What effect does expiration have on the SA node?

What effect does expiration have on the SA node?

• A. It has no effect
• B. It temporarily decreases the pacing
• C. It permanently increases the pacing
• D. It stabilizes the rate

Answer: (B)

During expiration, there is typically a decrease in intrathoracic pressure, which influences the autonomic nervous system's activity, particularly its parasympathetic component. This results in a temporary decrease in the pacing rate of the SA node, which is the natural pacemaker of the heart. The SA node is influenced by vagal tone, which is heightened during expiration.

As the diaphragm relaxes and the thoracic cavity compresses, the heart experiences an increase in vagal (parasympathetic) stimulation, leading to a reduction in heart rate. This phenomenon is an important physiological response that reflects the interplay between respiratory patterns and cardiac function. Thus, when one exhales, the SA node's pacing slows down momentarily in response to these changes, contributing to variations in heart rate throughout the breathing cycle, known as respiratory sinus arrhythmia. This understanding is key when studying heart rhythms and how external factors can influence cardiac pacing.