Which is the hormone that increases water reabsorption by inserting a water channel or aquaporins?

RRB Nsg. Superintendent-20 July 2019 (Shift-1)

Applied Physiology

Easy

Appeared in: RRB Nsg. Superintendent-20 July 2019 (Shift-1)

Explanation

Anti-diuretic hormone (ADH), also known as vasopressin, is the primary hormone that regulates the body's water balance.
Its main function is to make the collecting ducts and late distal tubules of the kidneys permeable to water.
ADH achieves this by binding to V2 receptors, which triggers a cellular process to insert pre-formed water channels, called Aquaporin-2, into the cell membrane.
This mechanism allows water to be reabsorbed from the urine back into the bloodstream, thus conserving water and concentrating the urine.

Option A: Angiotensin II is a potent vasoconstrictor that raises blood pressure. While it stimulates the release of ADH and aldosterone and promotes some sodium and water reabsorption in the proximal tubule, it does not directly cause the insertion of aquaporins.
Option B: Aldosterone's primary role is to increase the reabsorption of sodium in the distal convoluted tubule and collecting ducts. Water follows the reabsorbed sodium passively due to osmosis, but aldosterone does not directly insert water channels.
Option D: Atrial natriuretic peptide (ANP) has the opposite effect of ADH and aldosterone. It is released in response to high blood volume and pressure and promotes the excretion of sodium and water (natriuresis and diuresis) to lower blood pressure.

Visual 1: Diagram - An illustration of a nephron's collecting duct cell showing ADH binding to a V2 receptor, leading to the insertion of Aquaporin-2 channels in the apical membrane and subsequent water movement.
Visual 2: Flowchart - A diagram showing the feedback loop for ADH release, starting with increased plasma osmolality detected by the hypothalamus, leading to ADH release from the posterior pituitary, and its action on the kidney.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Hormonal regulation of renal water reabsorption as background academic context rather than a clinical decision trigger.
Understanding ADH is critical for managing conditions like Diabetes Insipidus (DI) and Syndrome of Inappropriate Antidiuretic Hormone (SIADH).
In DI, there is a deficiency of ADH or a renal insensitivity to it, leading to excessive thirst and excretion of large volumes of dilute urine. Nurses must monitor for dehydration and electrolyte imbalances.
In SIADH, there is excessive ADH production, causing water retention, dilutional hyponatremia, and concentrated urine. Nursing care involves fluid restriction and monitoring for signs of fluid overload and low sodium levels.

How to Approach the Question

First, identify the key terms in the question: 'increases water reabsorption' and 'inserting a water channel or aquaporins'.
The term 'aquaporins' is a very specific clue pointing to a direct mechanism of water transport, not a passive one.
Recall the primary functions of the four major hormones listed that regulate fluid and electrolyte balance.
Differentiate between the hormones. Aldosterone primarily acts on sodium, with water following osmotically. ANP causes water loss. Angiotensin II is mainly a vasoconstrictor and stimulus for other hormones.
Recognize that only Anti-diuretic hormone (ADH) has the specific function of directly inserting aquaporin water channels to increase water permeability in the collecting ducts.
Select the option that matches this specific mechanism.

Concept Tested & Keywords

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