How the Heart Works – blood pumped through the heart
The heart is an amazing organ; it beats 60-100 times per minute with the individual at rest. The amount of blood pumped out with each of these beats, called the “stroke volume”, is about 70 mL (a little more than a quarter of a cup). In women, due to their smaller body size, the average resting stroke volume is 50-60 mL. This means that the typical adult heart would pump more than 7000 litres of blood per day. This blood flow supplies oxygen and nutrients to the organs of the body as well as the equally important function of taking waste products away.
Figure 1: Anatomy of the heart.
The heart is divided into four muscular chambers with each side having one atrium and one ventricle. The bottom chamber, the ventricle, is the more important. It is responsible for pumping the blood out of the heart and helps to drive the pressure that keeps blood flowing. To achieve this important function the ventricles have thicker muscular walls. The upper chamber, the atrium (“atria” for plural), is smaller and helps pump blood into the adjacent ventricle. There are valves in the heart that to prevent backwards flow of blood from the ventricle into the atria, and other valves that prevent backwards flow into the ventricles.
A cardiac cycle is the rhythmic contraction and relaxation of the heart that occurs with each heart beat. This heart beat is often described as a “lub-dub… ...lub-dub…” sound. Initially the heart is relaxed and fills passively. During this time, blood flows through the vena cava, the right atrium, and passes through the triscupid valve which separates the right atria and right ventricle. On the left side oxygen-rich blood from the lungs flows through the pulmonary veins, the left atrium, and passes through the mitral valve which separates the left atria and ventricle.
Figure 2: Blood flow through the heart.
An electrical signal then spreads through the atria to stimulate contraction of their walls which squeezes in the last little blood into each the ventricle. This electrical signal then travels down to the lower part of the heart and spreads across both ventricles simultaneously causing them to contract and pump the blood out of the heart. At this point the tricuspid and mitral valves close to prevent blood flow back into the atria. This rapid closure of both valves produces the “lub” sound of the heart beat. The blood from the right ventricle exits through the pulmonary valve into the pulmonary arteries to reach the lungs. The blood exits from the left ventricle through the aortic valve, out through the aorta to the rest of the body. As the ventricles then relax, the pressure inside drops below the pressure of the aorta and pulmonary arteries, which causes the aortic and pulmonic valves to close. These two valves closing causes the “dub” noise of the heart beat. The blood from the aorta circulates through the systemic circulation before returning to the heart via the vena cava. The cardiac cycle repeats as the relaxed muscular walls allow the filling of ventricles.
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