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

What is the primary hormone involved in controlling urine concentration?

1. Insulin

2. Aldosterone

3. Anti-diuretic hormone (ADH)

4. Glucagon

The correct answer is: Anti-diuretic hormone (ADH)

The primary hormone involved in controlling urine concentration is anti-diuretic hormone (ADH), also known as vasopressin. ADH is produced in the hypothalamus and released from the posterior pituitary gland. Its main function is to regulate the amount of water the kidneys reabsorb while filtering waste from the blood. When the body is dehydrated, such as during times of increased physical activity or reduced fluid intake, the secretion of ADH increases. This hormone signals the kidneys to retain more water, resulting in concentrated urine and helping to restore the body's fluid balance. Conversely, when hydration levels are adequate or excessive, ADH release decreases, leading to more dilute urine. Other hormones mentioned, such as insulin and glucagon, play roles in glucose metabolism rather than in water regulation. Aldosterone, while it does influence electrolyte and fluid balance by promoting sodium reabsorption in the kidneys, does not directly control urine concentration in the same way that ADH does. Thus, the function of ADH specifically targets water reabsorption, making it the key hormone in urine concentration control.