"The amount of blood pumped by the heart per unit time (cardiac output) depends on the heart rate and the contractile force of the heart. Sympathetic activity increases the cardiac output; parasympathetic activity reduces it. The two parts of the autonomic nervous system thus have antagonistic effects on the spontaneously active heart. Of course, regulation of the cardiac output by the autonomous nervous system never occurs in the organism in just the... [that way], because the sympathetic and the parasympathetic systems influence the heart simultaneously. The heart is subjected continuously to inhibitory parasympathetic and excitatory sympathetic influences. Each change in activity in one or the other of the two autonomic systems results in changes in the heart rate and/or the contractile force. Thus, the cardiac output (amount of blood pumped by the heart per unit time) increases when there is a rise in sympathetic activity and/or a drop in parasympathetic activity. Conversely, the cardiac output declines when there is a drop in sympathetic activity and/or a rise in parasympathetic activity. Since the CNS [central nervous system] has these means at its disposal to regulate the cardiac output through the autonomic nervous system, the body can adapt the operation of the cardiovascular system to the requirements of the moment."
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