To pump the blood into blood vessels, heart regularly contracts and relaxes in a rhythmic manner. It is known as ‘heart beat’.
• Each heart beat includes one systole (contraction phase) and one diastole (relaxation phase) of the heart to distribute and receive blood to and from the body. The heart of a healthy person beats 72 times per minute.
• Beating is an inherent capacity of the heart. The heart of a resting human being pumps about 5 litres of blood per minute.
Origin of heart beat
• The mammalian heart is myogenic (myo = muscle, genic = originating from). It means the heart beat originates from a muscle, however, it is regulated by the nerves. The heart beat originates from the sinoatrial node (SA node) or pacemaker.
• The atrioventricular node (AV node or pacesetter) picks up the wave of contraction propagated by SA node. The bundle of His and the Purkinje fibres convey impulse of contraction from the AV node to the myocardium of the ventricles.
• When this system functions normally, the atria contract about one sixth of a second ahead of ventricular contraction, which allows filling of the ventricles before they pump the blood through the lungs and peripheral circulation.
Regulation of normal heart beat
• The basic means by which the volume pumped by the heart is regulated are of three types:
(i) Intrinsic regulation of heartbeat lyrics
• Under most conditions, amount of blood pumped per minute by heart is determined almost entirely by the rate of blood flow into heart from the veins i.e., venous return.
• This intrinsic ability of heart to adapt to increasing volumes of blood is called Frank-Starling mechanism. It states that the greater the heart muscle is stretched during filling, the greater is the quantity of blood pumped into the aorta i.e., within physiologic limits, the heart pumps all the blood that returns to it through veins.
(ii) Neural regulation
• The cardiac centre lies in the medulla oblongata of the brain. The cardiac centre is formed of cardio-inhibitor and cardio-accelerator parts.
• The former decreases the rate of heart beat and the latter accelerates it. The cardio-inhibitor is connected with the heart through vagus nerve (it carries-parasympathetic nerve fibres) and cardio accelerator through sympathetic nerve fibres.
• The vagal fibres are distributed mainly to the atria and not much to the ventricles, where the power contraction of the heart occurs. This explains the effect of vagal stimulation mainly to decrease heart rate rather than to decrease greatly the strength of heart contraction. Nevertheless, the great decrease in heart rate combined with a slight decrease in heart contraction strength can decrease ventricular pumping 50 per cent or more.
• Adrenaline (epinephrine) and noradrenaline (norepinephrine) hormones are secreted by the medulla region of the adrenal glands. Noradrenaline accelerates the heart beat under normal conditions while adrenaline does the function at the time of emergency. These hormones directly influence the SA node.
• Thyroxine hormone secreted by thyroid gland increases oxidative metabolism of the body cells. This requires more oxygen and thus indirectly increases heart beat. All these factors regulate heartbeat rate and are responsible for heart beats per minute.
Occasionally, some other part of the heart develops a rhythmical discharge rate that is more rapid than that of the sinus node. For instance, this sometimes occurs in the A-V node or in the Purkinje fibres when one of these becomes abnormal. In either case, the pacemaker of the heart shifts from the sinus node to the A-V node or to the excited Purkinje fibres. Under rarer conditions, a place in the atrial or ventricular muscle develops excessive excitability and becomes the pacemaker.
A pacemaker elsewhere than the sinus node is called an “ectopic” pacemaker. An ectopic pacemaker causes an abnormal sequence of contraction of the heart and can cause significant debility of heart pumping and this van be viewed as heart beats per minute chart.
Strong parasympathetic stimulation can completely stop either the excitation by SA node or transmission of cardiac impulse from atria to ventricles. The ventricles stop beating for 5 to 20 seconds, but then some point in the Purkinje fibres, usually in the ventricular septal portion of the A-V bundle, develops a rhythm of its own and causes ventricular contraction at a rate of 15 to 40 beats per minute. This phenomenon is called ventricular escape.
Pulse is the alternate expansion and elastic recoil of an artery with each systole. Pulse is strongest in the arteries closest to the heart. Normal pulse rate ranges 70-90 per minute.
• Tachycardia is the term applied to a rapid heart or pulse rate (over 100/ minute). Bradycardia is the term indicating a slow heart or pulse rate (under 50/ minute).
Pulse per minute is called as heart rate. Human heart beats 72 times per minute, this is designated as heart rate. It increases during exercise, fever, anger and fear.
Differences between heart beat and pulse
|1||It is the rhythmic contraction and relaxation of heart.||It is the rhythmic contraction and relaxation in aorta and its main arteries.|
|2||It is regulated by the nervous and endocrine systems.||It is due to the flow of blood from the heart and is dependent on the rate of heart beat.|
|3||One complete heart beat consists of one systole and one diastole and lasts for about 0.8 second.||Pulse is a regular jerk of an artery. It depends on the rate of heart beat.|
• Cardiac output is also the volume of blood flowing through either the systemic or the pulmonary circuit per minute.
It is determined by multiplying the heart rate with the volume of blood ejected by each ventricle during each beat, which is called the stroke volume.
• Cardiac output = Heart rate x Stroke volume = 72 beats/min x 0.07 litre/beat = 5.0 litres/min.
• During periods of strenuous exercise in well-trained athletes, the cardiac output may reach 35 l/min; the entire blood volume is pumped around the circuit seven times a minute!
• Cardiac index is the minute volume per sq.m, of body surface area. Its normal value is 3.3 lit/min/sq.m.