Cardiac Cycle

Sahra Abdulrahman

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.

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The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:

Atrial and ventricle systole

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The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:

Atrial and ventricle systole

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 goes 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 (atrio ventricular 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 second’s 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.

Atrial and ventricle Diastole

Once systole has finished, diastole begins. The transfer of blood through the heart is directed by the heart valves, the opening and closing of which in turn is controlled by the adjustments in blood pressure that happen within the chambers of the heart. Blood flows from an area of high pressure, to an area of low pressure.

During the diastole phase the atria and ventricles are relaxed and the atrioventricular valves are open. De-oxygenated blood from the superior and inferior vena cava flows into the right atrium. The open atrioventricular valves allow blood to pass through to the ventricles. The Sinoatrial Node (referred to as the pacemaker of the heart.) contracts triggering the atria to contract. The right atrium empties its contents into the right ventricle. The tricuspid valve prevents the blood from flowing back into the right atrium.

Systole- The contraction phase of the cardiac cycle

Diastole- The relaxation phase of the cardiac cycle

Bibliography

Third edition advanced Biology

By: J. Simpkins

J.I. Williams

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