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

How do the kidneys regulate water content in the body?

1. They filter blood to remove all substances

2. They control the volume and concentration of urine

3. They produce hormones to stimulate thirst

4. They absorb excess water from the environment

The correct answer is: They control the volume and concentration of urine

The kidneys play a vital role in maintaining the body’s water balance by carefully regulating the volume and concentration of urine. When the body is well-hydrated, the kidneys will produce a larger volume of dilute urine, allowing excess water to be excreted. Conversely, when the body is dehydrated or has a low water content, the kidneys conserve water by concentrating the urine and reducing its volume. This process involves specialized structures within the kidneys, such as the nephrons, which filter the blood and reabsorb water according to the body’s needs. By adjusting urine output in response to hydration levels, the kidneys ensure that the overall balance of water in the body is maintained, which is crucial for homeostasis. The other options do not accurately represent the kidneys' primary function in water regulation. For instance, filtering blood to remove all substances doesn't specifically address the regulation of water. Producing hormones related to thirst may play a role in the body’s overall hydration strategy, but it’s not the direct mechanism by which the kidneys regulate water content. Lastly, absorbing excess water from the environment does not describe a function of the kidneys; instead, it pertains more to organisms that can take in water directly from their surrounding environment.