Heart Beat | Manish Mevada

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Heart Beat

• What is heart beat ?

• Heart beat is the rhythmic contraction and relaxation of the heart 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 (average).
• Beating is an inherent capacity of the heart. The heart of a resting human being pumps about 5 litres of blood per minute.
• This means that a quantity of blood equal to the total amount contained in the body passes through the heart each minute.
• During exercise both the number of beats per minute and the amount of blood pumped per beat are greatly increased Types.
• The heart beat is of two types : neurogenic and myogenic.
• The neurogenic heart beat is initiated by a nerve impulse coming from a nerve ganglion (mass of nerve cells) situated near the heart . It is present in the heart of some annelids and most arthropods . The myogenic heart beat is initiated by a patch of modified heart muscle itself . It is found in hearts of molluscs and vertebrates including human beings.

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) - pace maker, which lies in the wall of the right atrium near the opening of the superior vena cava.
• The SA node is a mass of neuromuscular tissue. Sometimes, the SA-node may become damaged or defective.
• So the heart does not function properly.
• This can be remedied by the surgical grafting of an artificial pace maker in the chest of the patient.
• The artificial pace maker stimulates the heart at regular intervals to maintain its beat .

Conduction of heart beat.

• Another mass of neuromuscular tissue, the atrio-ventricular node (AV node) is situated in the wall of the right atrium.
• The AV node picks up the wave of contraction propagated by SA node.
• A mass of specialized fibres , the bundle of His , originates from the AV node.
• The bundle of His divides into two branches. one going to each ventricle.
• Within the mycocardium of the ventricles the branches of bundle of His divide into a net work of fine fibres called the Purkinje fibres.
• The bundle of His and the Purkinje fibres convey impulse of contraction from the AV node to the myocardium of the ventricles.

Regulation of heart beat ( = Regulation of Cardiac Activity ).

• The rate of heart beat is regulated by two mechanisms.

Neural Regulation

• The cardiac centre lies in the medulla oblongata of the brain.
• The cardiac centre is formed of cardio - inhibitor and cardioaccelerator 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.

Hormonal Regulation

• Adrenaline (epinephrine) and noradrenaline (norepineph rine) hormones are secreted by the medulla of the adrenal glands.
• Noradrenaline accelerates the heart beat under normal conditions while adrenaline does this 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.

Heart Rate

• Human heart beats about 72 times per minute in an adult person at rest.
• This is called heart rate of that person. The heart rate increases during exercise, fever, fear and anger . Because smaller animals have the higher metabolic rate, their heart rate is higher than larger animals.
• An elephant has normal heart rate of about 25 per minute whereas mouse has a normal heart rate of several hundreds per minute.

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