Maintaining a proper water balance in human body is important to avoid dehydration or over-hydration (hyponatremia) characterized by chronic, fluctuating elevations of serum sodium (and chloride) often to dangerously high levels. Water concentration of body is monitored by osmoreceptors in hypothalamus, which detect concentration of electrolytes in extracellular fluid.
The concentration of electrolytes in the blood rises when there is water loss caused by excessive perspiration, inadequate water intake, or low blood volume due to blood loss.
Antidiuretic hormone (ADH), also known as vasopressin, is a small peptide hormone which regulates retention of water in human body. It is one of only two hormones secreted by posterior pituitary gland in brain. Synthesis of ADH occurs in the supraoptic and paraventricular nuclei in the hypothalamus, with its release controlled by a negative feedback loop of osmoreceptors in hypothalamus.
A diuretic is an agent that increases the rate of urine formation. Antidiuretic hormone binds to receptors on cells in collecting ducts of kidney and promotes reabsorption of water back into circulation. In absence of antidiuretic hormone collecting ducts are virtually impermeable to water, and it flows out as urine.
When plasma osmolarity is below a certain threshold, osmoreceptors are not activated and secretion of antidiuretic hormone is suppressed. When osmolarity increases above threshold, osmoreceptors recognize it as signal to stimulate neurons that secrete antidiuretic hormone.
When osmolarity (plasma sodium concentration) increases above normal because of water deﬁcit, this feedback system operates as follows:
- An increase in extracellular ﬂuid osmolarity (which means an increase in plasma sodium concentration) causes special nerve cells called osmoreceptor cells, located in anterior hypothalamus near supraoptic nuclei, to shrink.
- Shrinkage of osmoreceptor cells causes them to activate, sending nerve signals to additional nerve cells in supraoptic nuclei, which then relay these signals down stalk of pituitary gland to posterior pituitary.
- These action potentials conducted to posterior pituitary stimulate release of ADH, which is stored in secretory granules (or vesicles) in nerve endings.
- ADH enters blood stream and is transported to kidneys, where it increases water permeability of late distal tubules, cortical collecting tubules, and medullary collecting ducts.
- The increased water permeability in distal nephron segments causes increased water reabsorption and excretion of a small volume of concentrated urine.
Thus, water is conserved in the body while sodium and other solutes continue to be excreted in urine. This causes dilution of solutes in the extracellular ﬂuid, thereby diluting initial excessively concentrated extracellular ﬂuid.
The opposite sequence of events occurs when extracellular ﬂuid becomes too dilute (hypo-osmotic). For example, with excess water ingestion and a decrease in extracellular ﬂuid osmolarity, less ADH is formed, renal tubules decrease their permeability for water, less water is reabsorbed, and a large volume of dilute urine is formed. This in turn concentrates body ﬂuids and returns plasma osmolarity toward normal.