Which of the following is a consequence of dehydration during exercise?

• A. Decreases plasma volume and impairs thermoregulation.
• B. Decreases heart rate.
• C. Decreases plasma volume, increases heart rate, decreases stroke volume, impairs thermoregulation and cognitive performance.
• D. Improves cognitive performance.

Answer: (C)

When you’re dehydrated during exercise, your blood volume drops, which lowers plasma volume. That reduces the amount of blood returning to the heart ( preload ), so the amount the heart can eject with each beat (stroke volume) falls. To compensate and keep enough blood circulating to the muscles and brain, the heart rate rises. The combination of lower plasma volume and higher core temperature makes it harder for the body to regulate heat, so thermoregulation is impaired. Dehydration also tends to worsen cognitive function, affecting attention and reaction time. Putting all of these together gives the most complete description: plasma volume decreases, heart rate increases, stroke volume decreases, thermoregulation is impaired, and cognitive performance declines.