Concentration of urine is controlled by?

NHM UP Staff Nurse - 2019

Applied Physiology

Easy

Appeared in: NHM UP Staff Nurse - 2019

Explanation

Antidiuretic Hormone (ADH), also called vasopressin, is the primary hormone responsible for regulating the body's water balance by controlling urine concentration.
ADH is released from the posterior pituitary gland when the body is dehydrated or plasma osmolarity is high.
It acts on the distal tubules and collecting ducts of the kidneys, increasing their permeability to water.
This increased permeability allows for greater reabsorption of water from the tubular fluid back into the blood.
The result is the excretion of a smaller volume of more concentrated urine, which helps the body conserve water.

Option A: Melanocyte-Stimulating Hormone (MSH) is primarily involved in stimulating the production and release of melanin in skin and hair, which determines pigmentation. It has no direct role in renal function or urine concentration.
Option C: Oxytocin is mainly known for its role in stimulating uterine contractions during childbirth and promoting milk ejection (let-down) during lactation. While it is released from the posterior pituitary like ADH, its primary targets are the uterus and mammary glands, not the kidneys for water balance.
Option D: Adrenocorticotropic Hormone (ACTH) is secreted by the anterior pituitary gland and its main function is to stimulate the adrenal cortex to produce and release cortisol. While cortisol can have minor effects on water and electrolyte balance, ACTH's primary role is not the direct control of urine concentration.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Hormonal regulation of urine concentration as background academic context rather than a clinical decision trigger.
Nurses frequently monitor urine output, color, and specific gravity as vital signs of a patient's hydration status and kidney function, which are directly influenced by ADH levels.
Understanding ADH is crucial for managing patients with conditions like Syndrome of Inappropriate Antidiuretic Hormone (SIADH), where excess ADH causes water retention and hyponatremia, and Diabetes Insipidus (DI), where deficient ADH causes severe dehydration and hypernatremia.
What if? If a patient has a traumatic brain injury affecting the pituitary gland, the nurse must anticipate the risk of Diabetes Insipidus. The priority action would be to closely monitor urine output (for polyuria), serum sodium levels (for hypernatremia), and signs of dehydration, and be prepared to administer synthetic ADH (desmopressin).

How to Approach the Question

First, identify the key physiological process in the question: 'concentration of urine'. This points to the body's mechanism for water balance.
Next, review the options, which are all hormones. The task is to match the correct hormone to the function of water retention in the kidneys.
Analyze each hormone's primary function: MSH (skin pigment), ADH (water balance), Oxytocin (childbirth/lactation), ACTH (adrenal/cortisol).
The name 'Antidiuretic Hormone' itself provides a strong clue: 'anti-' (against) and 'diuresis' (increased urination). Therefore, ADH acts to decrease urination by conserving water.
Based on this direct relationship, select ADH as the hormone that controls urine concentration.

Concept Tested & Keywords

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E6 Physiology Guyton 4SAE Part 2B pp. 40-41, 41-43