Final answer:
The heart beats faster during heart failure due to activation of the sympathetic nervous system in response to decreased blood pressure and increased carbon dioxide levels. Additional factors include the renin-angiotensin-aldosterone system, where aldosterone prompts sodium reabsorption to increase blood volume and stabilize pressure.
Step-by-step explanation:
The heart knows to beat faster during heart failure primarily due to activation of the sympathetic nervous system. This activation occurs in response to various physiological cues such as decreased blood pressure and increased carbon dioxide levels in the blood. When the heart is failing and cannot pump efficiently, the body attempts to compensate by increasing the heart rate and the force of contraction through the sympathetic nervous system, which releases epinephrine and norepinephrine. These catecholamines increase the heart rate to try and maintain adequate circulation.
Moreover, heart failure often leads to reduced blood volume which can stimulate the release of renin from the kidneys, not the liver, leading to a sequence of events in the renin-angiotensin-aldosterone system. This system causes the kidneys to reabsorb sodium and water, thereby increasing blood volume to help stabilize blood pressure. Aldosterone plays a key role in this process as it prompts the kidneys to reabsorb sodium, but it does not prompt an increased urine output as that would lower blood volume.
Sympathetic activation is associated not only with increased heart rate but also with pupil dilation (baring no relation to heart failure). Baroreceptors, which sense blood pressure drops, also play a role in stimulating the sympathetic nervous system when they detect decreased firing rates due to low blood volume or pressure.