Function of the Heart
There are many different areas and constituents of the heart, all with different functions to ensure the heart operates correctly.
The heart is a specialised muscle which contracts continuously in order to pump blood to the lungs and to the rest of the body, to supply oxygen and nutrients and at the same time remove carbon dioxide and other waste products. Contraction of the heart is stimulated by a flow of electricity through the heart which repeats itself in a cycle. This flow of electricity orginates from cells of heart itself. The cardiac muscle which the heart consists of can be described as myogenic, this refers to its ability to generate is own electrical impulse.
The cardiac action potential originates in a group of cells known as the sinoatrial node, located in the right atrium. These cells depolarise spontaniously producing many action potentials at a regular rate, normally between 60 and 100 times per minute. This intrinsic pacemaker automaticity can be influenced both by the parasympathetic and sympathetic nervous systems.
The action potential produced in the sinoatrial node propagates from cell to cell throughout the right atrial muscle and spreads to the left atrial muscle. Approximately 0.1 seconds after its origination, the action potential reaches the atrioventricular node. The impulse is prevented from spreading directly from atria to ventricles due to the presence of a fibrous atrioventricular ring. As a result of this the only available pathway for the impulse to travel along is the Bundle of His, a bundle of modified heart muscle that transmits the cardiac impulse from the atrioventricular node to the base of the ventricles causing them to contract, completing the heart beat. This cycle is continually repeated, keeping the heart beating 60-100 times every minute.
The diagram below shows the areas of the heart which are involved the producing and conducting the action potential that makes it beat.
Image by Madhero88 [CC-BY-3.0 (www.creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
The Sinoatrial node is a section of nodal tissue located in the right atrium of the heart and is the primary site of origin of the mammalian heart. It is the smallest electrical region of the heart and is the fastest normal pacemaker, with an intrinsic rate of 60bpm or faster. Therefore because it is the fastest pacemaker it overrides all other action potentials created from various different areas of the heart, making it the site of origin for the action potential which eventually causes the heart to beat.
The Atrioventricular node
The atrioventricular valve is located just above the atrioventricular ring and is the secondary point of origin of the electrical signal in the heart. The atrioventricular node is excited by an impulse reaching it through the atrial conduction pathways. Both the sinoatrial node and atrioventricular node rely on the interaction of 3 time-dependent voltage-gated currents. The intrinsic pace maker of the atrioventricular node has a rate of around 40bpm, considerably lower than the sinoatrial node. However if the sinoatrial node was to fail, the atrioventricular node takes over control and stimulates its own action potential, which has the ability to drive the heart succesfully.
The Bundle of His and Purkinje fibres
The bundle of His originates at the atrioventricular node, it then splits into 2 seperate branches to form the right and left bundle. The right bundle conducts signal to the right ventricle and the left bundle conducts signal to the left ventricle. As seen on the diagram above, the left bundle is considerably larger than the right bundle, this is because the left ventricle is larger than the right ventricle as it is required to pump blood with greater force.
Purkinje fibre cells have the slowest intrinsic pacemaker rate of all the cells in the heart, at around 20bpm. Therefore the purkinje fibre cells only become the functional pacemaker in the event of both the sinoatrial node and the atrioventricular node failing, they are considered tertiary pacemakers.
However the bundle of his and purkinje fibres are an effective conduction system within the ventricles as the conduct action potentials quicker than any other cell or tissue within the heart.
The Cardiac Cycle
The cardiac cycle is described as the sequence of events which occur in order to make the heart pump blood around the body succesfully. There are two phases of the cardiac cycle; diastole (relaxation) and systole (contraction).
1st Diastole Phase
During diastole both atria and ventricles are relaxed and the atrioventricular valves are open. Deoxgenated blood from the body returns to the heart via the inferior and superior vena cava and flows into the right atrium. The sinoatrial node then depolarises producing an action potential which propagates along the wall of the atria causing the blood to be pushed through the atrioventricular valve into the right ventricle. The tricuspid valve stops the blood passing back into the atria.
1st Systole Phase
When all the blood is transferred from the right atria to the right ventricle, an impulse produced via the atrioventricular node propagates down the bundle of His and splits into the right bundle branch of the purkinje fibres at the base of the ventricle, causing it to contract. The atrioventricular valves close and the semilunar valves open. The deoxygenated blood is pumped into the pulmonary artery which carries it to the heart to pick up oxygen, before it returns to the heart via the pulmonary vein.
2nd Diastole Phase
The oxygenated blood returns from the lungs via the pulmonary vein and fills up the left atrium. The sinoatrial node then produces an action potential which causes the atria to contract, pushing the blood through the open atrioventricular valve into the left ventricle. The mitral valve prevents the oxygenated blood flowing back into the left atrium.
2nd Systole Phase
Again an impulse propagates from the atrioventricular node, down the bundle of His to the left bundle branch of the purkinje fibres causing the ventricle to contract. This pumps the oxygenated blood through the semilunar valve into the aorta. The aorta then branches out to provide blood to all areas of the body. The oxygen depleated blood then returns to the heart via the vena cavae.
During the whole process of the cardiac cycle it is important to note that both diastole phases occur at the same time, and both systole phases occur at the same time. Therefore the different areas of the heart are either contracting all together, or relaxing all together.