Final answer:
Chronotropic agents are substances that influence the heart rate, either increasing it (positive chronotropic agents) or decreasing it (negative chronotropic agents). They act mainly through the autonomic nervous system and hormonal regulation such as thyroid hormones. The cardiovascular centers in the brain, including the cardiac control center, regulate these effects to maintain cardiac function.
Step-by-step explanation:
Chronotropic agents are substances that affect the heart rate. They can either be positive chronotropic agents, which increase the heart rate, or negative chronotropic agents, which decrease it. These effects are mediated through the autonomic nervous system and the endocrine system. For example, when epinephrine binds to adrenergic receptors on the heart, it increases the heart rate, whereas acetylcholine (ACh) decreases the heart rate by binding to muscarinic receptors.
The heart rate is crucial for determining the cardiac output, which is the volume of blood the heart pumps per minute. Cardiac output is calculated by multiplying the heart rate by the stroke volume (the amount of blood pumped out with each beat). During exercise, the heart rate increases to meet the body's increased demand for oxygen and nutrients, thus increasing cardiac output.
The heart rate is regulated by the cardiovascular centers in the brain, specifically the medulla oblongata, which include the cardiac control center and the vasomotor center. These centers respond to various stimuli, including exercise, emotional stress, and blood pressure changes, to maintain homeostasis.
Also, factors such as thyroid hormones can significantly affect the heart rate. Increased levels of thyroid hormone generally increase the heart rate and the heart's contractility, while excessive levels can lead to tachycardia. Beta-blockers, on the other hand, are a type of therapeutic chronotropic agents used to treat cardiovascular diseases by blocking beta-receptors and thus reducing heart rate and blood pressure.