Final answer:
When the heart rate increases above 160 bpm, the cardiac output decreases due to inadequate filling time. The body may compensate by increasing sympathetic stimulation and blood pressure.
Step-by-step explanation:
In a healthy young adult, when the heart rate increases above 160 bpm, the cardiac output decreases. This is because at higher heart rates, the heart does not have enough time to fill adequately with blood during each beat, decreasing the volume of blood pumped out of the heart per minute.
For example, if the heart rate is 180 bpm, there is less time for the ventricles to fill with blood, resulting in a decreased stroke volume and cardiac output. The body may also compensate for the increased heart rate by increasing sympathetic stimulation, which can lead to vasoconstriction and increased blood pressure.
In summary, when the heart rate increases above 160 bpm, the cardiac output decreases due to decreased filling time, and the body may compensate by increasing sympathetic stimulation and blood pressure.