nearly optimal degree of overlap for force generation. Therefore, the ventricle, because of its increased pumping, automatically pumps
the extra blood into the arteries.
This ability of stretched muscle, up to an optimal length, to contract with increased work output is characteristic of all striated
muscles. Another factor also increases heart pumping when its volume is increased. Stretch of the right atrial wall directly increases
the heart rate by 10 to 20 percent; this too, helps increase the amount of blood pumped each minute, although its contribution is
much less than that of the Frank-Starling mechanism.
(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.
Sensory fibres extend from the receptors present in the superior vena cava, aorta and carotid sinuses to the cardiovascular centre in
the medulla oblongata. The impulses received from the aorta and carotid sinuses decrease the heart rate, whereas, the impulses from
the vena cava increase the heart rate.
Ventricular escape
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 AV 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.(iii) Hormonal regulation : Adrenaline (epinephrine) and noradrenaline (norepinephrine) hormones are secreted by the
medulla of the adrenal glands. Noradrenaline accelerates the heartbeat 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 heartbeat.
PULSE
Pulse is the alternate expansion and elastic recoil of an artery with each systole. It is the strongest in the arteries closest to
the heart. Therefore, it is also called as arterial pulse. Normal pulse rate ranges from 70–90 per minute.
HEART RATE
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.
Table: Differences between heartbeat and pulse
Heartbeat Pulse- It is the rhythmic contraction and relaxation of heart. It is the rhythmic contraction and relaxation in aorta and its main
arteries. - 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 heartbeat. - One complete heart beat consists of one systole and one
diastole and lasts for about 0.8 second.
It is a regular jerk of an artery. It depends on the rate of heartbeat.- 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 is usually taken on the radial artery in the wrist but it can be taken on any artery.
- The pulse rate in children is more rapid than in adults.
- The pulse rate is more rapid in the female than in the male.
- When any strong emotion is experienced the pulse rate is increased, for example, anger excitement, fear, etc.
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