Which one of the following hormones increases the heartbeat in mammals?

Adrenaline (also known as epinephrine) is the primary hormone responsible for increasing the heartbeat in mammals during stress or 'fight-or-flight' situations. Secreted by the adrenal medulla, it acts directly on the heart to increase the rate and force of contractions, ensuring more oxygen is supplied to the muscles [1]. While thyroxine also influences cardiac function and can cause a rapid heart rate (tachycardia) when present in excess (hyperthyroidism), adrenaline is the classic physiological response for immediate heart rate elevation [1][3][4]. Thyroxine primarily regulates the basal metabolic rate and long-term cardiac sensitivity to catecholamines [2]. Insulin primarily functions to reduce blood glucose levels and does not typically increase heart rate as its primary action [3]. Therefore, adrenaline is the most direct and standard answer for a hormone that increases the heartbeat in response to immediate stimuli.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 6: Control and Coordination > 6.3 HORMONES IN ANIMALS > p. 109
2. [3] https://www.ncbi.nlm.nih.gov/books/NBK613062/
3. [4] https://en.wikipedia.org/wiki/Hyperthyroidism
4. [2] Science , class X (NCERT 2025 ed.) > Chapter 6: Control and Coordination > Do You Know? > p. 110