The effects of dehydration go beyond feeling thirsty. It begins when the body loses more water than it takes in, and may involve the loss of sodium and other electrolytes or an imbalance in them, particularly in cases of heavy sweating, vomiting and diarrhoea.
The progression of fluid loss and delayed thirst
Fluid loss can progress from headache, fatigue and poor concentration to impaired consciousness, reduced organ perfusion and hypovolaemic shock in severe cases. Thirst and dehydration are physiologically different: thirst is a signal that prompts a person to drink, whereas dehydration means that fluid loss has already begun.
This signal is not always a reliable early warning, particularly among older people, whose thirst response may be weakened. This increases the likelihood that fluid loss will continue before they recognise the need to replace it. Water plays an essential role beyond quenching thirst; it is the medium in which many vital processes take place inside the body.
Blood, lymph, the fluid surrounding cells and the fluids within them all depend on a water-based environment to transport nutrients and oxygen, remove waste and complete the biochemical reactions that maintain the body's balance and allow organs to continue functioning. The brain needs water to carry out its metabolic processes, while the heart depends on an adequate blood volume to keep pumping.
The lungs need moist surfaces for gas exchange, while the kidneys use water to regulate electrolytes and remove waste. The digestive system relies on it for digestion and absorption. Muscles and nerves also need a balance of water and electrolytes to generate electrical signals and enable contraction and movement.
A water molecule consists of two hydrogen atoms and one oxygen atom, and despite its simple structure, it is essential for life. The system that detects the need for water begins before any drink is consumed. When blood osmolality rises, meaning that the concentration of dissolved substances increases, specialised receptors detect the change, while other receptors monitor a fall in blood volume or a drop in circulatory pressure.
How the brain and kidneys respond to water loss
These signals travel to areas of the brain, including the hypothalamus, which plays a central role in regulating water balance, generating the desire to drink. At the same time, secretion increases of the antidiuretic hormone known as «vasopressin». This prompts the kidneys to reabsorb more water, reducing fluid loss through urine, which becomes more concentrated.
The body therefore responds to the deficit along two paths at the same time: it prompts a person to seek water and tries to retain the fluids already present until the required amount is obtained. After water is consumed, it begins to be absorbed through the digestive system and is then distributed between the blood, the fluid surrounding cells and the fluid inside cells, in a precise balance governed by the movement of water and dissolved substances across membranes.
Electrolyte concentrations are not equal on both sides of the cell membrane. Sodium is found in a higher concentration outside the cell, while potassium is more concentrated inside it. These differences are essential for regulating cell volume and maintaining cellular electrical functions and the ability to communicate.
One of the main mechanisms responsible for maintaining this difference is the «sodium-potassium pump», which uses cellular energy. In each cycle, the pump moves three sodium ions out of the cell in exchange for bringing two potassium ions in, consuming one molecule of cellular energy. This mechanism maintains the cells' osmotic balance and electrical potential.
Signs of dehydration and its effects on vital organs
When severe dehydration occurs alongside major electrolyte disturbances, cramps, weakness and impaired consciousness may develop, while the heartbeat and vital organ functions are affected. Early signs of dehydration include thirst, dry mouth, headache, fatigue and poor concentration, along with reduced urine output or urine becoming darker than usual.
As fluid loss increases, a person may experience dizziness, weakness and a rapid heartbeat. The body continues to reduce urine production in an attempt to preserve circulatory volume and ensure blood reaches vital organs. Severe dehydration may appear as confusion, fainting or extreme weakness, with a substantial reduction in urine or no urine at all. It may also be accompanied by a rapid pulse and breathing, and cold extremities.
These symptoms require urgent medical assessment, as they may indicate that fluid loss has exceeded the body's ability to compensate and is affecting the circulation and organs. Heat increases the risk of dehydration by increasing water loss through sweating, and losses may also rise in some dry environments and at high altitudes.
This highlights the importance of paying attention to hydration during heatwaves, while travelling and when working in harsh climatic conditions. The risk also increases with prolonged physical activity, particularly when exertion is accompanied by high temperatures. During exercise, the body loses not only water but also sodium through sweat, and the amount lost rises the longer the activity continues and the hotter it becomes.
People who sweat for hours may need to replace both water and electrolytes. Common medical causes of fluid loss include vomiting, diarrhoea and fever. Some medicines, particularly diuretics, may also increase the amount of water excreted by the body. Coffee, tea and other caffeinated drinks contribute to total daily fluid intake when consumed in moderation, and do not usually cause dehydration in people who are accustomed to them.
Those most at risk and common causes of fluid loss today
However, drinking excessive amounts of highly sugary or caffeinated beverages is not an ideal way to hydrate, and they should not be relied on permanently as a substitute for water. Infants and children face a particular risk because they can lose water and electrolytes rapidly during vomiting and diarrhoea, and dehydration can develop within a short time.
The risk is also higher among older people because of a weaker sense of thirst, as well as the effects of chronic diseases, limited mobility and some medicines on their ability to access or retain fluids. High blood sugar in some people with chronic diseases may increase water loss through urine.
By contrast, people with heart or kidney disease may need to restrict their fluid intake, making the determination of daily needs an individual matter rather than a universal rule. No single amount of water is suitable for everyone regardless of their health and daily circumstances. In the short term, fluid deficiency is associated with headache, fatigue and reduced attention and mental performance, and its effects may become more pronounced in heat and during physical exertion.
In the urinary system, a lack of water increases urine concentration, while consuming an adequate amount of fluid is one of the main ways to reduce the risk of some types of kidney stones. Fluid deficiency may also contribute to constipation in some people. In serious cases, severe fluid loss can reduce blood volume and pressure, followed by reduced blood flow to the organs and ultimately hypovolaemic shock.
An acute disturbance in the level of sodium or other electrolytes can cause neurological symptoms and seizures, showing that the effects of dehydration are not confined to thirst or dry mouth. Clear fluid deficiency may be accompanied by dry skin, but drinking more water when a person is already meeting their needs does not by itself guarantee more hydrated skin.
Skin is affected by multiple factors, including the integrity of the skin barrier, topical care, nutrition and sun protection. Evidence that drinking additional water improves skin hydration also remains limited. The rule of drinking eight glasses a day is not a medical law that applies to everyone, because water needs vary according to age, body size, diet, activity level, climate and health status.
But this range is not a general formula for determining how much every person should drink. Heat, sweating, illness, pregnancy and some heart and kidney diseases can substantially change fluid needs. For most healthy people, responding to thirst, monitoring urine colour and increasing fluid intake in hot conditions and during exertion help maintain hydration.
Water-rich foods, such as cucumber, watermelon, melon, citrus fruits and other fruit and vegetables, contribute to the body's total water intake and also provide fibre and a range of nutrients. During diarrhoea or vomiting, water alone may not be enough because the body loses fluids and electrolytes at the same time.
Standard oral rehydration solutions are the best option when needed. Preparing solutions at home requires precise adherence to the quantities, because adding too much salt or sugar can cause harm, particularly in children. As exposure to heatwaves increases, maintaining hydration becomes more important for outdoor workers, older people and children.
Increased sweating raises the loss of water and electrolytes and increases the strain on the heart and kidneys. The issue is not limited to how much an individual drinks, because access to safe and sufficient drinking water is a cornerstone of public health. Water shortages or contamination threaten people's ability to drink safely, maintain hygiene and prevent disease.
The importance of water therefore extends from regulating balance within cells, the blood and the circulation to ensuring continuous access to a safe source, protecting people from dehydration and supporting vital functions and public health.
