A Level Biology Practice Exam

Which component of the heart initiates contraction?

1. Atria

2. Ventricles

3. Sinoatrial node

4. Valve structures

The correct answer is: Sinoatrial node

The sinoatrial node (often referred to as the SA node) is the correct answer because it serves as the heart's natural pacemaker. This specialized group of cells is located in the right atrium and is responsible for generating electrical impulses that initiate each heartbeat. When the SA node fires, it sends out an electrical signal that causes the atria to contract, pushing blood into the ventricles. The other components listed do not initiate the contraction of the heart. The atria are involved in the receiving and pumping of blood but do not create the signal that starts the contraction. The ventricles contract in response to the signals received from the atria but rely on the SA node's impulse to do so. Valve structures, while crucial for directing blood flow and preventing backflow, have no role in initiating heart contractions. They operate based on the pressure differences created by the contractions of the heart's chambers.