Which one of the following hormones, released by the adrenal medulla, is primarily responsible for accelerating the rate of heartbeat during fear, stress or excitement?

Introduction:
The human body responds rapidly to stress, fear or excitement through changes such as increased heart rate, faster breathing and release of glucose into the blood. These changes are part of the fight or flight response and are controlled by specific hormones and neurotransmitters. This question asks which hormone mainly accelerates the heartbeat as part of this response.

Given Data / Assumptions:
- The effect to be explained is acceleration of heart rate. - The situation includes stress, fear or excitement. - Options include adrenaline, acetylcholine, thyroxine and pituitrin. - We assume normal functioning of the autonomic nervous system and endocrine system.

Concept / Approach:
Adrenaline, also known as epinephrine, is secreted by the adrenal medulla and acts as a key hormone in the sympathetic fight or flight response. It increases heart rate and force of contraction, raises blood pressure and prepares the body for rapid action. Acetylcholine is a neurotransmitter of the parasympathetic system that tends to slow heart rate. Thyroxine is a thyroid hormone that influences basal metabolic rate over a longer term but does not acutely accelerate heartbeat in sudden stress. Pituitrin is an older term for posterior pituitary hormones like vasopressin and oxytocin and is not the principal driver of acute heart rate increase. Hence, adrenaline is the correct answer.

Step-by-Step Solution:
Step 1: Identify that the question is about a rapid increase in heart rate during emotional or physical stress. Step 2: Recall that the adrenal medulla releases adrenaline into the blood in such situations. Step 3: Understand that adrenaline stimulates beta receptors in the heart, leading to faster and stronger heartbeats. Step 4: Compare this with acetylcholine, which slows heart rate, and thyroxine, which has more chronic metabolic effects rather than an immediate stress response. Step 5: Conclude that adrenaline is the hormone primarily responsible for accelerating the heartbeat in the described conditions.

Verification / Alternative check:
Physiology textbooks describe the sympathetic nervous system and adrenal medulla working together as a unit called sympathoadrenal system. When stimulated, this system releases adrenaline and noradrenaline, causing tachycardia (increased heart rate), raised blood pressure and increased blood flow to muscles. Pharmacology references also show that adrenaline injections cause rapid heart rate increases. These consistent observations confirm adrenaline as the correct answer.

Why Other Options Are Wrong:
Acetylcholine: A parasympathetic neurotransmitter that usually slows the heart rate by acting on the sinoatrial node. Thyroxine: A thyroid hormone that increases basal metabolism and can indirectly influence heart rate over time, but it is not the main acute stress hormone. Pituitrin: Refers to posterior pituitary extracts containing vasopressin and oxytocin, which primarily regulate water balance and uterine contraction, not acute heart rate changes.

Common Pitfalls:
Some learners confuse adrenaline and thyroxine because both increase metabolic activity, but they operate on different time scales and systems. Others may pick acetylcholine simply because it is a familiar term related to nerves, without recalling its slowing effect on the heart. A useful memory aid is to remember adrenaline rush as a popular phrase for the excited state with pounding heart, clearly linking adrenaline with increased heart rate in stressful situations.

Final Answer:
The hormone that accelerates the rate of heartbeat during stress is Adrenaline.