FLGX 312 SU4

Role of Antidiuretic Hormone (ADH) in Urine Concentration

  • Synthesis & Release

    • Synthesized in the supra-optic and paraventricular nuclei in the hypothalamus, transported to the posterior pituitary.

    • Factors influencing release: osmotic pressure changes, volume change, and triggers like exercise and angiotensin II.

    • Inhibition Factors: Atrial natriuretic peptide (ANP), alcohol, certain medications.

  • Mechanism of Action in Kidneys

    • Acts on the distal convoluted tubule and collecting ducts to increase permeability to water via Aquaporin-2 channels.

    • Effects:

    • Increased water reabsorption

    • Increased blood volume and normalization of osmolarity

    • At high concentrations, ADH increases peripheral vascular resistance, thus influencing blood pressure.

  • Feedback Mechanism

    • Osmoreceptors detect osmolarity changes, prompting ADH secretion to regulate fluid balance.

    • Reduced baroreceptor firing, increased angiotensin II and sympathetic tone also influence ADH release.

Mechanisms of Urine Formation

  • Formation of Dilute Urine

    • Occurs when excess water is present. Kidneys excrete nearly 20L/day of diluted urine.

    • Key Points:

    • Low ADH levels allow for significant water reabsorption in the proximal tubule; however, tubular fluid remains hypotonic.

    • The ascending limb of the Loop of Henle is impermeable to water, leading to hypomolar urine, irrespective of ADH levels.

  • Formation of Concentrated Urine

    • Essential for surviving in extreme conditions. Requires high ADH levels and a hyperosmotic renal medulla.

    • Counter-Current Mechanism:

    • The arrangement of Henle's loops and vasa recta allows the medulla to become hyperosmotic, facilitating solute trapping and increased osmolarity.

    • Urea plays a significant role in maintaining osmolarity within the medullary interstitium.

Clinical Application

  • Diabetes Insipidus

    • Characteristics include excessive thirst and dilute urine. Diagnosis involves high plasma osmolarity and low urine osmolarity.

    • Types:

    • Central (Cranial) diabetes insipidus: Lack of ADH secretion due to pituitary issues; treatable with ADH analogues.

    • Nephrogenic diabetes insipidus: Kidneys do not respond to ADH, challenging management often involving diuretics and other medications.

  • Factors Affecting Concentrated Urine Formation

    • Inappropriate ADH secretion, counter-current mechanism failure, or inadequate kidney response to ADH inhibit concentrated urine formation.