What is the primary use of atropine in medicine?

Atropine is primarily used in medicine to reduce heart rate, particularly in emergency situations where there is a risk of bradycardia, which is an abnormally slow heart rate. It achieves this by blocking the action of acetylcholine on muscarinic receptors in the heart, leading to an increase in heart rate. This action is critical in certain medical scenarios, such as during cardiac arrest or as a part of the resuscitation protocol.

In the context of other options, while atropine can affect various physiological processes, its primary and most relevant emergency application is in the cardiovascular system for heart rate management. It is not indicated for relieving diarrhea, promoting sleep, or stimulating appetite, as these functions are typically addressed by different classes of medications.