The question asks to identify the hormone among the given options that increases the heartbeat in mammals. Hormones are chemical messengers that travel through the bloodstream and regulate various bodily functions, including heart rate.

Understanding Hormones and Heartbeat Regulation

Several hormones influence the cardiovascular system, affecting factors like heart rate and blood pressure. When the body faces stress, excitement, or danger, it activates a response system that prepares it for action. This response involves the release of specific hormones.

Analyzing the Options

• Insulin: This hormone is produced by the pancreas and is primarily involved in regulating blood glucose levels by facilitating the uptake of glucose from the bloodstream into cells. It does not have a direct effect on increasing heartbeat.
• Melatonin: This hormone is produced by the pineal gland and plays a crucial role in regulating the sleep-wake cycle (circadian rhythm). It is not known to cause an increase in heartbeat.
• Thyroxine: This hormone, produced by the thyroid gland, regulates metabolism. While thyroid hormones can influence heart rate over time and hyperthyroidism can lead to increased heart rate, it is not the primary hormone responsible for an immediate, significant increase in heartbeat in response to acute stimuli like stress or danger.
• Adrenaline: Also known as epinephrine, adrenaline is produced by the adrenal glands. It is a key hormone in the body's "fight or flight" response. When released into the bloodstream, adrenaline acts on receptors in the heart, causing the heart to beat faster and with more force, thereby increasing heart rate and cardiac output. It also increases blood pressure, dilates airways, and redirects blood flow to muscles, preparing the body for intense physical activity or escape. This hormone is well-known for its immediate and strong effect on increasing heartbeat.

Based on the functions of these hormones, adrenaline is the one that most significantly and directly causes an increase in heartbeat in mammals, especially in response to stress or perceived danger.

Conclusion

The hormone that primarily increases the heartbeat in mammals, particularly during the 'fight or flight' response, is Adrenaline.

Revision Table: Hormones Affecting Heart Rate

Additional Information: The 'Fight or Flight' Response

The 'fight or flight' response is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. This response is mediated by the sympathetic nervous system and the endocrine system.

• Nervous System Component: The sympathetic nervous system rapidly stimulates organs, including the heart.
• Endocrine Component: The adrenal glands release adrenaline and noradrenaline into the bloodstream.

Adrenaline and noradrenaline act together to prepare the body for intense physical action by:

• Increasing heart rate and the force of heart contractions.
• Increasing blood pressure.
• Dilating bronchioles in the lungs to increase oxygen intake.
• Increasing blood flow to skeletal muscles.
• Stimulating the release of glucose from the liver for energy.

This coordinated response ensures that muscles and the brain receive sufficient oxygen and energy to either confront the threat ('fight') or escape from it ('flight'). Adrenaline's role in increasing heartbeat is central to this rapid response.