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Water is constantly supplied to the body through food and fluid intake. It is expelled out as urine while some water is lost by perspiration through the skin. The maintenance of water balance is essential for carrying on life processes. Over accumulation of water can lead to edema, while excessive loss or very limited intake of water can lead to dehydrated conditions both of which are not a sign of a healthy individual.
Water balance is majorly maintained by the excretory system which in turn is regulated by endocrine factors.
When there is a condition of dehydration in the body, a condition of low blood volume or low blood pressure prevails. There are certain mechanoreceptors called baroreceptors in the blood vessels near the heart. They sense the level of stretch on the blood vessel wall caused by the pressure of blood flowing inside. Any low blood pressure conditions are communicated to the brain. The brain switches on the hormone mechanisms that regulate the water balance in the body.
Antidiuretic hormone (ADH) also called vasopressin or arginine vasopressin is synthesized in the hypothalamus and stored in the posterior pituitary gland before being secreted into the bloodstream. Its main role is the regulation of fluid output by allowing kidneys to reabsorb water while filtering wastes from the blood. It is a peptide hormone with nine amino acids and for this reason, it is called nonapeptide.
The ADH regulates the amount of water to be reabsorbed from the kidneys. The brain receives signals of low blood volume. Osmoreceptors in the Hypothalamus sense the high solute concentration in blood (high solute is an indication of low fluid). Both the conditions trigger a release of ADH from the posterior pituitary gland, which is its storage site.
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Functions of Antidiuretic hormone
ADH maintains homeostasis which is necessary for a healthy cellular environment to carry on the life processes.
The term diuretic relates to substances capable of increasing urine formation. ADH is antidiuretic hormone that reduces urine output.
ADH conserves the fluid volume in the body by acting on kidneys and especially at the collecting ducts of nephrons. ADH action makes collecting tubules permeable to water and reabsorption can take place. Thus it allows only small amounts of concentrated urine to be excreted out of the body.
ADH helps to maintain the homeostasis which is needed for proper cell functioning.
ADH increases blood pressure by allowing blood vessels to constrict. Since the lumen is narrowed due to constriction, blood flows with increased pressure .
Regulation of ADH
Secretion of ADH is regulated by the hypothalamus which is otherwise considered the thirst centre of the body.
The conditions of heavy blood loss because of damage to a blood vessel leading to a temporary situation of hypovolemia are communicated to the brain by the baroreceptors that are present in blood vessels near to heart.
High solute concentration in blood arises during summers when there are extremely high day temperatures and excessive perspiration. Osmoreceptors in the hypothalamus detect the condition and communicate it to the brain.
The brain sets up a network to minimize the water loss through excretion in order to maintain a healthy cellular environment. The conditions of low fluid and high solute trigger the release of ADH from posterior pituitary gland.
ADH acts on nephrons in kidneys particularly at the collecting ducts allowing water retention.
Consumption of alcohol inhibits the ADH secretion which results in more fluid loss through urine.
Hormone levels and the risk factors of ADH
ADH levels are to be maintained by the body. Excessive secretion can lead to water retention at an alarming rate which can cause edematous conditions. Low secretion can lead to dehydration which in turn has negative effects on cellular functioning. The ADH levels are examined by carrying blood tests for samples obtained from veins.
Normal level of ADH is 1- 5 pg/mL or 0.9 - 4.6 pmol/L.
Possible risks associated with ADH levels can be excessive secretion or low secretion.
Few individuals may experience enough secretion of ADH but some possible resistance to its effects may hinder its action simulating the effects of low secretion .
Disorders of ADH
Syndrome of inappropriate ADH (SIADH) −
It arises when the body secretes excess ADH which results in more water retention by the kidneys. It affects water retention while the salts are excreted on a normal basis. This results in a cellular situation with a low salt concentration in high fluid content which in other words is called hyponatraemia (low sodium in the bloodstream).
It can be caused due to some kind of tumour that secrete ADH (lung cell tumours sometimes). This condition may arise as drug side effects also.
Central Diabetes insipidus
When the levels of ADH are low than the normal range due to decreased secretion, the collecting ducts are impermeable to water and water retention cannot take place. Therefore diluted urine with high fluid content is excreted out. This condition is termed central diabetes insipidus.
This can be due to damage to the hypothalamus or posterior pituitary gland caused due to brain trauma or surgery, infection, or inflammation in the hypothalamus.
Nephrogenic Diabetes Insipidus
Nephrogenic diabetes insipidus arises when kidneys are resistant to ADH either because of dysfunction of nephron receptors or signal mediators. The ADH secretion by the posterior pituitary gland is normal. However, this arises because the available ADH cannot bind to the target site and perform its biochemical function.
Patients diagnosed with Diabetes insipidus produce pale urine in large quantities and experience extreme thirst (polydipsia) relative to normal healthy individuals. Loss of body fluids in form of urine leads to hypernatremia (high sodium in the bloodstream) and hyperosmolarity.
The antidiuretic hormone (ADH) also called vasopressin or arginine vasopressin is secreted by the hypothalamus and stored in the posterior pituitary gland before being released into the blood. ADH is a nonapeptide with the primary function of water retention by kidneys. It is essential for the maintenance of homeostasis in the body. ADH allows water reabsorption from collecting ducts of the nephrons while they filter wastes from the blood. High levels of ADH in the blood called the syndrome of inappropriate ADH (SIADH) can lead to excessive water retention by the body producing highly concentrated urine. This results in edema. Low ADH can lead to Diabetes insipidus where there is excessive loss of water through urine and resulting in polydipsia and hypernatremia.
Q1. Can synthetic ADH treat Diabetes insipidus?
Ans. Patients suffering from Central Diabetes insipidus are injected with desmopressin (synthetic vasopressin). It acts on nephrons and allows water retention.
Q2. Is diabetes insipidus the same as diabetes mellitus?
Ans. Diabetes insipidus is the polyuria and polydipsia condition due to low ADH levels. Diabetes mellitus is a condition of high blood glucose due to low insulin secretion from the pancreas.
Q3. When is it recommended to test ADH levels?
Ans. ADH levels are tested when there are any of the following symptoms −.
Edema in body parts.
Excessive quantities of urine.
Low sodium levels in the blood.
Q4. How is ADH related to plasma osmolarity?
Ans. Plasma Osmolarity is the concentration of solutes in the blood. Hypoosmolar plasma has low solute in high fluid and hyperosmolar plasma has high solute in the low fluid. ADH regulates the fluid content. Imbalances in ADH lead to either high fluid or low fluid in the body causing hypoosmolar or hyperosmolar plasma.
Q5. How is alcohol intake related to ADH?
Ans. Alcohol consumption inhibits the secretion of ADH. Inhibiting fluid retention leads to increased urine output.