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14.08 Control of Water Content and the Role of ADH

Posted30/10/2024
Updated30/10/2024
ByBMF
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Osmoregulation: Overview and Key Components

  • Osmoregulation: The regulation of water potential in body fluids, crucial for maintaining homeostasis and ensuring cellular function.
  • Key Structures Involved:
    • Hypothalamus: Contains osmoreceptors that monitor changes in blood water potential.
    • Posterior Pituitary Gland: Releases antidiuretic hormone (ADH) in response to signals from the hypothalamus.
    • Kidneys: ADH targets the collecting ducts in the nephron to control water reabsorption.

Role of Osmoreceptors, Hypothalamus, and ADH

  1. Osmoreceptors:
    • Located in the hypothalamus, these sensory neurons detect decreases in blood water potential.
    • When water potential drops below a set point, osmoreceptors signal the posterior pituitary gland to release ADH.
  2. Antidiuretic Hormone (ADH):
    • A peptide hormone that conserves water by increasing its reabsorption in the kidneys.
    • ADH release is inhibited when blood water potential is high, such as after consuming large volumes of water, leading to the production of dilute urine.

How ADH Affects the Kidneys

  1. ADH and Water Reabsorption:
    • ADH specifically targets cells in the collecting ducts of the nephron.
    • It binds to receptors on these cells, initiating a series of enzyme-controlled reactions that increase water permeability.
  2. Mechanism of ADH Action:
    • Aquaporins: ADH causes vesicles containing water-permeable channels, called aquaporins, to fuse with the collecting duct cell membranes.
    • Water Movement: Aquaporins allow water to leave the collecting duct and enter the medullary tissue (where water potential is lower), concentrating the urine.
  3. Signalling Cascade:
    • ADH binding triggers the production of cyclic AMP (cAMP), a second messenger.
    • cAMP activates phosphorylation of aquaporin molecules, facilitating vesicle fusion with the cell membrane for enhanced water reabsorption.

When Blood Water Potential is High (Excess Water)

  • Inhibition of ADH Release: High blood water potential is detected by osmoreceptors in the hypothalamus, leading to decreased ADH secretion.
  • Removal of Aquaporins: Without ADH, aquaporins are removed from the collecting duct membrane, making it impermeable to water.
  • Production of Dilute Urine: Water stays in the collecting duct, resulting in a large volume of dilute urine that flows to the bladder for excretion.

Importance of Water Regulation

  • Homeostasis and Set Point: Maintaining a stable water potential (set point) is essential to prevent osmotic imbalances in cells.
  • Consequences of Imbalance:
    • Too High Water Potential: Cells may absorb too much water and burst.
    • Too Low Water Potential: Cells may lose water, shrink, and risk dehydration, impairing cellular function.

Diagrammatic Explanation of Water Regulation

  1. Set Point: The ideal water potential level.
  2. Receptors (Osmoreceptors): Detect changes in blood water potential.
  3. Input: Nerve impulses to the posterior pituitary gland.
  4. Effector (Posterior Pituitary Gland): Releases ADH when water potential is low.
  5. Output: ADH acts on the kidneys, promoting water reabsorption to stabilize water potential.

This cycle exemplifies negative feedback, where the body self-regulates to maintain stable water levels.

Questions for Understanding

Without ADH: Lower concentration in the collecting duct fluid, resulting in dilute urine as water remains in the duct.

Why are aquaporins common in cell membranes?

Aquaporins enable rapid water movement across membranes, essential for processes like osmoregulation and plant cell turgor.

Functional Advantage of Kidney Structure:

The parallel arrangement of structures like the loop of Henle and collecting ducts in the medulla helps create a high solute concentration, which is crucial for efficient water reabsorption, particularly in species adapted to arid environments.

Action of ADH as a Cell-Signaling Compound:

ADH binds to receptors on collecting duct cells, initiating a signaling cascade via cAMP, leading to the insertion of aquaporins and increased water reabsorption.

Graph Interpretation (Figure 14.20):

With ADH: High concentration of solutes in the collecting duct fluid as water is reabsorbed.

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