Introduction

Definition of dehydration

Dehydration is the process of losing body water and electrolytes like sodium, potassium, chloride, and bicarbonate through diarrhea, vomiting, sweating, or urination, without replacement.¹

Dehydration is the cause of many hospital admissions worldwide, and about 17 to 28% of adults suffer from dehydration in the United States.² Clinically, dehydration is classified from mild to severe depending on the quantity of fluid loss and the manifested symptoms.¹

• Mild dehydration: characterised by a history of acute water loss but no physical signs of dehydration. This can be corrected with more consumption of water⁵
• Moderate dehydration: in addition to a history of acute water loss, there is increased thirst, irritability, or restlessness, eyes may become sunken, and skin pinch fold regresses slowly. This may necessitate hospitalisation and correction of dehydration done with oral rehydration solution and zinc tablets⁵
• Severe dehydration: In addition to the history of acute fluid loss, severe dehydration presents with a lethargic or unconscious patient who may not be able to drink water orally, sunken eyes, and very slow regression of skin pinch fold. This is a medical emergency and necessitates immediate hospital admission and treatment with intravenous fluids⁵

Overview of thermoregulation

Thermoregulation is the ability of the body to regulate its temperature within appropriate parameters, which is within one or two degrees from 37^oC, even if the surrounding temperature is different. This is mediated through sweating and metabolic activities.³ The skin, through its sensory nerve activities to feel heat or cold, also plays an important role in thermoregulation.⁴ 

Understanding dehydration

Causes of dehydration

Water can be lost from the body through the gastrointestinal tract, the kidneys, the skin, and the lungs. This loss can be with or without electrolytes, notably sodium loss.² There are three forms of dehydration:

• Isotonic dehydration: loss of water occurs at the same time as the loss of sodium
• Hypertonic dehydration: the quantity of sodium lost is less than the loss of water. In this case, sodium is in the blood system, and osmolality is always high
• Hypotonic dehydration: loss of sodium exceeds loss of water. Osmolality and blood sodium are usually low² 

Depending on the body system through which the loss took place, the causes of dehydration include:

• Loss of water through the skin: excess heat, sweating during exercise, burns, and skin diseases
• Loss of water through the gastrointestinal tract: diarrhoea, vomiting, laxatives, and fistulas
• Loss of water through the kidney: the use of diuretics, renal disease whether acute or chronic, tubular disease, Addison's disease, hypoaldosteronism, hyperglycemia, etc
• Other losses could be intra-abdominal with ascites and peritonitis²

Failure to replace water loss can lead to an altered mental state, unconsciousness, and an altered thirst mechanism. ² 

The physiological impact of dehydration

Water constitutes about 65% of the human body. Two-thirds of this water is within the body cells, while the other one-third is external to the body cells, distributed between the extracellular space and the blood system. Water homeostasis refers to the method by which the body maintains the normal volume of body fluids. Water shortage is felt by water receptors in the brain, which then stimulates

• The hypothalamic thirst center increases thirst, leading to increased consumption of water
• The posterior pituitary gland produces the antidiuretic hormone, which causes increased water reabsorption by the kidneys²

Dehydration also causes hypotension (low blood pressure), which stimulates the secretion of renin by the kidneys. Renin then converts angiotensin I to angiotensin II, which stimulates the adrenal gland to increase the release of aldosterone. Aldosterone then increases sodium and water absorption by the kidney.²

Heat perception and its connection to dehydration

Mechanisms of heat perception

Heat perception is carried out by the skin through the somatosensory neurons.⁶ The somatosensory neurons consist of a dorsal root ganglion (DRG) that transmits information about the external environment to the central nervous system. The DRG neurons are composed of:

• Peripheral axons: which are found in the skin, muscle, or bone, and 
• Central axons: which are found in the spinal cord, visceral, and brainstem where they both relay information to the preoptic area of the anterior hypothalamus⁷ 

