Once your atria and ventricles contract, each part of the system resets electrically. When the left ventricle contracts, it pushes blood through the valve of the aortic crescent and into the aorta. Similarly, during periods of rest or sleep, when the body needs less oxygen, the heart rate decreases. Some athletes may actually have a normal heart rate well below 60 because their heart is very efficient and doesn`t need to beat as fast. Changes in your heart rate are therefore a normal part of your heart`s efforts to meet your body`s needs. These electrical impulses first cause the atria to contract. Then the pulses descend to the atrioventricular (or AV) node, which acts as a kind of relay station. From there, the electrical signal passes through the right and left ventricles, causing them to contract. The atria are separated from the ventricles by the atrioventricular valves: both atria are thin-walled chambers that absorb blood from the veins. Both ventricles are thick-walled chambers that powerfully pump blood from the heart. The differences in the thickness of the walls of the ventricle are due to variations in the amount of myocardium present, which reflects the amount of force that each chamber must generate. The sinus node is a banana-shaped structure whose size varies, usually between 10 and 30 millimeters (mm) long, 5 to 7 mm wide and 1 to 2 mm deep.   When the ventricles contract, the atrioventricular valves close to prevent blood from flowing back into the atria.
When the ventricles relax, the crescent-shaped valves close to prevent blood from flowing back into the ventricles. Blood flows from your right atrium into your right ventricle through the open tricuspid valve. When the ventricles are full, the tricuspid valve closes. This prevents blood from flowing backwards into the atria while the ventricles contract (compression). When the right ventricle contracts, blood is pushed into the pulmonary artery through the valve of the pulmonary crescent. Then it moves to the lungs. The electrical signal begins in a group of cells at the top of your heart called sinus nodes (SA). The signal then descends through your heart, triggering first your two atria and then your two ventricles.
In a healthy heart, the signal moves very quickly through the heart, allowing the chambers to contract smoothly and in an orderly manner. Although it is convenient to describe the blood flow through the right side of the heart and then through the left side, it is important to recognize that the atria and ventricles contract at the same time. The heart works as two pumps, one on the right and the other on the left, operating simultaneously. Blood flows from the right atrium to the right ventricle and is then pumped into the lungs to receive oxygen. From the lungs, blood flows into the left atrium, and then into the left ventricle. From there, it is pumped into the systemic cycle. Blood passes through the superior vena cava into the right atrium of the heart. The right atrium contracts and pushes blood cells through the tricuspid valve into the right ventricle. The right ventricle then contracts and pushes blood through the pulmonary valve into the pulmonary artery, bringing it into the lungs. In the lungs, blood cells exchange carbon dioxide for oxygen.
Oxygenated blood returns to the heart through the pulmonary veins and enters the left atrium. The left atrium contracts and pumps blood through the mitral valve into the left ventricle. Eventually, the left ventricle contracts and pushes blood into the aorta. The aorta branches into several different arteries that pump oxygenated blood to different parts of the body. The heart wall myocardium is a functioning muscle that needs a continuous supply of oxygen and nutrients to function effectively. For this reason, the heart muscle has an extensive network of blood vessels to bring oxygen to the contracting cells and eliminate waste. The main task of a sinus node cell is to trigger heart action potentials, which can pass through heart muscle cells and cause contraction. An action potential is a rapid change in the membrane potential produced by the movement of charged atoms (ions). In the absence of stimulation, non-cardiac stimulator cells (including ventricular and atrial cells) have a relatively constant membrane potential; This is called rest potential.
This resting phase (see cardiac action potential, phase 4) ends when an action potential reaches the cell. This leads to a positive change in the membrane potential, called depolarization, which spreads through the heart and initiates muscle contraction. However, pacemaker cells have no resting potential. Instead, immediately after repolarization, the membrane potential of these cells automatically begins to depolarize again, a phenomenon known as pacemaker potential. As soon as the pacemaker`s potential reaches a fixed value, the threshold potential, it creates an action potential.  Other heart cells (including Purkinje fibers and the atrioventricular node) may also trigger action potentials; However, they do this more slowly, and if the SA node is working properly, its action potentials usually outweigh those that would be produced by other tissues.  The electrical signal travels through the network of conductive cellular pathways, which stimulates the contraction of your upper chambers (atria) and lower chambers (ventricles). The signal is able to pass through these pathways through a complex reaction that allows each cell to activate one next to it and stimulate it to “relay” the electrical signal in an orderly manner. When one cell at a time quickly transmits the electrical charge, the whole heart contracts into a coordinated movement, creating a heartbeat.
Pumps require a series of valves to keep fluid flowing in one direction, and the core is no exception. The heart has two types of valves that allow blood to flow in the right direction. The valves between the atria and ventricles are called atrioventricular valves (also called kuspid valves), while those at the base of the large vessels that leave the ventricles are called crescent valves. The sinus node (also called sinuatrial node, SA node or sinus node) is a group of cells called pacemaker cells, located in the wall of the right atrium of the heart.  These cells can generate an electrical impulse (action potential) that travels through the heart`s electrical conduction system, causing it to contract. In a healthy heart, the SA node continuously produces action potentials, adjusts the rhythm of the heart (sinus rhythm) and is therefore called the heart`s natural pacemaker. The rate of action of the generated potentials (and therefore the heart rate) is influenced by the nerves that feed them.  The sinus node was first discovered by a young medical student, Martin Flack, in the heart of a mole, while his mentor, Sir Arthur Keith, was on a bike ride with his wife. They made the discovery in a makeshift laboratory on a farm in Kent, England, called Mann`s Place.
Their discovery was published in 1907.   Your heart has 4 rooms. The upper chambers are called the left and right atrium and the lower chambers are called the left and right ventricles. A muscle wall called the septum separates the left and right atria, as well as the left and right ventricles. These are called the atrial and ventricular septums. You may have heard that your doctor refers to a condition called a “hole in the heart.” It simply means a tiny hole in the atrial septum that separates the atria (called PFO – Patent Foramen ovale or ASD – Atrial Septal Defect) or in the ventricular septum that separates the ventricles (called VSD – Ventricular Septal Defect). The left ventricle is the largest and strongest chamber in your heart. Blood first enters the right atrium of the heart. A muscle contraction forces blood through the tricuspid valve in the right ventricle. Two valves also separate the ventricles from the large blood vessels that carry blood from the heart: After the electrical signal causes your atria to contract and blood to pump into your ventricles, the electrical signal arrives at a group of cells at the bottom of the right atrium called the atrioventricular node, or AV node. The AV node briefly slows down the electrical signal, giving the ventricles time to receive blood from the atria. .