Adrenaline triggers sudden heart rate increase in fight-or-flight response

Steps:

• Adrenaline is released during sudden stress like a shock, rapidly increasing heart rate to prepare for action.
• At point X (the jump), heart rate spikes immediately due to adrenaline's effect on the sinoatrial node.
• The increase is short-term; heart rate then gradually returns to baseline as the hormone levels drop.
• Select the graph showing a steady baseline, abrupt rise at X, and subsequent decline.

Why A is correct:

• It depicts a sudden, sharp increase in heart rate at X followed by a gradual return to normal, matching adrenaline's acute stimulatory effect on cardiac output.

Why the others are wrong:

• B: Shows a gradual rise before X, not a sudden shock response.
• C: Indicates a decrease in heart rate, opposite to adrenaline's excitatory action.
• D: Displays no change or sustained high rate without decline, ignoring recovery phase.

Final answer: A