Biological Systems MCAT Practice Exam

The sinoatrial (SA) node serves as the heart's primary pacemaker, initiating the electrical impulses that dictate the heartbeat. It is located in the right atrium and generates action potentials at a regular rhythm, typically around 60-100 beats per minute under normal physiological conditions. This automaticity is crucial for maintaining a consistent heart rate and coordinating the contraction of the heart chambers. The impulses generated by the SA node spread through the atria, causing them to contract and push blood into the ventricles. Once the electrical signal reaches the atrioventricular (AV) node, it is briefly delayed before passing into the ventricles, allowing them enough time to fill with blood after atrial contraction. While the AV node plays an important role in heart conduction, it does not initiate the heartbeat. Structures such as the Bundle of His and Purkinje fibers are further down the conduction pathway and are responsible for transmitting the electrical impulse from the AV node through the ventricles. However, these components do not possess the pacemaking abilities of the SA node. Thus, the SA node is recognized as the key structure responsible for setting the rhythm of the entire heart.