U01.08.06 Antidiuretic hormone

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance by increasing water reabsorption in the kidneys. Understanding its function, regulation, and clinical implications is essential for USMLE Step 1.

Function of ADH

ADH increases water permeability in the distal convoluted tubule and collecting duct of the kidney, leading to enhanced water reabsorption and reduced urine output.

Action of ADH	Effect on Kidney
Increases water permeability	Enhances reabsorption in the distal tubule and collecting duct
Reduces urine volume	Concentrates urine to prevent dehydration
Helps maintain plasma osmolality	Adjusts water excretion based on body needs

Regulation of ADH Secretion

ADH secretion is primarily regulated by plasma osmolality and blood volume changes.

A. Stimuli for ADH Secretion

Increased Plasma Osmolality (Main Stimulus)
- Osmoreceptors in the hypothalamus detect increased osmolality (e.g., dehydration).
- ADH is released from the posterior pituitary to promote water retention.
Hypovolemia (Low Blood Volume)
- Baroreceptors in the carotid sinus and aortic arch detect low blood pressure.
- ADH is secreted to enhance water retention and restore blood volume.
Stress, Pain, and Drugs
- Factors like nausea, nicotine, and angiotensin II can also increase ADH release.

ADH in Syndrome of Inappropriate ADH Secretion (SIADH)

In SIADH, ADH secretion is inappropriately high despite low plasma osmolality, leading to excessive water retention and dilutional hyponatremia.

Feature	SIADH
Plasma Osmolality	Low (due to water retention)
Urine Osmolality	High (concentrated urine despite low plasma osmolality)
Serum Sodium	Low (dilutional hyponatremia)
ADH Levels	Inappropriately High

Key Points to Remember

ADH increases water permeability in the distal convoluted tubule and collecting duct, leading to increased water reabsorption.
The primary stimulus for ADH secretion is increased plasma osmolality (except in SIADH).
In SIADH, ADH remains elevated despite low plasma osmolality, leading to hyponatremia and concentrated urine.

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