Response to Hyperosmolarity (ADH)
When a person consumes salty food, the plasma osmolarity increases. This rise (hyperosmolarity) acts as a powerful stimulus detected by hypothalamic osmoreceptors.
In response to excitation, the hypothalamus actively synthesizes antidiuretic hormone — vasopressin (ADH). It is important to note that the hormone is not released directly into the blood: it is transported via axons to the posterior pituitary gland (neurohypophysis), and only secreted into the bloodstream from there.
Upon reaching the kidneys, vasopressin binds to V2-receptors on the membrane of collecting duct cells, initiating an intracellular cascade:
- Adenylyl cyclase is activated.
- cAMP concentration increases.
- Protein kinase A is activated and phosphorylates proteins.
- Water channels — aquaporin-2 — are inserted into the apical membrane of the cells.
As a result, water reabsorption sharply increases: it returns to the bloodstream, plasma osmolarity is restored, and diuresis (urine volume) decreases. Additionally, in very high concentrations, vasopressin can cause peripheral artery vasoconstriction.
Renin-Angiotensin-Aldosterone System (RAAS)
This system is activated by other triggers: blood loss, dehydration (leading to a decrease in circulating blood volume and blood pressure), or renal artery stenosis (renal ischemia).
The RAAS cascade operates step-by-step:
- In response to a drop in pressure, the juxtaglomerular apparatus of the kidneys releases a proteolytic enzyme into the blood — renin.
- Renin acts on angiotensinogen (a protein synthesized in the liver), cleaving a fragment from it to form Angiotensin I.
- ACE converts it into active Angiotensin II, which is the primary effector of the system.
Angiotensin II acts comprehensively. It directly causes vasoconstriction, stimulates the thirst center in the hypothalamus, and triggers the adrenal cortex to produce aldosterone. Aldosterone, in turn, travels to the distal convoluted tubules of the kidneys, where it enhances the reabsorption of sodium ions. Water passively follows sodium along an osmotic gradient. The net result of RAAS activity is the restoration of blood volume, increased blood pressure, and a return to homeostasis.
Sites of Synthesis for Regulators
To understand fluid and electrolyte balance, it is essential to know where its main participants are synthesized. This information is frequently tested on board exams.
| Substance | Site of Synthesis | Additional Details |
|---|---|---|
| Vasopressin | Hypothalamus | Transported to the neurohypophysis |
| Oxytocin | Hypothalamus | Follows the same pathway as vasopressin |
| Aldosterone | Adrenal glands | Zona glomerulosa of the cortex |
| Renin | Kidneys | Juxtaglomerular apparatus |
| Angiotensinogen | Liver | Continuously circulates in the blood |
Clinical Examples of Disorders
During brain tumor resections, the pituitary stalk is often damaged. This interrupts the transport of vasopressin from the hypothalamus to the neurohypophysis. In the absence of ADH, water is no longer reabsorbed in the collecting ducts, leading to central diabetes insipidus, the main symptom of which is polyuria — a sharp and uncontrollable increase in urine output.
Interestingly, polyuria also occurs in the exact opposite condition: primary hyperaldosteronism (aldosterone excess). Although the hormone is expected to retain fluid, its hyperproduction leads to severe potassium loss. This causes hypokalemic nephropathy, impairing the kidneys' ability to concentrate urine, paradoxically resulting in excessive water excretion.