The heart is a hollow muscle in our chest that pumps our blood through the body. Together with the arteries and the veins, it forms the circulation system of our blood. The heart pumps oxygenated blood to the muscles and organs. These, in turn, send de-oxygenated blood back to the heart. An adult person's heart pumps about 70 times per minute. About 4 or 5 litres of blood is circulated every minute.

The chambers of the heart

The heart consists of four separate pockets: two antechambers (the atria) and two chambers (the ventricles). Between each ventricle and atrium there is a valve which ensures that the blood flows in only one direction (from atrium to ventricle) and which prevents the blood from flowing back to the antechambers when the heart contracts. The heart is also divided into two halves, separated by the septum (a kind of dividing wall). These two halves differ remarkably from each other. The blood that flows from the body enters the right atrium and goes to the lungs through the right ventricle. Here it is supplied with oxygen and then goes to the left atrium. From there it goes back to the body through the left ventricle.

The electrical activity

The heart muscle functions mainly as a pump. To do this, the heart contracts at a particular rate and according to a fixed pattern: the atria contract first, followed by the ventricles. In order to maintain this speed and coordination, the heart has an electrical conduction system. The electrical conduction begins at the sinoatrial node (the pacemaker of the heart), to ensure that the antechambers contract and pump the blood to the chambers. The signal is transmitted to the atrioventricular node (AV node), which in turn activates the chambers and causes them to contract through the bundle branches and Purkinje fibres. The blood is pumped through the ventricles to the rest of the body.

After the contraction of the heart the muscle cells are 'discharged’ and ‘recharged’ in this resting phase in order to be ready for the next contraction. The sinoatrial node starts the whole cycle again, at a speed of about 60 to 100 times per minute. A normal heart rhythm is regular. Under effort, the sinus node will fire off electrical impulses at a higher rate, making the heart beat faster to meet the increased oxygen needs of the body. The cardiologist can assess your heart rhythm using an electrocardiogram or ECG.

The heart valves

The heart has four valves which ensure that the blood can flow in one direction only. The mitral valve and tricuspid valve between the atria and the ventricles and the aortic valve and pulmonary valve at the place where the blood leaves the heart. Remarkably enough, the valves work without actively moving themselves. In fact, heart valves do not have muscle cells and wobble in the bloodstream. The valves between ventricles and atria are attached to the wall of the heart by the chordae tendineae (small tendons). When a heart valve is working properly, the valve leaves close perfectly and will open fully as well. If a valve is not working properly, the blood can flow back (e.g. from the right ventricle back to the right atrium).

The blood comes from the body through two large veins (superior vena cava and the inferior vena cava) into the right ventricle. The blood flows through the open tricuspid valve into the right ventricle. Then the tricuspid valve closes and the pulmonary valve opens. The contraction of the right ventricle pumps the blood to the lungs. The blood enters the left atrium (oxygen-rich blood) and flows into the left ventricle via the open mitral valve. The mitral valve closes and the aortic valve opens. The contraction of the left ventricle pumps the blood through the body via the aorta and the other arteries. This is illustrated in figure 3. The oxygen-starved blood flows through the blue portion and the oxygenated blood flows through the red part.

The coronary arteries

The heart pumps oxygen-rich blood to the rest of the body but it also needs oxygen for itself. The blood vessels that supply the heart muscle with oxygenated blood are called the coronary arteries. These run along the surface of the heart, between the muscle layer and the pericardium. They both branch out from the aorta on the left half of the heart, just above the aortic valve. There are two major coronary arteries: one on the right side (the right coronary artery or the RCA) and one on the left side (the left coronary artery or LCA). The coronary artery on the left side splits into two major branches: the LAD and the CFX.

The blood circulation

Both heart halves differ from each other very much. The right half of the heart takes care of the small blood circulation (heart, lungs and heart) and ensures that the oxygen-starved blood, which comes from the body, is supplied with oxygen in the lungs and that carbon dioxide (CO2) is expelled. The left half of the heart takes care of the large blood circulation (heart - body - heart) and ensures that the absorbed oxygen and nutrients are delivered to all parts of the body. Because the left ventricle pumps the oxygen-rich blood throughout the whole body, the muscle wall of the left ventricle is much thicker. In figure 5, the oxygen-rich blood is shown in red, and the oxygen-starved blood is shown in blue.

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