BIOL 1002 Chapter : Ch 32 Circulation Part 2

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15 Mar 2019
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The structure of cardiac muscle: electrical signals are spread rapidly by gap junctions (fig 32-4) The heart muscles must beat in smooth coordinated contractions: keep blood moving and avoid random contractions (fibrillations) These coordinated contractions require a pacemaker: pacemaker is an area of muscle that sets the pace for the other muscle cells. *defibrillator machine re-syncs the contractions of the ventricles. The sinoatrial node (sa node) is the heart"s primary pacemaker (fig. It is a small mass of muscle cells in the wall of the right atrium. It generates electrical impulses that cause both atria to contract. The sa node pacemaker maintains a steady beat of 60-100 beats/minute: at rest, then nerve impulses are low, heart rate slows to about 50-70 beats/minute, nervous impulses and hormones can influence heart rates. Exercise or stress can accelerate the heart rate. Epinephrine (flight or fight hormone) increases the heart rate. Rbcs (erythrocytes) lose nuclei as they mature and cannot divide.

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