The Human Cardiac Cycle

The Human Cardiac Cycle

Organisms require a transport system for several reasons. As the organism gets larger, so the transport system becomes more complex in order to fulfil its requirements. The surface area to volume ratio becomes smaller to the point where a long-distance transport system is required in order to move substances more rapidly. These systems have two primary functions: to link the interior of the organism with the outside world and to link each cell and organ of the organism to each other.

Humans are fairly large mammals and so they naturally have a more intricate transport system. The type of system in a human is known as a closed vascular circulatory system. It involves pumping blood around the body at sustained high pressure which eventually flows back to the heart. The heart is the organ that pumps blood around the body. The transport in humans is also known as a double circulatory system. This means that the system is in two parts: deoxygenated blood is first pumped from the heart to the lungs and then returns back to the heart as oxygenated blood. The second part is when the blood is pumped again by the heart but to the rest of the body to carry out its functions.

Below is a diagram of the heart outlining the main parts of this four-chambered organ as a reference for the rest of this paper:

A common phrase used is 'heartbeat'. A 'heartbeat' occurs when blood is pumped simultaneously from both ventricles in the heart to the lungs and the body. The human cardiac cycle is the sequence of events that occur when the heart 'beats' once. The entire cycle can be simplified into three periods: first the atria contract (atrial systole), this is followed by the contraction of the ventricles (ventricular systole), then finally both the atria and ventricles remain at rest (atrial and ventricular diastole). In humans an average heart rate may be 72 beats/minute. This means that the cardiac cycle normally has a duration of 0.8 seconds. Atrial systole requires 0.1 seconds while ventricular systole requires 0.3 seconds, so the heart is completely at rest for about 0.4 seconds, or half of the cardiac cycle.

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The cardiac cycle can be followed externally in a few different ways. It can be followed by monitoring either the pressure in the arteries, ventricles and atria, or the electrical potential produced at each phase, or even the audible sounds produced by the heartbeat. We can document the cycle beginning at the stage when the heart (all four chambers) are full of blood. It is useful to remember that the right and left sides of the heart work in the same way and so it is easier to consider just what happens on one side for the sake of this depiction. Now, at the beginning stage blood is flowing passively from the atria into the ventricles so the pressure in both chambers slowly increases. The pressure in the arteries is decreasing because the blood pumped from the previous cycle is emptying from the vessels.

When atrial systole takes place an electrical potential is produced and so an increase in the readings, from an electrocardiogram monitoring electric currents, is recorded. The purpose of the systole is to squeeze the last few drops of blood from the atrium into the ventricle. Then at 0.15 seconds the atria go into diastole (relax) and the ventricles begin systole so the ventricular pressure increases. Like the atria, when the ventricles contract they also create an electrical potential but this time a relatively large current is recorded on the ECG. This increase is very rapid because a great pressure is required to pump the blood to its destinations. Immediately the pressure in the ventricle exceeds the low pressure in the atrium and so the valve between the two (atrioventricular valve) is forced shut. When the valve closes a heart sound can be recorded from the vibrations made by the closing of the valve. This sound is dull and protracted. Also after the valve closes the pressure in the atrium drops because it is empty. It then slowly increases as blood passively flows into and refills the atrium. The ventricular pressure increases very rapidly and when it also exceeds the pressure in the arteries, the semilunar valve is opened.

When this valve is opened the artery and the ventricle effectively become one chamber and so the pressure of both are equal. The pressure continues to increase further as ventricular systole carries on. Then at 0.4 seconds when systole ceases and blood from the ventricle is emptied, the pressure in the ventricle decreases. At 0.47 secs the ventricular pressure falls below that of the artery and so the semilunar valve is forced shut. This causes another heart sound to be recorded. The sudden closure creates a much shorter and sharper sound. Surprisingly the pressure in the artery again increases because of the elastic recoil of the walls of the artery so as blood empties the artery springs back into shape and increases the pressure momentarily until it again decreases until the next cycle. This elastic recoil also creates a third electric current. This may be an induced potential due to the recoil being similar to a muscle contraction. The pressure in the ventricle decreases as quickly as it increased and by 0.6 it falls below the atrial pressure so that the atrioventricular valve reopens. The pressure built up in the atrium decreases as blood passively flows into the ventricle and as the ventricle fills up the pressure in there slowly increases too. This concludes the cardiac cycle where it now rejoins back to the beginning and starts again.

At one extreme, the heart of a hibernating mammal may beat only a few times a minute; at the other, the hummingbird has a heart rate of 2000 heartbeats per minute. However, what is it that co-ordinates this cardiac cycle so precisely in such a small space of time? The rate and the strength of the heartbeat is controlled by the vagus and sympathetic nerves through a series of reflexes that speed it up or slow it down. However, the impulse to contraction (systole) of the atria and ventricles is not dependant on external nervous stimuli. The heart is myogenic in that it possesses its own in built mechanism for initiating the contraction of the cardiac muscles. Each cardiac muscle cell has the ability to beat in isolation provided it is supplied by the transport system. This property is called intrinsic rhythmicity. But something is required to co-ordinate the beating of each muscle cell.

The origin of the heartbeat is situated at the right atrium where there is a small bit of specialised tissue called the sinoatrial node. A cardiac impulse is formed at the node which causes the atrium to contract. This wave of excitation spreads out across the top of the heart also causing the left atrium to contract. The wave then reaches another node in the septum between the atria. This is called the atrioventricular node and it is in turn excited by the wave. The atrioventricular bundle (also known as the Bundle of His) conducts the impulse to the bottom of the heart where it again spreads out through fine branches called purkinje fibres. The impulse reaches the muscles of the ventricles and induces systole there, and in this way the contraction and relaxation of the heart are co-ordinated. It is these waves which give rise to the electrical potential that is recorded by the electrocardiogram.

Milan Shah - 12E

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pictureperfect

7th of Aug 2012

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This essay is well written, the candidate uses correct terminology throughout, there are no problems with grammar and only a few spelling errors. You should make sure that you take the time to look over your work so that you can avoid basic errors.

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It is clear from the additional information that the candidate has discussed that they have taken the time to do some research around this topic. This is a good idea as it shows interest in your subject and the extra information this allows you to include in your essay can make it more enjoyable to read. That said, it is important that you reference any resources you have used during your research as this can show that your work is not plagiarised. On a less positive note, the candidate has not written a conclusion therefore their essay comes to a rather abrupt end. It is essential that you conclude your essay, if you done properly, a conclusion allows you to leave the reader with a good impression of your work. To write a good conclusion you should summarise key points from your essay and state why this are relevant as this helps to tie together any loose ends. You should also write a personal response to your topic as your teacher/examiner is interested in what you have to say. You can do this by making sure that you answer any questions you set yourself with a personal opinion or for a science based essay like this one, you could discuss what you have leant form researching your topic.

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The candidates response is very good, they provide a great deal of detail and it is clear that they have a good understanding of this topic. However, this essay doesn’t have an appropriate introduction, which is a mistake, you should always take the time to introduce your work. This is your opportunity to catch the readers attention and get them interested in what you have to say. A good introduction should include a few sentences that introduce your topic and a clear statement of what you plan to discuss. There are many ways in which you can engage the reader, these include adding an interesting fact or quote, though this must be relevant to your topic.

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