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

What occurs in the kidneys when there is excess water intake?

1. They retain more water

2. They excrete more water leading to dilute urine

3. They stop filtering blood

4. They increase salt absorption

The correct answer is: They excrete more water leading to dilute urine

When there is an excess of water intake, the kidneys respond by excreting more water to maintain the body's fluid balance. The process involves filtering the blood to remove excess water, which leads to the production of dilute urine. This mechanism helps prevent potential issues such as overhydration or water intoxication, which can disturb electrolyte balance and cellular function. The body's regulation of water is influenced by hormones, particularly antidiuretic hormone (ADH). When a person drinks a lot of water, the levels of ADH decrease, leading the kidneys to absorb less water back into the bloodstream and consequently produce a larger volume of diluted urine. This response is essential for homeostasis, ensuring that the body's fluid levels remain stable and optimal for various physiological processes. The other choices reflect mechanisms that would not occur during excess water intake, such as increased water retention or changes in blood filtration and salt absorption.