The Best Heart Animation

The heart generates pressure gradients that drive blood flow through the vascular system. Although the heart appears to be a single structure, it is divided by a central septum into two separate pumps, each consisting of two chambers, the atrium and the ventricle, that alternately contract and empty during systole, then relax and fill during diastole. The timing of the electrical and mechanical events in these chambers must be precisely controlled to ensure efficient cardiac function. The timing of atrial and ventricular contractions relies on two primary cell types, cardiac myocytes, which do the mechanical pumping, and auto-rhythmic cells, which initiate and conduct electrical impulses that drive contractile activity.

Auto-rhythmic cells cyclically depolarize on their own, then fire an action potential each time they reach threshold, a process known as pacemaker activity. This activity originates from primary pacemaker cells in the sinoatrial or SA node in the upper right atrial wall. The pacemaker potentials swiftly move through the interatrial pathway to the left atrium and spread throughout the atrial myocardium via gap junctions. As a result, both atria depolarize and contract as a single unit. The impulses generated by the SA node also rapidly spread through the inter-nodal pathway to the atrial ventricular or AV node at the base of the right atrium.

The AV node is the only point of electrical contact between the atria and ventricles. Here, the impulse is briefly slow, allowing the atria to contract and empty their contents into the ventricle before ventricular depolarization and contraction occur. The impulse travels quickly through the bundle of hits within the interventricular septum. The bundle branches into smaller pockinji fibers that radiate throughout the ventricular myocardium. Action potentials travel quickly through the pockinji fibers, then spread to the surrounding ventricular myocytes via gap junctions. Depolarization triggers contraction. Repolarization triggers relaxation. The swift spread of the impulse through the specialized conduction system ensures that the ventricles become excited and contract as a unit.

The spread of cardiac electrical activity may be recorded using electrodes placed on the body's surface. The recording, called the electrocardiogram, or ECG, has several distinct components. The P-Wave corresponds to atrial depolarization. The QRS complex appears with ventricular depolarization. The T-Wave represents ventricular repolarization. The P-Wave initiates atrial contraction, which completes ventricular filling. The QRS complex initiates ventricular contraction, which pumps blood out of the heart. During this time, the atria repolarized and relaxed. The T-Wave initiates ventricular relaxation, during which the ventricles fill once again. One cardiac cycle is complete.