Excretion

Concept: The volume and concentration of urine produced by the kidneys are carefully regulated by hormones to maintain water and electrolyte balance in the body (osmoregulation). Step 1: Key Hormone in Water Reabsorption and Urine Concentration The primary hormone involved in regulating water reabsorption in the kidneys, and thus the volume and concentration of urine, is Anti-diuretic Hormone (ADH). ADH is also known as vasopressin.
Production and Release: ADH is produced in the hypothalamus (a part of the brain) and stored and released from the posterior pituitary gland.
Action: ADH acts on the collecting ducts and distal tubules of the nephrons in the kidneys, increasing their permeability to water. This allows more water to be reabsorbed from the filtrate back into the blood.
Effect on Urine:
When ADH levels are high (e.g., during dehydration), more water is reabsorbed, resulting in a smaller volume of concentrated urine.
When ADH levels are low (e.g., when well-hydrated), less water is reabsorbed, resulting in a larger volume of dilute urine. "Diuresis" means increased or excessive production of urine. "Anti-diuretic" means it acts against diuresis, i.e., it reduces urine output by promoting water reabsorption. Step 2: Analyzing the options
(1) Adreno-corticotropic hormone (ACTH): Produced by the anterior pituitary gland, ACTH stimulates the adrenal cortex to produce corticosteroid hormones (like cortisol). It's not the primary regulator of urine volume/concentration.
(2) Gonadotropin hormone (e.g., FSH, LH): These hormones (Follicle-Stimulating Hormone, Luteinizing Hormone) are produced by the anterior pituitary and regulate the function of the gonads (testes and ovaries), primarily involved in reproduction. Luteinizing hormone is listed as option (4).
(3) Anti-diuretic hormone (ADH): Correct. ADH (vasopressin) directly regulates water reabsorption in the kidneys and thus controls urine volume and concentration.
(4) Leutenizing hormone (LH): A gonadotropin hormone, primarily involved in reproduction (e.g., ovulation in females, testosterone production in males). Therefore, Anti-diuretic hormone (ADH) is the key hormone that regulates urine.