Heart although this description is disputed, as the heart is likely to be slightly larger. Well-trained athletes can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle. The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. This distinction is visible also on the surface of the heart as the coronary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial
Short QT syndrome activation lose their pumping ability - fibrillation of the cardiac atria in atrial fibrillation leads to an irregular pulse, and fibrillation of the cardiac ventricles in ventricular fibrillation renders the heart unable to pump blood at all. There are several possible mechanisms by which short action potentials might promote fibrillation. The link between these mechanisms is how the duration of the action potential influences how frequently a heart muscle cell can be excited. A shorter action potential generally allow a heart muscle cell to be excited more frequently - the refractory period is shorter. The first mechanism, referred to as