Final answer:
Increased sympathetic output from the cardioacceleratory center boosts both heart rate and the force of the heart's contractions, which in turn increases stroke volume and cardiac output, particularly during stress situations like blood loss.
Step-by-step explanation:
Increased sympathetic output from the cardioacceleratory center increases heart rate and force of contraction, which are critical components in managing cardiac output. The cardiac accelerator nerves, which are preganglionic sympathetic fibers, play a significant role in this process by initiating a signal that causes the heart rate to increase when the cardiovascular center in the medulla is stimulated.
In addition to increasing heart rate, sympathetic stimulation is a positive inotrope, which means it increases the force of the heart's contractions. This is achieved through the neurotransmitter norepinephrine (NE) at the neuromuscular junction from the cardiac nerves and the hormone epinephrine. Both NE and epinephrine act on alpha and beta receptors of the cardiac muscle cell membrane to also increase metabolic rate, further enhancing the force of contraction.
The net result of sympathetic stimulation is not only an increased heart rate but also a higher stroke volume (SV) due to a more powerful contraction, leaving a smaller residual end-systolic volume (ESV) in the ventricles. This response is particularly important in situations such as blood loss, where the body needs to increase cardiac output and blood pressure to ensure adequate circulation to vital organs.