DAVID GIFFORD / SCIENCE PHOTO LIBRARY DAVID GIFFORD / SCIENCE PHOTO LIBRARY
Illustration of a section through the human heart, showing (in yellow) the nerves that govern the pumping cycle. Blood flows from right and left ventricles (bottom left & right respectively on image). The electrical impulse that provokes ventricular contraction begins in the Sinoatrial (SA) node, the heart's pacemaker (yellow bundle at top left), spreading to both left & right atria (top) and down to the atrioventricular (AV) node (located between right atrium & ventricle). The AV node delays passage of the impulse to allow time for the ventricles to fill before effecting ventricular contraction via branches of the bundle of HIS (yellow fibres between ventricles).
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