The sequence of events, which take place during the completion of one heartbeat, is known as cardiac cycle. It involves repeated contraction and relaxation of the heart muscles. A contraction is termed systole and a relaxation is termed diastole. One complete heartbeat or cardiac cycle consists of one systole and one diastole. It lasts for about 0.8 seconds.
The events that take place during the completion of one heartbeat are—the auricular systole, the ventricular systole and the joint diastole or complete cardiac diastole.
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Auricular systole:
The two auricles act as a pump to force the venous blood into the ventricles.
The ventricles are in diastole at this time. During auricular systole, the auricular muscles contract and the openings of venae cavae and pulmonary vein close. The tricuspid valve and bicuspid valve open and the blood enter the ventricles through these valves. At the end of an atrial systole as the two auricles relax, more venous blood is passed from the great veins into the atria to fill them up.
Ventricular systole:
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With the onset of atrial diastole the ventricles start contracting. The bicuspid and tricuspid valves (also known as atrio-ventricular or AV valves) close to prevent the backward flow of blood from ventricles to the atria. When the pressure in the ventricles increases, semilunar valves open and blood is passed into the great arteries.
Joint diastole:
At the end of a ventricular systole, the ventricles relax. Also at this stage the atria are still in their diastole. Since all the heart chambers are in diastole, this stage is called joint diastole.
During this phase, blood continues to flow into the auricles from the great veins. As the atria fill with blood, the pressure in them rises and becomes greater than that in the relaxed ventricles. Blood then flows from the auricles to respective ventricles through the AV valves.
1. Detection of Normalcy of Heartbeat Electrocardiogram (ECG):
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The muscle cells or fibres of the heart are specialised at certain parts of heart to generate electric currents that cause normal rhythmic heartbeat. The electrical events that control the cardiac cycle are recorded by placing electrodes on certain parts of the body. The electrodes of the electrocardiogram taped to the chest pick up the electrical events of the cardiac cycle as they pass through the body.
This makes the ECG a useful diagnostic tool. Heart diseases like tachycardia (a high pulse rate above 100/minute), brachycardia (a slow pulse rate below 60/minute), ventricular fibrillation and heart block can be diagnosed using the ECG.
People with heart problems can now transmit their ECG trace down the telephone to a specialist who can detect problems at a distance.
2. Pulse Rate:
The rhythmic contraction of the heart can be felt as a jerk in certain arteries which are superficial in position. This is the arterial pulse. Each ventricular systole starts a new pulse. It proceeds as a wave of expansion throughout the arteries and disappears in the capillaries.
Pulse rate is the same as the heartbeat rate. The pulse rate in a normal adult man is about 64-72 beats per minute while in a woman 72-80 beats per minute. The pulse rate in infants is about 140 beats per minute.
3. Heart Sounds Lubb and Dubb:
The various valves present at the outlet and inlet of ventricles determine the direction of flow of blood in the heart. The opening and closing of these valves cause heart sounds. These sounds can be heard by using a stethoscope (an instrument which magnifies sounds and conducts them to the ear). These sounds are heard as ‘lubb’ and ‘dubb’ sounds.
The lubb sound (first heart sound):
This sound is caused by the vibrations set upon by the closure of tricuspid and bicuspid valves accompanied by the contraction of ventricular muscles. It is also called systolic sound. Lubb sound is low-pitched, not very loud and of a long duration.
The dubb sound (second heart sound):
This sound is caused by the closure of the semilunar valves and marks the end of ventricular systole. The dubb sound is high-pitched, louder and shorter in duration.
The sounds ‘lubb-dubb’ and their quality indicates the state of the valves.
4. Blood Pressure:
It is the force with which blood pushes against the walls of the arteries. It is generally measured in terms of how high it can push a column of mercury. When the ventricles contract, pressure of blood inside the arteries is highest and this pressure is termed as systolic blood pressure.
In a healthy young human being, it is about 120 millimetres of mercury (120 mm Hg). When ventricles relax, pressure of blood inside the arteries is comparatively less and this pressure is termed as diastolic pressure. It is about 80 millimetres of mercury (80 mm Hg) in a healthy young man. Thus, a healthy young man is said to have a normal blood pressure of 120/80 mm of Hg. The instrument used to measure blood pressure is called sphygmomanometer.
