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. As we do this, keep in mind that the heart consists of two pumps side-by-side. Let's follow a single spurt of blood as it's pumped through the cardiovascular system. We'll do this by making the rest of the blood invisible and slowing the heart down. Let's begin with blood returning to the heart from the systemic circuit, that is, from everywhere in the body except for the lungs. Three main vessels bring blood back into the heart. The superior vena cava returns blood from body areas superior to the diaphragm. The inferior vena cava returns blood from areas inferior to the diaphragm. The coronary sinus returns blood from the heart itself. Blood from all three vessels flows into the right atrium. Blood then flows from the right atrium through the tricuspid valve into the right ventricle. Contraction of the right atrium pushes a bit more blood into the right ventricle just before the ventricles begin to contract. As the right ventricle contracts, the tricuspid valve closes and blood is forced out through the pulmonary semilunar valve into the pulmonary trunk. The pulmonary trunk is the first part of the pulmonary circuit. It splits into two pulmonary arteries that carry blood to the right and left lungs. In this schematic view, we have combined the two pulmonary arteries. Usually veins are shown as blue. Notice that the pulmonary arteries are blue here. 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). The pulmonary arteries carry oxygen-poor blood to the lungs. In the lungs, blood picks up oxygen and unloads carbon dioxide before it returns to the heart. The four pulmonary veins carry oxygen-rich blood from the lungs back to the heart. Blood from the pulmonary circuit enters the left atrium. Blood then flows from the left atrium through the mitral valve into the left ventricle. Contraction of the left atrium pushes a bit more blood into the left ventricle just before the ventricles begin to contract. As the left ventricle contracts, the mitral valve closes and blood is forced out through the aortic semilunar valve into the aorta. The aorta is the first part of the systemic circuit. The systemic circuit delivers oxygen-rich blood to all tissues in the body. In the tissues, blood delivers oxygen and picks up carbon dioxide. This oxygen poor blood then returns to the heart and brings us back to where we started. Think about the heart as a double pump serving these two circuits. Which of the following do you think is true about the volume of blood pumped by each ventricle? Is it (a) the left ventricle pumps more blood than the right ventricle; (b) the left and right ventricles pump the same amount of blood; or (c) the left ventricle pumps less blood than the right ventricle? The correct answer is (b). Both ventricles pump the same amount of blood. To see why this is the case, let's rearrange the plumbing so we can see our double pump system more clearly. Notice that what we really have is one big loop with two pumps that pump at the same time. That means that what flows into one pump must be the same as what flows out of the other pump, because there is nowhere else for the blood to go. Notice that the systemic circuit is much longer than the pulmonary circuit. Which circuit do you think operates at a higher blood pressure? Is it true that (a) the systemic circuit operates at a higher pressure; (b) the pulmonary circuit operates at a higher pressure; or (c) both circuits operate at the same pressure? The correct answer is (a), the systemic circuit operates at a higher blood pressure than the pulmonary circuit. This is because the total resistance in the systemic circuit is much higher than in the pulmonary circuit. This is why the left ventricle wall is thicker than the right ventricle wall. We've now followed a spurt of blood step by step along the path it takes through the heart. Take some time to cement these events in your mind. A firm grasp of this information will help you understand the physiology of the heart, which is the next topic of this chapter.