What happens to the sinoatrial node when the body is relaxed?

The sinoatrial (SA) node is the natural pacemaker of the heart, generating electrical impulses that initiate each heartbeat. When the body is relaxed, such as during rest or sleep, the demand for oxygen decreases since the metabolic rate slows down. In this relaxed state, the autonomic nervous system adjusts the heart rate accordingly.

The parasympathetic nervous system, which promotes relaxation and conservation of energy, becomes more active. This system releases acetylcholine, which slows down the firing rate of the SA node. As a result, the node fires less frequently, leading to a decrease in heart rate. This physiological response helps the body conserve energy and allows various restorative processes to take place during periods of rest.

Thus, saying that the sinoatrial node fires less frequently during relaxation accurately reflects how the body adjusts heart activity in relation to its energy demands.