 In this video, I will describe the path of blood flow through the heart, compare pressure and oxygen concentration of blood flowing out of the left and right ventricles into the systemic and pulmonary circuits. The primary function of the heart is to pump blood, the heart pumps blood into arteries generating blood pressure, which is the force that drives the flow of blood throughout the body. Blood then drains through veins into the heart. The heart is really two separate pumps connected to two separate circuits of blood vessels, the pulmonary circuit and the systemic circuit. The pulmonary circuit refers to the blood vessels of the lungs, as well as the arteries transporting blood from the heart to the lungs and the veins transporting blood from the lungs back to the heart. The systemic circuit refers to the rest of the blood vessels of the body. The pulmonary circuit is where oxygen moves into the blood, where blood becomes oxygenated. This occurs in the pulmonary capillaries, so the pulmonary capillaries in the lungs is the location where exchange of gases occurs where oxygen moves into the blood and carbon dioxide moves out of the blood. We represent oxygen-rich blood with a red color, and so you'll see here that the pulmonary veins are shown in red. So here's the left pulmonary veins that transport blood from the left lungs to the heart, and here's the right pulmonary veins. Blood flows through the pulmonary arteries to the heart, and the blood in the pulmonary arteries has a relatively low oxygen concentration. So here we see the left pulmonary arteries represented in blue, which transport blood that has a relatively low oxygen concentration from the heart to the left lung. There's also right pulmonary arteries transporting blood to the right lung. So in the systemic circuit, the arteries have a relatively high oxygen concentration. The largest systemic artery is known as the aorta. Then the aorta branches into numerous smaller arteries. Blood then flows from these arteries into systemic capillaries. So here we can see the systemic capillaries of the lower body, and up here we see systemic capillaries of the upper body. So you'll notice that the color of the blood in the illustration changes at the capillaries. At the systemic capillaries blood changes from red to blue, and then the blood that's draining into the veins and returning to the heart from the systemic capillaries is shown in the blue color here in the illustration. Here are the systemic veins from the upper body, and here are the systemic veins from the lower body. So the systemic veins drain blood into a chamber of the heart known as the right atrium. So the atria are the more superior chambers of the heart that receive blood from the veins. The right atrium receives blood from the systemic veins, and the largest systemic veins are known as the inferior and superior vena cava. So here we see the inferior vena cava that drains blood coming from the systemic veins of the lower region of the body. And then here we see the superior vena cava that drains the blood coming from the veins of the upper body. But both the superior and the inferior vena cava drain blood with a relatively low oxygen concentration into the right atrium. Then this blood is pumped from the right atrium down into the right ventricle. So the ventricle is the larger, more inferior chamber that pumps the blood into the arteries. The right ventricle pumps blood into the pulmonary arteries, and this blood then flows into the lungs where it becomes oxygen rich at the pulmonary capillaries and then drains through the pulmonary veins back to the heart where this blood will enter into the left atrium. So here we see the left atrium receives the blood from the pulmonary veins. Both the left pulmonary veins and the right pulmonary veins drain blood into the left atrium. Then the left atrium pumps blood down into the left ventricle and the left ventricle pumps blood out into the aorta, into the arteries of the systemic circuit. And so the arteries of the systemic circuit have a relatively high oxygen concentration, the veins of the systemic circuit have a relatively low oxygen concentration, the arteries of the pulmonary circuit have a relatively low oxygen concentration, and the veins of the pulmonary circuit have a relatively high oxygen concentration. The other major difference between the pulmonary and systemic circuit has to do with the length of blood vessels. So the distance that the blood will travel from the left ventricle to the right atrium through the systemic circuit is much longer than the distance that blood will travel from the right ventricle to the left atrium through the pulmonary circuit. For this reason, the blood pressure of the systemic arteries is much higher than the blood pressure of the pulmonary arteries. This blood pressure is the force that drives blood flow, and a higher blood pressure is required to force blood to flow a longer distance through a longer system of blood vessels. The thickness of the walls of the ventricles reflects this fact. The left ventricle has much thicker walls than the right ventricle. Here we see in the left ventricle, the wall is very thick, and then over here the right ventricle has a relatively thinner wall. That is because the right ventricle is pumping blood into the pulmonary arteries and produces a lower blood pressure relative to the left ventricle that pumps blood into the systemic arteries and generates a greater blood pressure.