chambers of the heart). Coronary arteries receive perfusion of blood from the aorta during diastole when the ventricular myocardium is resting. This allows for more time to oxygenate the myocardium between ventricular contractions.
The right coronary artery (RCA) extends off the right side of the aorta down the inferior side of the heart across the right atrium and the right ventricle. It perfuses the right atrium and the right ventricle. In 70% of the population, the RCA is the dominant coronary artery perfusing the posterior wall of the heart. The SA node is perfused by the RCA in approximately 55% of the population, and the AV node is perfused by the RCA in approximately 90% of the population.
The left coronary artery (LCA) branches immediately into the left anterior descending artery (LAD) and the left circumflex artery (LCX). The LAD perfuses the anterior wall of the left ventricle and 2/3 of the interventricular septum. The circumflex artery perfuses the lateral wall of the left ventricle. In 45% of the population, the LCX perfuses the SA node, and in 10% it perfuses the AV node. Also, the LCX perfuses the inferior-posterior surface of the heart in 10% of the population.
Coronary collaterals may exist between the RCA and the LCX to extend perfusion when one of these arteries is narrowed. Collateral circulation generally develops when coronary artery disease is more advanced in older patients.