Which consequence of dehydration contributes to reduced performance during heat stress?

• A. Increases blood viscosity and impairs CNS function
• B. Reduces blood viscosity
• C. Improves cognitive function
• D. Increases sweating efficiency

Answer: (A)

Dehydration concentrates the blood because fluid leaves the plasma, so blood becomes thicker (increased viscosity). That makes it harder for blood to flow, especially to the brain and working muscles, which raises cardiac workload and can reduce cerebral perfusion. With less blood flow to the brain, CNS function can be impaired and perception of effort rises, leading to poorer decision-making, slower reaction times, and reduced endurance in heat. In the heat, this combination—slower circulation, higher strain on the heart, and impaired cooling—directly undermines performance. The other statements describe effects that don’t occur with dehydration: viscosity isn’t reduced (it’s increased), cognitive function isn’t improved (it’s often worsened), and sweating efficiency isn’t increased (it’s typically reduced due to lower plasma volume).