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Sino-atrial node is called the pacemaker...

Sino-atrial node is called the pacemaker of our heart. Why?

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The S.A. node is located in the wall of right auricle slightly below the opening of the superior vena cava. It has a unique property of self-excitation which enables it to act as the pacemaker of the heart. It spontaneously initiates a wave of contraction which spreads over both the auricles more or less simultaneously along the muscle fibres that fan out from the pacemaker.
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Assertion : Sino-atrial node (SAN) is called the pacemaker Reason : SAN generates the maximum number of action potentials and is responsible for intiating and maintaining the rhythmic contractions of the heart.

Assertion : The sino-atrial node (SAN) is called the pacemaker. Reason : SAN is responsible for initiating and maintaining the rhythmic contractile activity of the heart.

Assertion : The AV node is also called as the pacemaker of the heart. Reasoning : It is because of the fact that the AV node determines the rate of heartbeat.

Explain the following : (a) Why does the atrial systole normally precedes the ventricular systole ? (b) Why is S-A node called the pacemaker of the heart ? (c ) Why does the ventricle relax as a closed chamber in the early phase of its diastole ? (d) Why is there no mixing of deoxygenated and oxgenated blood in mammalian heart ? (e ) Why can you palpate the pulse on an artery in each heart beat ? (f) Why does the lymph contain much less proteins than the plasma ? (g) Why does the ventricle contract as a closed chamber in the early phase of its systole ? (h) Why is the closed circulatory system more efficient than the open one ? (i) Why does the left ventricle has a thicker wall than the right ventricle ?