Thirst

Last updated September 11, 2023

Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance.^[1] It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.^[2]

There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration.^[1]^[2] Some sources 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.^[2]

Detection

It is vital for organisms to be able to maintain their fluid levels in very narrow ranges. The goal is to keep the interstitial fluid, the fluid outside the cell, at the same concentration as the intracellular fluid, the fluid inside the cell. This condition is called isotonic and occurs when the same levels of solutes are present on either side of the cell membrane so that the net water movement is zero. If the interstitial fluid has a higher concentration of solutes (or a lower concentration of water) than the intracellular fluid, it will pull water out of the cell. This condition is called hypertonic and if enough water leaves the cell, it will not be able to perform essential chemical functions. The animal will then become thirsty in response to the demand for water in the cell. After the animal drinks water, the interstitial fluid becomes less concentrated of solutes (more concentrated of water) than the intracellular fluid and the cell will fill with water as it tries to equalize the concentrations. This condition is called hypotonic and can be dangerous because it can cause the cell to swell and rupture. One set of receptors responsible for thirst detects the concentration of interstitial fluid. The other set of receptors detects blood volume.^[2]

Decreased volume

This is one of two types of thirst and is defined as thirst caused by loss of blood volume (hypovolemia) without depleting the intracellular fluid. This can be caused by blood loss, vomiting, and diarrhea. This loss of volume is problematic because if the total blood volume falls too low the heart cannot circulate blood effectively and the eventual result is hypovolemic shock. The vascular system responds by constricting blood vessels thereby creating a smaller volume for the blood to fill. This mechanical solution, however, has definite limits and usually must be supplemented with increased volume. The loss of blood volume is detected by cells in the kidneys and triggers thirst for both water and salt via the renin-angiotensin system.^[2]^[3]

Renin-angiotensin system

Hypovolemia leads to activation of the renin angiotensin system (RAS) and is detected by cells in the kidney. When these cells detect decreased blood flow due to the low volume they secrete an enzyme called renin. Renin then enters the blood where it catalyzes a protein called angiotensinogen to angiotensin I. Angiotensin I is then almost immediately converted by an enzyme already present in the blood to the active form of the protein, angiotensin II. Angiotensin II then travels in the blood until it reaches the posterior pituitary gland and the adrenal cortex, where it causes a cascade effect of hormones that cause the kidneys to retain water and sodium, increasing blood pressure.^[3] It is also responsible for the initiation of drinking behavior and salt appetite via the subfornical organ.^[2]

Others

Arterial baroreceptors sense a decreased arterial pressure, and signal to the central nervous system in the area postrema and nucleus tractus solitarii.^[2]
Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema and nucleus tractus solitarii.^[2]

Cellular dehydration and osmoreceptor stimulation

Osmometric thirst occurs when the solute concentration of the interstitial fluid increases. This increase draws water out of the cells, and they shrink in volume. The solute concentration of the interstitial fluid increases by high intake of sodium in diet or by the drop in volume of extracellular fluids (such as blood plasma and cerebrospinal fluid) due to loss of water through perspiration, respiration, urination and defecation. The increase in interstitial fluid solute concentration causes water to migrate from the cells of the body, through their membranes, to the extracellular compartment, by osmosis, thus causing cellular dehydration.^[1]

Clusters of cells (osmoreceptors) in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), which lie outside of the blood brain barrier can detect the concentration of blood plasma and the presence of angiotensin II in the blood. They can then activate the median preoptic nucleus which initiates water seeking and ingestive behavior.^[3] 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.^[4]^[5] In addition, there are visceral osmoreceptors which project to the area postrema and nucleus tractus solitarii in the brain.^[2]

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.^[3] This is also a result of the renin-angiotensin system activation.^{[ medical citation needed ]}

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.^[6] 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 an intake.^[7]^[8]^[9] In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time.^[10] 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.^[10]^[11]

Thirst quenching

According to preliminary research, quenching of thirst – the homeostatic mechanism to stop drinking – occurs via two neural phases: a "preabsorptive" phase which signals quenched thirst many minutes before fluid is absorbed from the stomach and distributed to the body via the circulation, and a "postabsorptive" phase which is regulated by brain structures sensing to terminate fluid ingestion.^[12] The preabsorptive phase relies on sensory inputs in the mouth, pharynx, esophagus, and upper gastrointestinal tract to anticipate the amount of fluid needed, providing rapid signals to the brain to terminate drinking when the assessed amount has been consumed.^[12] The postabsorptive phase occurs via blood monitoring for osmolality, fluid volume, and sodium balance, which are collectively sensed in brain circumventricular organs linked via neural networks to terminate thirst when fluid balance is established.^[12]

Thirst quenching varies among animal species, with dogs, camels, sheep, goats, and deer replacing fluid deficits quickly when water is available, whereas humans and horses may need hours to restore fluid balance.^[12]

Neurophysiology

The areas of the brain that contribute to the sense of thirst are mainly located in the midbrain and the hindbrain. Specifically, the hypothalamus appears to play a key role in the regulation of thirst.

