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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 sino-atrial (SA) node is a specialised bundle of neurons located in the upper part of the right atrium of the heart. The cardiac impulse originating from the SA node triggers a sequence of electrical events in the heart, thereby controlling the sequence of muscle contraction that pumps blood out of the heart. Since the SA node initiates and maintains the rhythmicity of the heart, it is known as the natural pacemaker of the human body.
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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 ?