At high altitude with reduced oxygen, what sequence of changes occurs in heart rate, blood pressure, and cardiac output before acclimation?

• A. Heart rate up; blood pressure up; cardiac output up
• B. Heart rate down; blood pressure up; cardiac output down
• C. Heart rate up; blood pressure down; cardiac output up
• D. Heart rate down; blood pressure down; cardiac output no change

Answer: (C)

When oxygen is scarce at high altitude, the body responds by increasing heart rate to boost oxygen delivery to tissues. This tachycardia is driven by sympathetic activation and helps raise cardiac output even if the amount pumped with each beat drops a bit. At the same time, there’s a loss of plasma volume from diuresis and a reduction in venous return (preload), which tends to lower systolic blood pressure. The bigger push in heart rate compensates for the smaller stroke volume, so overall cardiac output rises despite the drop in blood pressure.