In human body, the hormone which is secreted into blood and which controls the rate of heart beat is

Correct — A, Adrenaline. Adrenaline is released into the blood by the medulla of the adrenal gland when the body is startled, frightened or under strain, and among its effects is a direct increase in the rate and force of the heartbeat. It also widens the airways, diverts blood to skeletal muscle and raises blood sugar, which together make up the fight-or-flight response.

• (b)Thyroxine — Thyroxine sets the general metabolic rate of the body over long periods. An excess does speed the heart, but its action is slow and metabolic rather than the immediate control the question describes.
• (c)Insulin — Insulin, from the beta cells of the pancreas, lowers blood glucose by helping cells take it up. It has no role in setting heart rate.
• (d)Testosterone — Testosterone is the principal male sex hormone, governing the development of male secondary sexual characters. Heart rate is not among its functions.

The adrenal gland has two functionally separate parts. The inner medulla secretes adrenaline and noradrenaline, which act within seconds and are effectively the chemical arm of the sympathetic nervous system. The outer cortex secretes steroid hormones — cortisol, aldosterone and small amounts of sex steroids — which act more slowly on metabolism and on salt and water balance.

The heart has its own pacemaker, the sinoatrial node, and will beat without any nerve or hormone reaching it. What adrenaline does is modulate that intrinsic rhythm upward. The distinction the examiner is testing is between an emergency hormone that acts in seconds and background hormones that act over hours or years, so identifying the timescale each option works on settles the item quickly. Adrenaline's clinical use in anaphylaxis and in cardiac arrest follows directly from these same actions.

• Adrenaline, also called epinephrine, is secreted by the adrenal medulla and raises heart rate and force of contraction.
• It is the hormone of the fight-or-flight response, widening airways and raising blood glucose alongside its cardiac effects.
• The adrenal cortex is a separate region, secreting cortisol and aldosterone rather than adrenaline.
• Thyroxine from the thyroid controls the basal metabolic rate and contains iodine.
• The heartbeat originates in the sinoatrial node, the heart's own pacemaker, which hormones and nerves then speed up or slow down.

• Choosing thyroxine because it also affects the heart; its action is metabolic and slow.
• Placing adrenaline in the adrenal cortex instead of the medulla.
• Forgetting that the heartbeat is generated by the sinoatrial node and only modulated by hormones.

Hormone-to-function matching. Building a short table of gland, hormone and one-line function covers most items from this area.

• practice — not a real PYQ

  Adrenaline is secreted by

  • (a)the adrenal cortex
  • (b)the adrenal medulla
  • (c)the anterior pituitary
  • (d)the thyroid gland

  Answer(b) the adrenal medulla — the cortex secretes the steroid hormones cortisol and aldosterone instead.

• practice — not a real PYQ

  The pacemaker of the human heart is

  • (a)the atrioventricular node
  • (b)the sinoatrial node
  • (c)the bundle of His
  • (d)the Purkinje fibres

  Answer(b) the sinoatrial node — it sets the intrinsic rhythm, which nerves and hormones such as adrenaline then adjust.