To maintain homeostasis, the amount of blood pumped by the heart must vary dramatically. For example, during exercise the cardiac output can increase several times over resting values. The cardiac output is controlled by regulatory mechanisms that can be classified as either intrinsic or extrinsic. Intrinsic regulation results from the normal functional characteristics of the heart and does not depend on either innervation or hormonal regulation. It functions when the heart is in place in the body or is removed and maintained outside the body under proper conditions. On the other hand, extrinsic regulation involves neural and hormonal control. Neural regulation of the heart results from sympathetic and parasympathetic reflexes, and the major hormonal regulation comes from epinephrine and norepinephrine secreted from the adrenal medulla.
