Contraction of the left atrium pushes a bit more blood into the left ventricle just before the ventricles begin to contract. Blood then flows from the left atrium through the mitral valve into the left ventricle. Blood from the pulmonary circuit enters the left atrium. The four pulmonary veins carry oxygen-rich blood from the lungs back to the heart. In the lungs, blood picks up oxygen and unloads carbon dioxide before it returns to the heart. The pulmonary arteries carry oxygen-poor blood to the lungs. Why do you think that is? Is it because (a) the pressure in these arteries is low (b) these arteries return blood to the heart or (c) these arteries contain blood that is low in oxygen? The correct answer is (c). Notice that the pulmonary arteries are blue here. In this schematic view, we have combined the two pulmonary arteries. It splits into two pulmonary arteries that carry blood to the right and left lungs. The pulmonary trunk is the first part of the pulmonary circuit. As the right ventricle contracts, the tricuspid valve closes and blood is forced out through the pulmonary semilunar valve into the pulmonary trunk. Contraction of the right atrium pushes a bit more blood into the right ventricle just before the ventricles begin to contract. Blood then flows from the right atrium through the tricuspid valve into the right ventricle. Blood from all three vessels flows into the right atrium. The coronary sinus returns blood from the heart itself. The inferior vena cava returns blood from areas inferior to the diaphragm. The superior vena cava returns blood from body areas superior to the diaphragm. Three main vessels bring blood back into the heart. Let's begin with blood returning to the heart from the systemic circuit, that is, from everywhere in the body except for the lungs. We'll do this by making the rest of the blood invisible and slowing the heart down. Let's follow a single spurt of blood as it's pumped through the cardiovascular system. As we do this, keep in mind that the heart consists of two pumps side-by-side. INSTRUCTOR: Now that you know the basic anatomy of the heart, let's follow the path that blood takes through the heart, the pulmonary circuit, and the systemic circuit.
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