3.2: Conduction System and Cardiac Cycle

The heart's ability to pump blood relies on a highly coordinated sequence of electrical and mechanical events. The electrical blueprint that dictates the timing of each heartbeat is known as the cardiac conduction system. This system consists of specialised cardiac muscle cells that generate and transmit electrical impulses throughout the heart wall, forcing the chambers to contract. Without this precise electrical timing, the heart could not function as an efficient pump to supply oxygen to working muscles.

The electrical impulse begins at the sinoatrial (SA) node, located in the wall of the right atrium. Often referred to as the heart's natural pacemaker, the SA node generates an electrical signal that spreads rapidly across the muscle walls of both the left and right atria. This electrical stimulation causes the atria to contract simultaneously, pushing blood downward into the ventricles. The impulse then reaches the atrioventricular (AV) node, situated in the septum between the atria. The AV node deliberately delays the electrical signal for about 0.1 seconds. This brief pause is mechanically crucial, as it ensures the atria have completely emptied their blood into the ventricles before the ventricles begin to contract.

Once the delay is complete, the electrical impulse travels down from the AV node into the interventricular septum via the Bundle of His. This bundle quickly splits into right and left bundle branches, which carry the signal down to the apex (the bottom) of the heart. Finally, the impulse spreads upward through the ventricular walls via a network of specialised pathways called Purkinje fibres. The arrival of the signal at the Purkinje fibres triggers a powerful, coordinated contraction of the ventricles, which begins at the apex and squeezes upward, efficiently ejecting blood out into the pulmonary artery and the aorta.

This electrical activity directly dictates the mechanical phases of the cardiac cycle, which represents one complete heartbeat and lasts roughly 0.8 seconds at rest. The cardiac cycle is divided into two distinct mechanical phases: diastole and systole. Diastole is the relaxation phase of the cycle. During atrial and ventricular diastole, the heart muscle relaxes, the internal pressure drops, and blood flows passively into the atria and down into the ventricles. The semilunar valves snap shut during this phase to prevent blood from leaking back into the heart, producing the second sound of the heartbeat (the "dub").

Systole is the contraction phase of the cycle, during which pressure within the chambers rises dramatically to propel blood. During athletic activity, the atria contract to top off the filling of the ventricles. This is immediately followed by ventricular systole, triggered by the Purkinje fibres. As the ventricles contract, the sudden rise in pressure slams the atrioventricular (tricuspid and bicuspid) valves shut to prevent backflow into the atria, creating the first sound of the heartbeat (the "lub"). The high pressure forces the semilunar valves open, driving blood out to the lungs and the rest of the body. During exercise, the conduction system fires more rapidly, shortening diastole to ensure a faster, continuous supply of oxygenated blood to meet the athlete's elevated metabolic demands.