Thirst is the craving for fluids, resulting in the basic instinct

Instinct

Instinct or innate behavior is the inherent inclination of a living organism toward a particular behavior.The simplest example of an instinctive behavior is a fixed action pattern, in which a very short to medium length sequence of actions, without variation, are carried out in response to a...

 of animals to drink

Drinking

Drinking is the act of consuming water or a beverage through the mouth. Water is required for many of life’s physiological processes. Both excessive and inadequate water intake are associated with health problems.-Physiology:...

. It is an essential mechanism involved in fluid balance

Fluid balance

Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...

. It arises from a lack of fluids and/or an increase in the concentration of certain osmolites, such as salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 signals thirst.

Continuous dehydration

Dehydration

In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

 can cause many problems, but is most often associated with neurological problems such as seizures and renal

Kidney

The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

 problems.

Excessive thirst, known as polydipsia

Polydipsia

Polydipsia is a medical symptom in which the patient displays excessive thirst. The word derives from the Greek πολυδιψία, which is derived from πολύς + δίψα...

, along with excessive urination, known as polyuria

Polyuria

Polyuria is a condition usually defined as excessive or abnormally large production or passage of urine . Frequent urination is sometimes included by definition, but is nonetheless usually an accompanying symptom...

, may be an indication of diabetes.

There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration. They signal to the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

, where central processing succeeds. Some sources, therefore, distinguish "extracellular thirst" from "intracellular thirst", where extracellular thirst is thirst generated by decreased volume and intracellular thirst is thirst generated by increased osmolite concentration. Nevertheless, the craving itself is something generated from central processing in the brain, no matter how it is detected.

Detection

There are many different receptors for sensing decreased volume or an increased osmolite concentration.

Decreased volume

Further reading: Hypovolemia

Hypovolemia

In physiology and medicine, hypovolemia is a state of decreased blood volume; more specifically, decrease in volume of blood plasma...

• Renin-angiotensin system

Hypovolemia leads to activation of the renin angiotensin system (RAS) and a decrease in atrial natriuretic peptide

Atrial natriuretic peptide

Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...

. These mechanisms, along their other functions, contribute to elicit thirst, by affecting the subfornical organ

Subfornical organ

The subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina , is one of the circumventricular organs of the brain.-Relations with other circumventricular organs:...

. For instance, angiotensin II, activated in RAS, is a powerful dipsogen

Dipsogen

A dipsogen is an agent that causes thirst. -Physiology:Angiotensin II is thought to be a powerful dipsogen, and is one of the products of the renin-angiotensin pathway, a biological homeostatic mechanism for the regulation of electrolytes and water.-External references:...

 (ie it stimulates thirst) which acts via the subfornical organ.

• Other
  • Arterial baroreceptors sense a decreased arterial pressure, and signals to the central nervous system
    Central nervous system
    The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
    
     in the area postrema
    Area postrema
    The area postrema is a medullary structure in the brain that controls vomiting. Its privileged location in the brain also allows the area postrema to play a vital role in the control of autonomic functions by the central nervous system.-Anatomy:...
    
     and nucleus tractus solitarius.
  • Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema
    Area postrema
    The area postrema is a medullary structure in the brain that controls vomiting. Its privileged location in the brain also allows the area postrema to play a vital role in the control of autonomic functions by the central nervous system.-Anatomy:...
    
     and nucleus tractus solitarius as well.

Cellular dehydration and osmoreceptor stimulation

Osmosis is the movement of liquid from one side of a semipermeable membrane with lower concentrations of dissolved substances, to the other side of the membrane with higher concentrations, resulting in dilution of the higher concentration solution. In living cells the transport of water from low concentration to high concentration solutions through the cell membrane tends to continue until equilibrium is achieved. The pressure needed to halt the movement through the membrane is the "osmotic pressure

Osmotic pressure

Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....

."

