Cardiac Conduction System and Cardiac Cycle for the MCAT

The heartbeat starts in the sinoatrial (SA) node, spreads across the atria, pauses at the atrioventricular (AV) node, then travels down the bundle of His, the bundle branches and the Purkinje fibers to contract the ventricles from the bottom up.

The conduction pathway

1. SA node in the right atrium is the pacemaker. It depolarizes spontaneously at the fastest rate, so it sets the rhythm.
2. Impulse spreads through both atria, which contract.
3. AV node delays the signal by about 0.1 second so the ventricles can fill before they contract.
4. Bundle of His and bundle branches carry the signal along the septum.
5. Purkinje fibers spread it through the ventricular walls for a coordinated contraction.

Sympathetic input speeds the SA node, and parasympathetic input through the vagus nerve slows it.

The ECG

The cardiac cycle

• Diastole: the ventricles relax and fill. The AV valves are open and the semilunar valves are closed.
• Systole: the ventricles contract. The AV valves close first (the "lub" sound), then pressure rises until the semilunar valves open and blood is ejected. When the ventricles relax, the semilunar valves close (the "dub" sound).

Cardiac output

Cardiac output = heart rate x stroke volume. Stroke volume is the blood ejected per beat. Typical resting cardiac output is about 5 L per minute. Stroke volume rises with greater venous return (preload) and with stronger contractility.

Common traps

• Placing the pacemaker at the AV node. The SA node sets the pace; the AV node is a delay.
• Saying the T wave is atrial. It shows ventricular repolarization.
• Mixing up which valves make which sound. AV closure is the first sound; semilunar closure is the second.

One-minute recap

• SA node, atria, AV node (delay), bundle of His, Purkinje fibers.
• P atrial depolarization, QRS ventricular depolarization, T ventricular repolarization.
• Cardiac output = heart rate x stroke volume.