Final answer:
During SNS dominance, CO initially increases with HR and SV but can decrease if HR exceeds about 160 bpm due to reduced ventricular filling time and consequent drop in SV.
Step-by-step explanation:
When the Sympathetic Nervous System (SNS) is dominant, there are significant effects on Cardiac Output (CO). The SNS, part of the autonomic nervous system, stimulates the heart rate (HR) and thus can initially increase CO due to an increase in stroke volume (SV). During the SNS dominance, which often occurs during situations such as exercise or when responding to a threat, the HR can increase to enhance blood circulation to vital organs and muscles.
However, as HR continues to rise significantly, the time for ventricular filling during diastole decreases, which eventually leads to a decrease in SV. When HR exceeds approximately 160 beats per minute (bpm), the SV may decrease more rapidly than the HR can compensate, leading to a reduction in CO.