The heart
General information about the heart
The heart is a muscle that may be influenced to make it stronger and more enduring. Once your fitness level improves, your heart becomes stronger. Or you might want to think of it this way: one of the reasons for becoming fitter is that your heart becomes stronger, making it able to transport more blood throughout your body with each beat.
Stroke volume is the amount of blood that your heart is able to contain before beating again, i.e. it also equals the amount of blood being pumped into the body with each beat. The stroke volume is increased through regular fitness training because it strengthens the heart muscle and makes it able to expand more, and because the heart will be filled more and in a better way as blood circulation becomes more effective.
When the stroke volume is increased, the relative heart rate frequency decreases, i.e. the number of heart rates needed to perform a certain job is decreased. In this way, less pressure is placed on your heart in general.
The heart consists of 4 chambers
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Right atrium. This chamber receives blood from the body, i.e. deoxygenated blood.
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Right ventricle. This chamber receives deoxygenated blood from the right atrium and pumps the blood along to the small circulation. The small circulation includes the lungs where the blood deposits CO2 and receives O2.
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Left atrium. This chamber receives oxygenated blood from the small circulation.
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Left ventricle. This last chamber pumps blood into the large circulation, i.e. the body.
The heart is a muscular, hollow organ located towards the front of the thorax between the lungs. It pumps out blood into the body in the systole and is filled with blood in the diastole. It consists of a left and a right part, each of those divided into an atrium and a ventricle. Thus, the heart consists of four chambers: right atrium and right ventricle as well as the left atrium and left ventricle.
Heart valves: Between each atrium and ventricle, a set of heart valves makes sure to rectify the blood flow direction by only opening in one direction. The pressure difference between atrium and ventricle decides whether or not the valve is open or closed.
During a heart beat, the atriums contract for a short while within the heart chambers. The pressure within the atrium is now at its maximum, causing the valves to open up. When the heart chambers contract, the pressure is increased and the valves are closed. Because the pressure increases significantly during heart chamber contraction, the valves are anchored to various bundles of muscles (papillary muscles) that reach all the way down through the heart chamber to make sure that the valves are not pushed too far back, which would allow the blood to flow back into the atriums. The blood flow through the heart is rectified by means of the valves: cuspidal valves are located between atrium and ventricle, kept in place by papillary muscles, and semilunar valves are located between heart chambers and arteries. Those semilunar valves are extended by the blood pressure (causing them to close), once the blood flows backwards. Once the blood is pushed out of the heart chambers to the main arteries (from the right part of the heart to the lung arteries and from the left part to the main artery (aorta)), it passes by another set of valves. Contrary to the valves located inside the heart, these valves only function through the pressure difference between heart chambers and veins. Once the heart chambers are relaxed, the pressure within the heart chambers is lower than in the arteries, causing the blood to start flowing backwards. The blood flow will then expand the valves, causing them to stop the blood from flowing backwards.
Electrical and mechanical activities
The heart muscle consists of several types of muscle cells: partly cells that are able to activate an electrical impulse or pass it on, partly cells that conduct impulse and are able to contract if hit by an electrical impulse. The heart also contains certain groups of impulse-creating and bundles of impulse-conductive cells that make sure that the heart contracts at the right rhythm and in the correct way. Thus, the heart is able to beat without receiving constant nerve impulses from the central nervous system. Under normal circumstances, the heart is being carefully monitored by the central nervous system (the autonomous nervous system). The heart’s own steering system makes sure that the heart muscles contract in the correct sequence. Apart from ensuring that the atriums contract slightly earlier than the heart chambers, this also ensures that the chambers contract in order to push the blood in the right direction.
The impulse-conductive system within the heart
Within the sino-atrial-node (the SA node), impulses are created that spread out into the atrium walls (black arrows). Once the impulse reaches the atrioventricular node (AV node), it continues through the bundle of His, the branchesand finally to the Purkinje fibres where the heart muscle cells are influenced. The heart’s own steering system consists of two nodes: one located in the wall of the right atrium (SA node) and another located in the wall between the right atrium and the left heart chamber (AV node), along with impulse-conducting fibres (Purkinje-fibres). The anodes contains cells (nodale cells), which create impulses with a certain frequency (pacemaker function). The AV node runs at a frequency rhythm of 15-35 impulses per minute, whereas the SA node creates a frequency of 60-100 per minute. The impulses leave the SA node and move into the atriums to hit the AV node. From here, the AV node conducts the electrical impulses relatively slowly compared to the rest of the impulse-conductive system, causing a delay within the electrical impulse that makes up the base for the heart chambers to be able to contract slightly later than the atriums.
From the AV node, a strong bundle of fibres (the His bundle) connects via two large bundles to the two heart chambers (branches). The central nervous system (the autonomous system) regulates the heart rate by influencing the SA and AV nodes. Under normal circumstances, the AV node’s own frequency would be of no importance, given that the SA node frequency is higher, meaning that the SA node impulses hit the AV node before it is able to create its own impulses. Similarly, under normal circumstances, the SA node frequency would be guided by the central nervous system, rarely being equal to its own frequency.
The pumping function of the heart is also influenced in a mechanical manner. When the heart is filled during the relaxation phase (diastole), the heart muscle cells are stretched. The more they are stretched, the stronger the next contraction will be. If the reverse flow to the heart is increased (e.g. during inhalation), the next heart beat will cause more blood to be pumped out using a stronger force. The adjustment of the relationship between the heart stroke volume and the filling of the heart (called the Starling mechanism) is an important factor within the circulation. The two halves of the heart function like two pumps in a series. By placing electrodes onto the body, we can measure the total tension change within the heart. By measuring heart activities (by creating an ECG), we are able to obtain a picture of how many cells are contracting and in which sequence. Usually, such a picture is a reliable way of knowing how the heart pumps the blood around within the body (the mechanical activity).
Just like every other part of the body, the heart itself needs to be supplied with blood in order to have sufficient energy and oxygen to perform its tasks and in order to get rid of waste products. Blood input into the heart is carried out by two large arteries (coronaries) that originate at the beginning of the main artery (aorta). During the systole phase, the pressure within the heart musculature is so high that the blood is unable to flow through the arteries within the heart. During the diastole phase, pressure is released, allowing the blood to spread out to the heart musculature. The heart is filled with blood from the coronaries, which originate from the main artery just before the place where the main artery originates from the heart. The SA node and the AV node locations are marked by dotted circles. Please note that great differences exist between individual cardiovascular structures. The two coronaries quickly split into smaller veins, supplying each part of the heart with blood. Around 6/7th of the entire blood input into the heart happens through the left coronary, which then supplies the front and left part of the heart as well as a large part of the septum between the left and right parts of the heart. The remaining 1/7th is divided by means of the right coronary to the rest of the heart. In a few places, smaller veins from the coronary meet; however, each part of the heart musculature is supplied with blood from only one of the coronaries – or from one of the smaller veins. The heart’s impulse-generating and conductive system (the AV and SA nodes as well as the His bundle and the branches make up an exemption from this rule: they all receive their blood supply from both coronaries.This structure is an important factor for blood supply to each part of the heart, i.e. the steering system features a higher level of supply security that each single part of the heart musculature on its own.
