Body Fluids and Circulation

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Atrio-ventricular node (AVN) in the lower left corner

of the right atrium close to the atrio-ventricular

septum.

From the AVN, a bundle of fibres atrio-ventricular bundle (AVB)

passes through atrio-ventricular septa and divides into right & left

branches. Each branch passes through the ventricular walls of its

side. In the ventricular wall, it breaks up into minute fibres, purkinje

fibres. These fibres along with the bundles are known as bundle of

His. (AIPMT 2011)

Nodal tissues generate action potential without any external

stimuli, i.e. it is auto excitable SAN initiates and maintains contraction

of heart by generating action potentials (70-75/min). So, it is called

the pacemaker.

ÇÅRDÏÅÇ ÇÝÇLË

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It is the cyclic contraction and relaxation of heart for pumping blood. It

involves 3 stages:

(Å) JØÏÑT DÏÅSTØLË:

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It is the relaxed state of all chambers of heart. When the tricuspid and

bicuspid valves open, blood from pulmonary vein and vena cava flows

into left & right ventricles respectively through left and right atria.

Semilunar valves are closed at this stage.

(B) ÅTRÏÅL (ÅÜRÏÇÜLÅR) SÝSTØLË:

SAN generates an action potential. As a result,

both the atria contract. It is called atrial systole.

This increases the flow of blood into the ventricles

by about 30%.

(Ç) VËÑTRÏÇÜLÅR SÝSTØLË:

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The action potential is conducted to ventricular side by AVN & AV bundle

from where bundle of His transmits it through the ventricular musculature.

As a result, ventricles contract. It is called ventricular systole.