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1-3. FLOW OF BLOOD THROUGH THE HEART
It is helpful to follow the flow of blood through the heart in order to understand the relationship of the heart structures. Remember, the heart is the pump and is also the connection between the systemic circulation and pulmonary circulation. All the blood returning from the systemic circulation must flow through the pulmonary circulation for exchange of carbon dioxide for oxygen. Blood from the upper part of the body enters the heart through the superior vena cava and blood from the lower part of the body enters through the inferior vena cava.
a. Blood from the superior vena cava and inferior vena cava enters the heart at the right atrium. The right atrium contracts, and blood is forced through the open tricuspid valve into the relaxed right ventricle.
b. As the right ventricle contracts, the tricuspid valve is closed, preventing back flow into the atrium. The pulmonary semilunar valve opens as a result of the force and movement of the blood, and the right ventricle pumps the blood into the pulmonary artery.
c. The blood is carried through the lung tissues, exchanging its carbon dioxide for oxygen in the alveoli. This oxygenated blood is collected from the main pulmonary veins and delivered back to the left side of the heart to the left atrium.
d. As the left atrium contracts, the oxygenated blood flows through the open bicuspid (mitral) valve into the left ventricle.
e. As the left ventricle contracts, the bicuspid valve is closed. The aortic semilunar valve opens as a result of the force and movement of the blood, and the left ventricle pumps oxygenated blood through the aortic semilunar valve into the aorta, the main artery of the body. Oxygenated blood now starts its flow to all of the body cells and tissues. The systemic circulation starts from the left ventricle, the pulmonary circulation from the right ventricle.
