Final answer:
The pacemaker site in a first-degree block in the heart is the AV node. This node takes over the role of initiating contractions and maintaining the heart rhythm in the event of a delay in conduction from the SA node.
Step-by-step explanation:
In a first-degree block, there is a delay in conduction between the SA (sinoatrial) and AV (atrioventricular) nodes of the heart. The pacemaker site in a first-degree block is the AV node. This is because, in the event of a delay in conduction from the SA node, the AV node takes over the role of initiating contractions and maintaining the heart rhythm.