The Circulation System — Page 26, Lesson 37

I want to walk you through the circulation system, starting at its centre — the heart. The heart is a muscular pump, located in the chest cavity. If you look at a section through a human heart, you can see it has four chambers. There are two ventricles, which are the most powerful parts of the heart, and two auricles, also called atria — 'atrium' is the singular of 'atria' — which are the upper chambers of the organ. Let's talk about what the ventricles do. The ventricles supply the main force that propels the blood through the lungs and the circulatory system. Specifically, the right ventricle pumps deoxygenated blood to the lungs, and the left ventricle pumps the oxygenated blood around the body. So the right side handles blood that needs oxygen, sending it to the lungs to pick up oxygen, and the left side handles the freshly oxygenated blood, pushing it out to the rest of the body. Now the atria — their job is different. The atria act principally as entryways to the ventricles. They also pump weakly to help move the blood on through the atria into the ventricles. By doing that, they increase the effectiveness of the ventricles as pumps. So think of the atria as receiving chambers that give the ventricles a little extra help filling. Leading into and out of the heart are various blood vessels. The arteries carry blood from the heart at high pressure, and the veins return blood to the heart at low pressure. Now, because the heart itself is a muscle, it needs its own blood supply system. That system is provided by the coronary arteries and veins. A narrowing or blockage of the coronary arteries or veins is the cause of one of the major diseases that may affect the heart — that's coronary artery disease. Let's move on to oxygen carriage. A small amount of oxygen is dissolved in the blood plasma, but the major vehicle for oxygen carriage is haemoglobin, which is contained in the red blood cells. Like all cells in the body, red blood cells die and are replaced in a regular cycle. Red blood cells contain no nuclei, and they have an average life in man of about 108 days. New red blood cells and their haemoglobin are made mainly in the bone marrow, but some are also produced in the liver and spleen. Here's how oxygen gets transported: oxygen from the lungs combines with the haemoglobin in the red blood cells to form oxyhaemoglobin. That oxyhaemoglobin will then release the oxygen to tissues that require it. Blood containing a large amount of oxygen is a red colour, while blood with a shortage of oxygen is of a bluer tinge. This is why if an artery is cut, bright red blood spurts out — because arterial blood is oxygenated. If a vein is cut, then dark blood oozes out — that's deoxygenated blood returning to the heart. Finally, let's look at the carriage of carbon dioxide. A small amount of the carbon dioxide may be dissolved in the plasma, but the majority is carried in chemical combination with water in the form of carbonic acid. So carbon dioxide is not primarily carried by haemoglobin — it's mostly converted into carbonic acid in the blood for transport back to the lungs to be exhaled.