Osmoreceptors are sensory receptor primarily found in hypothalamus of most homoeothermic organisms that detects changes in osmotic pressure. They contribute to osmoregulation controlling fluid balance in the body. They are defined functionally as neurons that have an intrinsic ability to detect changes in osmolality.
Osmoreceptors are located in vascular organ of lamina terminalis, a circumventricular organ which lacks a blood-brain barrier. Osmoreceptors are also found in kidneys where they modulate osmolality.
An osmole is one mole of any fully dissociated substance dissolved in water. Osmolality is concentration of osmoles in a mass of solvent. In biologic systems, osmolality is expressed as mOsm/kg of water. Osmolarity is concentration of osmoles in a volume of solvent, and expressed as mOsm/L of water in biologic systems.
Osmoles that do not cross cell membrane freely are considered effective osmoles, whereas those that cross the membrane freely are termed ineffective osmoles. The most abundant osmoles in the extracellular fluid are sodium and potassium (and their accompanying anions chloride and bicarbonate), glucose, and urea etc. Every molecule dissolved in the total body water contributes to osmolality regardless of size, weight, charge, or composition.
The primary osmoreceptors that control AVP secretion and thirst are located in anterior hypothalamus. Hypothalamic osmoreceptors sense changes in plasma osmolality, and changes of only 2 to 3 mOsm/kg induce compensatory mechanisms to return plasma osmolality to its hypothalamic set point.
The two major physiological mechanisms for controlling plasma osmolality are antidiuretic hormone (ADH) system and thirst.
Anti-diuretic hormone is a small peptide secreted by posterior pituitary gland. ADH (also known as vasopressin) causes direct water reabsorption from the kidney tubules, salts and wastes are concentrated in what will eventually be excreted as urine.
There are two major stimuli for ADH release: elevated plasma osmolality and decreased effective circulating volume. Increased plasma osmolality causes shrinkage of a specialized group of cells in the hypothalamus called osmoreceptors.
When their cell volume decreases, these hypothalamic osmoreceptors send impulses via neural afferents to posterior pituitary, leading to ADH release.
Thirst is craving for fluids resulting the urge or instinct to drink. It is an essential mechanism involved in fluid balance which arises from a lack of fluids or an increase in concentration of certain osmolites such as salt. If water volume of body falls below a certain threshold or osmolite concentration becomes too high, brain signals thirst.
As blood becomes more concentrated, thirst response, a sequence of physiological processes is activated. Osmoreceptors, sensory receptors in thirst center in hypothalamus monitor the concentration of solutes (osmolality) of blood.
Hyper osmolality and decreased effective circulating volume also stimulate thirst. The mechanisms by which hyperosmolality and hypovolemia stimulate thirst are similar to those that stimulate ADH release. Thirst and resultant water consumption are main physiologic determinants of free water intake.