As the volume of extracellular fluids such as blood plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

 and cerebrospinal fluid

Cerebrospinal fluid

Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

 drops due to loss of water through perspiration, respiration, urination and defecation, the concentration of sodium (the major substance affecting osmosis in the extracellular fluid) and hence the osmotic pressure, rises; water migrates from the cells of the body, through their membranes, to the extracellular compartment, and cellular dehydration occurs. (Infusion of hypertonic saline solution into the blood stream has the same cellular dehydration effect.)

In the mammalian brain, the posterior surface of the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

 forms the front wall of the third ventricle

Third ventricle

The third ventricle is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid ....

 (a cerebrospinal fluid-filled cavity) and clusters of cells (osmoreceptors) on this surface, notably in the organum vasculosum of the lamina terminalis

Organum vasculosum of the lamina terminalis

The organum vasculosum of the lamina terminalis is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ and the area postrema in the brainstem.-AV3V region:...

 (OVLT) and subfornical organ

Subfornical organ

The subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina , is one of the circumventricular organs of the brain.-Relations with other circumventricular organs:...

 (SFO), signal this cellular dehydration to other parts of the brain, and thirst is experienced. Destruction of this part of the hypothalamus in humans and other animals results in partial or total loss of desire to drink even with extremely high salt concentration in the extracellular fluids.

In addition, there are visceral osmoreceptors. These project to the area postrema

Area postrema

The area postrema is a medullary structure in the brain that controls vomiting. Its privileged location in the brain also allows the area postrema to play a vital role in the control of autonomic functions by the central nervous system.-Anatomy:...

 and nucleus tractus solitarius in the brain.

Salt craving

Because sodium is also lost from the plasma in hypovolemia, the body's need for salt proportionately increases in addition to thirst in such cases. This is also a result of the renin-angiotensin system activation.

Elderly

In adults over the age of 50 years, the body’s thirst sensation reduces and continues diminishing with age, putting this population at increased risk of dehydration

Dehydration

In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

. Several studies have demonstrated that elderly persons have lower total water intakes than younger adults, and that women are particularly at risk of too low intake.
The European Food Safety Authority

European Food Safety Authority

The European Food Safety Authority is an agency of the European Union that provides independent scientific advice and communication on existing and emerging risks associated with the food chain, created by European Regulation 178/2002....

 (EFSA) recently included water as a macronutrient in its dietary reference values for the first time. Recommended intake volumes in the elderly are the same as for younger adults (2.0 L/day for females and 2.5 L/day for males) as despite lower energy consumption, the water requirement of this group is increased due to a reduction in renal concentrating capacity.

Central processing

The area postrema

Area postrema

The area postrema is a medullary structure in the brain that controls vomiting. Its privileged location in the brain also allows the area postrema to play a vital role in the control of autonomic functions by the central nervous system.-Anatomy:...

 and nucleus tractus solitarius signal, by 5-HT, to lateral parabrachial nucleus

Lateral parabrachial nucleus

The lateral parabrachial nucleus induces thirst by stimulating the median preoptic nucleus.It gets its name from its location relative to the superior cerebellar peduncles, which is also known as the "brachia conjunctiva"....

, which in turn signal to median preoptic nucleus. In addition, the area postrema and nucleus tractus solitarius also signal directly to subfornical organ.

Thus, the median preoptic nucleus and subfornical organ receive signals of both decreased volume and increased osmolite concentration. They signal to higher integrative centers, where ultimately the conscious craving arises. However, the true neuroscience

Neuroscience

Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

 of this conscious craving is not fully clear. In general, the end-result is towards behavior of drinking

Drinking

Drinking is the act of consuming water or a beverage through the mouth. Water is required for many of life’s physiological processes. Both excessive and inadequate water intake are associated with health problems.-Physiology:...

 for hydration, but this can to some degree be resisted, such as in voluntary fluid restriction.

In addition to thirst, the organum vasculosum of the lamina terminalis

Organum vasculosum of the lamina terminalis

The organum vasculosum of the lamina terminalis is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ and the area postrema in the brainstem.-AV3V region:...

 and the subfornical organ

Subfornical organ

The subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina , is one of the circumventricular organs of the brain.-Relations with other circumventricular organs:...

 contribute to fluid balance

Fluid balance

Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...

 by vasopressin

Vasopressin

Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...

release.