In physiology, dehydration is a deficit of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake due to exercise, disease, or high environmental temperature. Mild dehydration can be caused by immersion diuresis, which may increase risk of decompression sickness in divers. Most people can tolerate a 3-4% decrease in total body water without difficulty or adverse health effects. A 5-8 % decrease can cause dizziness. Loss of over ten percent of total body water can cause physical and mental deterioration, accompanied by severe thirst. Death occurs at twenty-five percent of the body water. Mild dehydration is characterized by thirst and general discomfort and is resolved with oral rehydration. Dehydration is distinct from hypovolemia; the hallmarks of dehydration include thirst and neurological changes such as headaches, general discomfort, loss of appetite, decreased urine volume, unexplained tiredness, purple fingernails and seizures.

The symptoms of dehydration become severe with greater total body water loss. A body water loss of 1-2%, considered mild dehydration, is shown to impair cognitive performance. In people over age 50, the body's thirst sensation continues diminishing with age. Many senior citizens suffer symptoms of dehydration. Dehydration contributes to morbidity in the elderly population during conditions that promote insensible free water losses, such as hot weather. A Cochrane review on this subject defined water-loss dehydration as "people with serum osmolality of 295 mOsm/kg or more" and found that the main symptom in the elderly was fatigue. Risk factors for dehydration include but are not limited to: exerting oneself in hot and humid weather, habitation at high altitudes, endurance athletics, elderly adults, infants and people living with chronic illnesses. Dehydration can come as a side effect from many different types of drugs and medications. In the elderly, blunted response to thirst and/or inadequate ability to access free water in the face of excess free water losses seem to be the main causes of dehydration.

Excess free water or hypotonic water can leave the body in two ways – sensible loss such as osmotic diuresis, sweating and diarrhea, insensible water loss, occurring through the skin and respiratory tract. In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body; these occur through either impaired thirst/water access or sodium excess. Dehydration occurs when water intake is not enough to replace free water lost due to normal physiologic processes, including breathing and perspiration, or other causes, including diarrhea and vomiting. Dehydration can be life-threatening when severe and lead to seizures or respiratory arrest, carries the risk of osmotic cerebral edema if rehydration is overly rapid; the term "dehydration" itself has sometimes been used incorrectly as a proxy for the separate, related condition hypovolemia, which refers to a decrease in volume of blood plasma. The two are regulated through independent mechanisms in humans.

For routine activities, thirst is an adequate guide to maintain proper hydration. Minimum water intake will vary individually depending on weight, environment and genetics. With exercise, exposure to hot environments, or a decreased thirst response, additional water may be required. In athletes in competition drinking to thirst optimizes performance and safety, despite weight loss, as of 2010, there was no scientific study showing that it is beneficial to stay ahead of thirst and maintain weight during exercise. In warm or humid weather or during heavy exertion, water loss can increase markedly, because humans have a large and variable capacity for the active secretion of sweat. Whole-body sweat losses in men can exceed 2 L/h during competitive sport, with rates of 3–4 L/h observed during short-duration, high-intensity exercise in the heat; when such large amounts of water are being lost through perspiration, electrolytes sodium, are being lost. In most athletes and sweating for 4–5 hours with a sweat sodium concentration of less than 50 mmol/L, the total sodium lost is less than 10% of total body stores.

These losses appear to be well tolerated by most people. The inclusion of some sodium in fluid replacement drinks has some theoretical benefits and poses little or no risk, so long as these fluids are hypotonic; the treatment for minor dehydration, considered the most effective is drinking water and stopping fluid loss. Plain water restores only the volume of the blood plasma, inhibiting the thirst mechanism before solute levels can be replenished. Solid foods can contribute to fluid loss from diarrhea. Urine concentration and frequency will customarily return to normal as dehydration resolves. In some cases, correction of a dehydrated state is accomplished by the replenishment of necessary water and electrolytes; as oral rehydration is less painful, non invasive and easier to provide, it is the treatment of choice for mild dehydration. Solutions used for intravenous rehydration must be isotonic or hypertoni

P. V. Kurian

P. V. Kurian was an Indian revolutionary and social activist under the British Raj. An eminent freedom fighter, Socialist thinker and activist and a close associate of Dr. Ram Manohar Lohia, Kurian came to the freedom movement by joining Congress in 1938, he became the joint secretary of the Travancore branch of All India Forward Bloc, founded by Subhas Chandra Bose. After Bose left India, Mr. Kurian joined Congress Socialist Party and became associated with Dr. Ram Manohar Lohia. After Lohia's death in 1967, Mr. Kurian founded Lohia Study Centre at Thiruvananthapuram, renamed as Lohia Vichāra Vedi, he used to write in Socialist journals in Malayalam such as Swathanthra Bhāratham, Kerala Nādu, Pōrāttam, Māttam, Samājavādi. A major contribution of Mr. Kurian is his voluminous book on Lohia: Dr. Ram Manohar Lohia Enna Sārvadeśeeya Viplavakari, pages 2210


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