External heat is felt by the sensory nerves of the skin, muscles, or bone by the peripheral axon, and internal heat is felt by the central axons of the spine, brainstem, and viscera. When your body feels heat from the outside, like from the sun or a hot surface, nerves in your skin, muscles, or bones pick it up. When the heat comes from inside your body, like from a fever, deeper nerves in your spine, brainstem, or internal organs detect it. These nerves send signals to a part of your brain called the hypothalamus, which acts like the body’s thermostat. The hypothalamus then decides what to do: if you are too cold, it works to warm you up by making more heat and keeping it in; if you are too hot, it helps you cool down by releasing heat.⁷,⁸

Effects of dehydration on heat perception

When the central and peripheral thermoreceptors sense a change in body temperature, they transmit the message to the anterior hypothalamus, which, in turn, stimulates an increase or decrease in body temperature to regulate the temperature change.⁸

Dehydration reduces the sweating rate and the flow of blood in the skin, thus increasing heat storage. Hypertonicity and low blood volume contribute to decreased heat loss and thus increase the storage of heat.^8,9

Sweating and its role in thermoregulation

Sweating as a primary cooling mechanism

When faced with a hot environment or increased metabolism, sweating becomes a means of survival. Sweating, which is evaporative heat loss, is a primary means of sending out heat when the temperature of the environment is higher than the temperature of the skin. Exercise also increases the temperature of the skin and internal body, in turn increasing sweat rate and blood flow of the skin.¹⁰ 

Impact of dehydration on sweating

When a dehydrated individual is in a heated environment, thermoregulation is partially suppressed to conserve body fluid and circulation.¹¹ Sweating without replacement causes increased loss of water and electrolytes. Sweat composition, however, varies as dehydration will cause an increased quantity of sodium and chloride in sweat related to elevated extracellular sodium.¹² 

Feedback loop: dehydration, heat perception, and sweating

Interrelated effects

Water and electrolyte balances are essential for thermoregulation and performing exercises in heat. During this heated exercise,

• The body heats up, causing an increased heat perception, which activates the body's thermoregulatory system to respond by sending out heat through sweating⁸
• Sweating causes the loss of water and electrolytes. These losses, without replacement, cause dehydration¹²
• Dehydration, through a series of mechanisms, causes increased thirst and water retention by the kidneys to regulate body fluid volume.² If no corrective measures are taken, dehydration causes the sweat composition to change to more sodium chloride, worsening the dehydration¹²

Potential outcomes

The potential outcomes of the above loop include: 

• Increased irritability and possibility of unconsciousness due to electrolyte imbalance and severe dehydration¹,² 
• Decreased physical performance due to dehydration²,¹³

Practical considerations

Preventing dehydration in heat-prone environments

Prevention of dehydration is primarily done by drinking a lot of water. The quantity of water needed by a healthy individual per day is:

• 2 to 4 l/day in a cool climate, and 
• 8–16 l/day in very hot climates⁹

Recognising signs of dehydration

Signs of dehydration in an individual with a history of fluid loss through vomiting, diarrhea, etc., include:

• Sunken eyes
• Slow regression of skin pinch fold
• Increased thirst manifesting in the manner in which the individual drinks water
• Dry skin
• Irritable or unconscious⁵

Implications for specific groups

The following specific groups are severely affected by dehydration and should be taken into consideration:

• Children who are not able to describe their feelings of thirst,
• Athletes should rehydrate often during exercises
• Burn patients should be rehydrated accordingly due to their excess loss of fluid
• People living in hot climatic zones should rehydrate according to their daily needs¹³

Summary

Dehydration is a medical emergency that needs immediate treatment. Daily water needs vary by the climate of the region. Thermoregulation is how the body adapts to a very hot or cold region. Perception of heat stimulates perspiration, and in the absence of replenishment, may lead to dehydration. Athletes, children, individuals in hot climate zones, and all people have to rehydrate every day to enjoy a healthy life.