The Cardiac Cycle

Challenges to clear

• Put one full heartbeat in order.
• Lay the AV node relay in its correct place.

The task

One complete heartbeat takes about 0.8 seconds. Put its events in order.

What this lesson covers

• The SA node fires — The sino-atrial node in the wall of the right atrium generates the impulse on its own. This is why it is called the pacemaker — the heartbeat is myogenic, originating in the heart muscle itself.
• Atrial systole — Both atria contract together and push blood down into the ventricles through the open atrio-ventricular valves.
• The AV node relays the impulse — The atrio-ventricular node picks up the impulse and passes it, through the bundle of His and the Purkinje fibres, into the ventricle walls. The slight delay lets the ventricles fill completely first.
• Ventricular systole — The ventricles contract. Pressure rises and the atrio-ventricular valves snap shut to stop blood flowing back into the atria.
• Blood is ejected — The semilunar valves open and blood is forced into the aorta and the pulmonary artery.
• Ventricular diastole — The ventricles relax. Pressure falls and the semilunar valves close, preventing blood in the arteries from flowing back.
• Joint diastole — Atria and ventricles are both relaxed. Blood flows in from the veins and the heart refills, ready for the next beat.

Hint

The two heart sounds are valves closing, not opening. LUBB is the atrio-ventricular valves shutting at the start of ventricular systole; DUPP is the semilunar valves shutting as the ventricles relax.