Homeostasis is derived from a Greek term that means "same state." Biologically it refers to the way your body attempts to maintain constant internal conditions in an ever-changing external environment. The body can control internal conditions such as temperature, heart rate and blood pressure. It uses a monitoring system that detects outside conditions and makes the necessary hormonal changes to maintain a steady set point. The body can control heart rate using a variety of methods.
The body can regulate its blood pressure to directly control heart rate through a "negative feedback loop." This simply means an increase in one function will cause a decrease in another. For example, the body can constrict its vessels to increase blood pressure, causing pressure receptors to send a message to the brain, which will then release a chemical to slow the heart rate.
During stress or exercise, your body will cause the heart rate to increase in order to meet metabolic needs. The adrenal gland, which lies on top of both kidneys, releases a hormone called epinephrine; this hormone travels to the heart and attaches to certain receptors, causing the heart rate to increase.
During rest, your body will keep the heart rate in a low range to match the body's metabolic needs. When your body is resting, the adrenal gland continually releases a chemical known as acetylcholine, which acts on blood vessels and the heart to decrease blood pressure and heart rate, respectively.
When your body loses a large amount of blood volume (i.e. bleeding), the adrenal gland and the hypothalamus release norepinephrine and vasopressin, respectively. These hormones will constrict blood vessels to maintain an adequate blood pressure and increase the heart rate so organs receive blood.