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The Human Cardiac Cycle

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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. ...read more.


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. ...read more.


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 ...read more.

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