Biology - Body Fluids and Circulation Question with Solution | TestHub

The rate of heart beat is regulated by the autonomic nervous system and hormones. Sympathetic nerve endings release norepinephrine, which binds to ${\beta }_1$-adrenergic receptors in the heart. This increases the rate of spontaneous depolarization at the sinoatrial (SA) node, thereby increasing the heart rate and the strength of ventricular contraction.

Other factors like hypoxia, acidosis, and alkalosis generally depress cardiac function or have variable effects depending on severity, but they are not the primary physiological factors that increase heart rate.

Answer: Secretion of Norepinephrine