The area postrema and nucleus tractus solitarii signal to the subfornical organ and to the lateral parabrachial nucleus.^[2] The latter signaling relies on the neurotransmitter serotonin. The signal from the lateral parabrachial nucleus is relayed to the median preoptic nucleus.^[2]

The median preoptic nucleus and the subfornical organ receive signals of decreased volume ^{[ clarification needed ]} and increased osmolite concentration. Finally, the signals are received in cortex areas of the forebrain ^[2] where thirst arises. The subfornical organ and the organum vasculosum of the lamina terminalis contribute to regulating the overall bodily fluid balance by signalling to the hypothalamus to form vasopressin, which is later released by the pituitary gland.^[2]

Related Research Articles

In biology, homeostasis is the state of steady internal, physical, chemical, and social conditions maintained by living systems. This is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance, being kept within certain pre-set limits. Other variables include the pH of extracellular fluid, the concentrations of sodium, potassium, and calcium ions, as well as the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life.

The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman's capsule. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of epithelial cells with a lumen. A healthy adult has 1 to 1.5 million nephrons in each kidney. Blood is filtered as it passes through three layers: the endothelial cells of the capillary wall, its basement membrane, and between the foot processes of the podocytes of the lining of the capsule. The tubule has adjacent peritubular capillaries that run between the descending and ascending portions of the tubule. As the fluid from the capsule flows down into the tubule, it is processed by the epithelial cells lining the tubule: water is reabsorbed and substances are exchanged ; first with the interstitial fluid outside the tubules, and then into the plasma in the adjacent peritubular capillaries through the endothelial cells lining that capillary. This process regulates the volume of body fluid as well as levels of many body substances. At the end of the tubule, the remaining fluid—urine—exits: it is composed of water, metabolic waste, and toxins.

Renin, also known as an angiotensinogenase, is an aspartic protease protein and enzyme secreted by the kidneys that participates in the body's renin–angiotensin–aldosterone system (RAAS)—also known as the renin–angiotensin–aldosterone axis—that increases the volume of extracellular fluid and causes arterial vasoconstriction. Thus, it increases the body's mean arterial blood pressure.

The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure, fluid and electrolyte balance, and systemic vascular resistance.

Angiotensin is a peptide hormone that causes vasoconstriction and an increase in blood pressure. It is part of the renin–angiotensin system, which regulates blood pressure. Angiotensin also stimulates the release of aldosterone from the adrenal cortex to promote sodium retention by the kidneys.

<span class="mw-page-title-main">Aldosterone</span> Mineralocorticoid steroid hormone

Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon. It plays a central role in the homeostatic regulation of blood pressure, plasma sodium (Na⁺), and potassium (K⁺) levels. It does so primarily by acting on the mineralocorticoid receptors in the distal tubules and collecting ducts of the nephron. It influences the reabsorption of sodium and excretion of potassium (from and into the tubular fluids, respectively) of the kidney, thereby indirectly influencing water retention or loss, blood pressure, and blood volume. When dysregulated, aldosterone is pathogenic and contributes to the development and progression of cardiovascular and kidney disease. Aldosterone has exactly the opposite function of the atrial natriuretic hormone secreted by the heart.

In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 50–60% of total body weight; women and the obese typically have a lower percentage than lean men. Extracellular fluid makes up about one-third of body fluid, the remaining two-thirds is intracellular fluid within cells. The main component of the extracellular fluid is the interstitial fluid that surrounds cells.

Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by excessive unsuppressible release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source. Unsuppressed ADH causes an unrelenting increase in solute-free water being returned by the tubules of the kidney to the venous circulation.

An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure. Osmoreceptors can be found in several structures, including two of the circumventricular organs – the vascular organ of the lamina terminalis, and the subfornical organ. They contribute to osmoregulation, controlling fluid balance in the body. Osmoreceptors are also found in the kidneys where they also modulate osmolality.

<span class="mw-page-title-main">Subfornical organ</span>

The subfornical organ (SFO) is one of the circumventricular organs of the brain. Its name comes from its location on the ventral surface of the fornix near the interventricular foramina, which interconnect the lateral ventricles and the third ventricle. Like all circumventricular organs, the subfornical organ is well-vascularized, and like all circumventricular organs except the subcommissural organ, some SFO capillaries have fenestrations, which increase capillary permeability. The SFO is considered a sensory circumventricular organ because it is responsive to a wide variety of hormones and neurotransmitters, as opposed to secretory circumventricular organs, which are specialized in the release of certain substances.

Fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes in the various body fluids are kept within healthy ranges. The core principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in; for example, in humans, the output must equal the input. Euvolemia is the state of normal body fluid volume, including blood volume, interstitial fluid volume, and intracellular fluid volume; hypovolemia and hypervolemia are imbalances. Water is necessary for all life on Earth. Humans can survive for 4 to 6 weeks without food but only for a few days without water.

The vascular organ of lamina terminalis (VOLT), organum vasculosum of the lamina terminalis(OVLT), or supraoptic crest is one of the four sensory circumventricular organs of the brain, the others being the subfornical organ, the median eminence, and the area postrema in the brainstem.

Circumventricular organs (CVOs) are structures in the brain characterized by their extensive and highly permeable capillaries, unlike those in the rest of the brain where there exists a blood–brain barrier (BBB) at the capillary level. Although the term "circumventricular organs" was originally proposed in 1958 by Austrian anatomist Helmut O. Hofer concerning structures around the brain ventricular system, the penetration of blood-borne dyes into small specific CVO regions was discovered in the early 20th century. The permeable CVOs enabling rapid neurohumoral exchange include the subfornical organ (SFO), the area postrema (AP), the vascular organ of lamina terminalis, the median eminence, the pituitary neural lobe, and the pineal gland.

In renal physiology, reabsorption or tubular reabsorption is the process by which the nephron removes water and solutes from the tubular fluid (pre-urine) and returns them to the circulating blood. It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the intestines) and the body is reclaiming them from a postglomerular fluid stream that is on its way to becoming urine (that is, they will soon be lost to the urine unless they are reabsorbed from the tubule into the peritubular capillaries. This happens as a result of sodium transport from the lumen into the blood by the Na⁺/K⁺ATPase in the basolateral membrane of the epithelial cells. Thus, the glomerular filtrate becomes more concentrated, which is one of the steps in forming urine. Nephrons are divided into five segments, with different segments responsible for reabsorbing different substances. Reabsorption allows many useful solutes (primarily glucose and amino acids), salts and water that have passed through Bowman's capsule, to return to the circulation. These solutes are reabsorbed isotonically, in that the osmotic potential of the fluid leaving the proximal convoluted tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport channels driven by the sodium gradient.

<span class="mw-page-title-main">Area postrema</span> Medullary structure in the brain that controls vomiting

The area postrema, a paired structure in the medulla oblongata of the brainstem, is a circumventricular organ having permeable capillaries and sensory neurons that enable its dual role to detect circulating chemical messengers in the blood and transduce them into neural signals and networks. Its position adjacent to the bilateral nuclei of the solitary tract and role as a sensory transducer allow it to integrate blood-to-brain autonomic functions. Such roles of the area postrema include its detection of circulating hormones involved in vomiting, thirst, hunger, and blood pressure control.

In the physiology of the kidney, tubuloglomerular feedback (TGF) is a feedback system inside the kidneys. Within each nephron, information from the renal tubules is signaled to the glomerulus. Tubuloglomerular feedback is one of several mechanisms the kidney uses to regulate glomerular filtration rate (GFR). It involves the concept of purinergic signaling, in which an increased distal tubular sodium chloride concentration causes a basolateral release of adenosine from the macula densa cells. This initiates a cascade of events that ultimately brings GFR to an appropriate level.

<span class="mw-page-title-main">Median preoptic nucleus</span> Nucleus in the anterior hypothalamus

The median preoptic nucleus is located dorsal to the other three nuclei of the preoptic area of the anterior hypothalamus. The hypothalamus is located just beneath the thalamus, the main sensory relay station of the nervous system, and is considered part of the limbic system, which also includes structures such as the hippocampus and the amygdala. The hypothalamus is highly involved in maintaining homeostasis of the body, and the median preoptic nucleus is no exception, contributing to regulation of blood composition, body temperature, and non-REM sleep.

The human body and even its individual body fluids may be conceptually divided into various fluid compartments, which, although not literally anatomic compartments, do represent a real division in terms of how portions of the body's water, solutes, and suspended elements are segregated. The two main fluid compartments are the intracellular and extracellular compartments. The intracellular compartment is the space within the organism's cells; it is separated from the extracellular compartment by cell membranes.

Sodium ions are necessary in small amounts for some types of plants, but sodium as a nutrient is more generally needed in larger amounts by animals, due to their use of it for generation of nerve impulses and for maintenance of electrolyte balance and fluid balance. In animals, sodium ions are necessary for the aforementioned functions and for heart activity and certain metabolic functions. The health effects of salt reflect what happens when the body has too much or too little sodium. Characteristic concentrations of sodium in model organisms are: 10 mM in E. coli, 30 mM in budding yeast, 10 mM in mammalian cell and 100 mM in blood plasma.

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