What is a primary factor that changes heart rate?

The primary factor that changes heart rate is sympathetic nervous system activation. The sympathetic nervous system is responsible for the "fight or flight" response, which prepares the body to respond to stress or danger. When activated, it releases neurotransmitters such as norepinephrine that bind to receptors on the heart, causing an increase in heart rate and contractility. This response is crucial for enhancing blood flow to muscles and organs during periods of increased demand, such as physical activity or stress.

The other factors, while they can influence cardiovascular function, do not have the same direct and immediate effect on heart rate. Vascular resistance relates more to the pressure the heart must work against to pump blood but does not directly change the heart rate itself. Electrolyte balance is important for various physiological functions, including muscle contractions, but changes in heart rate are primarily mediated through neural responses rather than solely by electrolyte levels. Blood viscosity can affect the workload on the heart and overall circulation, but it does not directly trigger alterations in heart rate like sympathetic nervous system activation does.