The Circulation System — Page 26, Lesson 41

I want to walk you through the circulation system, starting with a critical point about where gas exchange actually happens. It is emphasized that this exchange—the movement of oxygen and carbon dioxide between the blood and the tissues—can only take place at the capillaries. Even the smallest arteries and veins have walls that are too thick to allow the exchange of gases. So the capillaries are the only site where that vital transfer occurs. Now let's trace the path of de-oxygenated blood. De-oxygenated blood passes from the venous capillaries into veins, which progressively increase in size as they merge together. That blood returns eventually to the right atrium of the heart. From the right atrium, it passes into the right ventricle. The right ventricle then pumps the blood via the pulmonary artery to the lungs. Once in the lungs, the carbon dioxide and water that are carried in the blood are released to the lungs, and at the same time, the blood is re-oxygenated—it picks up fresh oxygen. The now oxygenated blood then returns to the heart via the pulmonary vein and enters the left auricle. From there, it is pumped back into the aorta by the left ventricle, ready to be distributed to the rest of the body. Let's move on to a key concept: cardiac output. Cardiac output is defined as the quantity of blood pumped by the left ventricle into the aorta each minute. It is the product of two factors: the stroke volume and the heart rate, which is also called the pulse rate. The normal pulse rate for a healthy average individual is 70 beats a minute. The stroke volume of a healthy heart remains constant—it doesn't change much from beat to beat under normal conditions. Because stroke volume stays constant, cardiac output will increase linearly with an increase in pulse rate. In other words, if your heart beats faster, you pump more blood per minute, in direct proportion. However, there is a limit. The limit of effective heart rate in man is about 180 beats a minute. Above this rate, the heart cannot fill up quickly enough between beats, and as a result, cardiac output will actually decrease. So pushing your heart rate beyond that point becomes counterproductive. For the average human being at rest, the cardiac output is 5.0 to 5.5 litres a minute. That's the volume of blood the left ventricle sends into the aorta every minute when you're sitting still.