OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

How do kidneys respond when sweat causes significant water loss?

1. They conserve water and produce concentrated urine

2. They flush out all salts

3. They decrease urine production completely

4. They stop functioning temporarily

The correct answer is: They conserve water and produce concentrated urine

The kidneys play a crucial role in maintaining the body's balance of fluids and electrolytes, particularly when there is a significant loss of water, such as through sweating. When sweat leads to decreased water levels in the body, the kidneys respond by conserving water, which is essential for preventing dehydration. To achieve this, the kidneys adjust their filtration processes. They reduce the amount of water that is excreted in urine, resulting in more concentrated urine. This concentration helps the body retain as much water as possible while still removing waste products. The hormone responsible for this process is antidiuretic hormone (ADH), which signals the kidneys to increase water reabsorption back into the bloodstream. In contrast, flushing out all salts could disrupt electrolyte balance. Decreasing urine production entirely is impractical, as the body still needs to eliminate waste. Temporarily stopping kidney function would lead to severe health risks and is not a mechanism the kidneys use to respond to dehydration. Consequently, the kidneys effectively conserve water and produce concentrated urine as a response to significant fluid loss.