First 305 words of the document:
1) Atrial systole atria contract, reducing the volume inside each one and increasing the
pressure, which forces the blood into the ventricles by following the pressure gradient.
2) Ventricular systole ventricles contract, reducing the volume and raising the pressure of each
one, which forces the blood into the aorta and pulmonary artery by following the pressure
gradient. The blood cannot flow backwards because the atrioventricular valve closes.
3) Diastole the atria and ventricles all relax, making the volumes increase again and therefore
decrease the pressure inside each compartment, which makes blood flow into the atria again by
the vena cava and pulmonary vein. The blood in the aorta and pulmonary artery doesn't reenter
the ventricles as the semilunar valves close.
1) > 2) > 3) > 1) > 2).......
The heart is myogenic (coordinates itself, not by nerve impulses). The sinoatrial node (SAN) is
the pacemaker of the heart. It produces a wave of electrical excitation that cause each chamber
of the heart to contract. The SAN is situated in the right atrium, and the atrium itself is insulated
by collagen so the wave doesn't cause irregular contractions. The wave causes both right and
left atria to contract together and then reaches the atrioventricular node (AVN), which delays the
wave by a fraction of a second to stop the atria and ventricles contracting at the same time. The
wave is then conducted by purkyne tissue in the centre of the heart, until it reaches the heart's
apex, where it goes up the walls of both ventricles, making them both contract from the bottom
P shows the excitation of the atria. QRS show the excitation of the ventricles, and T shows