Cambridge A-Level Biology – Homeostasis in Mammals
Homeostasis in Mammals: Osmoregulation
Osmoregulation is the process by which mammals maintain the osmotic pressure of body fluids within a narrow range (≈ 285–295 mOsm kg⁻¹). The hypothalamus, posterior pituitary gland, antidiuretic hormone (ADH), aquaporins and the collecting ducts of the kidney work together to achieve this balance.
Key Structures and Their Roles
Hypothalamus (osmoreceptor region) – Detects changes in plasma osmolality via specialised osmoreceptor neurons.
Posterior Pituitary Gland – Stores and releases ADH into the bloodstream in response to hypothalamic signals.
Antidiuretic Hormone (ADH, also called vasopressin) – Hormone that increases water re‑absorption in the kidney.
Aquaporins (AQP2) – Water‑channel proteins inserted into the apical membrane of collecting‑duct cells, allowing rapid water movement.
Collecting Ducts – Segment of the nephron where final adjustment of urine volume and concentration occurs.
Physiological Sequence of Osmoregulation
Plasma osmolality rises (e.g., after dehydration).
Hypothalamic osmoreceptors swell, increasing their firing rate.
Signal travels via hypothalamo‑hypophyseal tract to the posterior pituitary.
Posterior pituitary releases ADH into the systemic circulation.
ADH binds to V₂ receptors on the basolateral membrane of collecting‑duct cells.
Binding activates a cAMP cascade, causing insertion of AQP2 channels into the apical membrane.
Water moves from the tubular lumen into the interstitium down its osmotic gradient, concentrating the urine and diluting plasma.
When plasma osmolality returns to normal, ADH secretion declines, AQP2 channels are removed, and water re‑absorption decreases.
Key Equations
The osmotic pressure (π) of plasma can be approximated by:
\$\pi = iCRT\$
where i = van ’t Hoff factor, C = molar concentration of solutes, R = gas constant, and T = absolute temperature.
Summary Table
Component
Location
Primary Function in Osmoregulation
Effect of Increased Activity
Hypothalamic Osmoreceptors
Anterior hypothalamus
Detect plasma osmolality changes
Stimulates ADH release → ↑ water re‑absorption
Posterior Pituitary
Neurohypophysis
Store and secrete ADH
More ADH → ↑ AQP2 insertion
ADH (Vasopressin)
Blood plasma
Bind V₂ receptors on collecting‑duct cells
↑ cAMP → ↑ AQP2 channels
Aquaporin‑2 (AQP2)
Apical membrane of collecting‑duct cells
Facilitate water movement into interstitium
Urine becomes more concentrated
Collecting Ducts
Kidney medulla
Final site of water re‑absorption
Reduced urine volume, increased plasma volume
Suggested diagram: Flow of the osmoregulatory feedback loop from hypothalamic osmoreceptors to ADH release, AQP2 insertion, and water re‑absorption in the collecting ducts.
Common Examination Questions
Explain how plasma osmolality is sensed and how this information is transmitted to the posterior pituitary.
Describe the molecular mechanism by which ADH increases water permeability of the collecting duct.
Predict the effect on urine volume and concentration if ADH secretion is inhibited (e.g., by a vasopressin antagonist).
Understanding these mechanisms provides a solid foundation for tackling A‑Level questions on homeostasis and renal physiology.