Please note that the content of this book primarily consists of articles
available from Wikipedia or other free sources online. Isovolumetric
contraction is a term used in cardiac physiology to refer to an event
occurring in early systole, during which the ventricles contract with no
corresponding volume change. In a healthy young adult, blood from the
venous return enters the atria and flows to the ventricles via the
opened tricuspid and mitral valves. Atrial contraction rapidly follows,
actively pumping about 20% of the returning blood. As diastole ends, the
ventricles start depolarizing and, while ventricular pressure starts to
rise due to contraction, the atrioventricular valves close, so as to
prevent back flow to the atria. At this stage, which corresponds to the
R peak or the QRS complex seen on an ECG, the aortic and pulmonary
valves are also closed. The net result of this situation is that, while
contraction causes ventricular pressure to rise sharply, there is no
overall change in volume because of the closed valves.
