Tasks of the atria
In the atria, the heart collects the blood from the preceding circulatory segments. Through the upper and lower vena cava, the blood from the body’s circulation reaches the right atrium. From there it is pumped through the tricuspid valve into the right ventricle.
The atrium itself has hardly any pumping function. Rather, the blood is sucked into the right ventricle by the negative pressure that is created in the right ventricle during the relaxation phase. The blood that ends up in the left atrium comes from the pulmonary circulation.
From the left atrium it is pumped through the mitral valve into the left ventricle. The atria, like the ventricles, have a tensing and a relaxing phase. However, these phases run in the opposite direction to those of the ventricles. In the relaxation phase of the ventricles, the atria must contract so that they can pump blood into the ventricles. As the ventricles contract, the atria fill up again with the blood from the previous stages of circulation.
Role of the heart in the circulatory system
The blood flow divides the heart into a right and a left half. Colloquially one speaks of the “right” and “left heart”. While the right half of the heart collects the blood from the entire body’s circulation and pumps it into the pulmonary vessels, the left half of the heart receives the blood from the pulmonary circulation and from there it flows back into the rest of the body.
Although both halves of the heart have to handle the same amount of blood, the left ventricle is much more muscular. This is due to the fact that it has to pump the blood against a higher pressure. Depending on the condition of the body, the heart has to meet different demands.
In a lying person, the heart has comparatively little to do. When standing, part of the blood must be pumped into the brain against gravity. This requires a little more force.
Anyone who does sport drives his heart to peak performance. Because during sport, the muscles of the body must be supplied with more nutrients and oxygen. This requires a boost to the cardiovascular system, which means more work for the heart.
Task of the excitation conduction system from the heart
For the heart to pump blood reliably and evenly into the circulation, all the muscle cells of the heart must be coordinated. This is what the excitation conduction system is for. This consists of nerves that transport information from one heart muscle cell to the next.
The excitation conduction system begins at the sinus node in the atria. When the electrical signal reaches the muscle cells there, they tense up and pump the blood further into the heart chambers. The muscle cells then relax again.
Meanwhile, the signal continues to run in the excitation conduction system. It passes through the cardiac septum to the tip of the two ventricles and then, along the outer wall of the heart, back to the base of the heart. In the ventricles, the signal also leads to tension in the heart muscle cells, which causes the blood to be pumped from both chambers into the circulation.
While the electrical signal returns in the ventricles and the muscles relax there, the signal for the next heartbeat is generated at the sinus node. The sinus node is the pacemaker of the heart. This means that the electrical impulses that make the heart beat are generated here.
The sinus node is located in the right atrium. From there, the excitation spreads to the AV node and is then passed on to the ventricles. Normally, the sinus node sets a rhythm of about 60 to 80 beats per minute.
During physical exertion, for example, in stressful situations or when anxiety is present, the heart beats faster. In order to control these processes, the sinus node receives information from the brain and converts it into faster or slower pulses. The AV node has a watchdog function in the excitation conduction system of the heart.
The excitation of the heart muscle cells spreads from the sinus node through the atria and ends up at the AV node. This node transmits the given heart rate to the heart chambers. The AV node has an important function when the sinus node is no longer functioning properly.
If it gives impulses too quickly, as is the case with atrial fibrillation, for example, the AV node does not transmit all excitation into the ventricles. In this way, it controls the frequency and ensures that the ventricles continue to work at a normal speed. If the sinus node fails completely, the AV node steps in. It then itself generates the impulses that make the heart beat. However, in this case the heartbeat is a bit slower.