Fluid balance

Fluid balance—or fluid homeostasis—is a biological concept that expresses the relationship between a person’s fluid input and output. When fluid balance is optimized, input and output are equal, and the body can maintain stable levels of water, oxygen, nutrients, and electrolytes. Proper fluid balance also helps the body perform critical metabolic processes while preventing the potentially serious complications of both dehydration and overhydration—hyperhydration.

People maintain their fluid balance by consuming as much fluid as their bodies use each day. Numerous environmental factors, such as humidity, atmospheric pressure, and radiation levels, can affect fluid output in humans; therefore, exact fluid input requirements often vary among individuals. Furthermore, certain activities and health conditions expel important minerals called electrolytes at higher levels, and in such cases, mineral-rich fluids are required to restore the correct fluid balance. In healthcare settings, medical professionals use documentation tools known as fluid balance charts to track and analyze a patient’s fluid input and output levels over specific periods to identify whether the patient needs any restorative or regulatory interventions.

Background

At birth and during early childhood, approximately 70 percent of the human body’s weight is comprised of fluids. By adulthood, fluid accounts for 60 percent of a typical male’s body weight and 52 to 55 percent of a typical female’s body weight. Fluid levels must be carefully maintained to ensure that they do not rise above or fall below these baseline norms, as both dehydration and hyperhydration can have serious health consequences.

Water is the primary fluid required by the human body. The body obtains most of its water through the digestive tract, absorbing it from the food a person eats and the fluids a person drinks. It also collects small amounts of water released by the metabolic processing of certain nutrients. Under normal circumstances, the body naturally regulates its own fluid balance by absorbing what it needs from food and drink intake and excreting the excess as urine. However, illness symptoms, such as diarrhea and vomiting, can eject large quantities of fluid from the body, leaving the individual unable to replace them in adequate quantities. Excessive sweating, certain disorders of the liver and/or kidney(s), and the side effects of some medications can also disrupt the body’s normal water balance.

Electrolytes, which are mineral salts that generate an electrical charge when dissolved in water in the body, also have an important function in fluid balance. Present in blood, urine, bodily tissues, and many other bodily systems, electrolytes help the body regulate water and pH levels, deliver nutrients to cells, remove waste byproducts from cells, and support the proper function of major organs. The main classes of electrolytes are calcium, chloride, magnesium, phosphate, potassium, and sodium. Their levels usually reflect the body’s overall fluid balance, becoming too high during periods of hyperhydration and too low during periods of dehydration.

Overview

Medical professionals generally advise adults to drink about 1.5 to 2 quarts (2 liters) of fluids per day to optimize their fluid balance. This prevents dehydration and overhydration, and also helps maintain proper electrolyte levels. Doctors also note that taking in too many fluids is generally preferable to not taking in enough—the body can readily rid itself of excess fluid in urine, but has a far more difficult time preserving proper fluid balance during periods of low input.

In most cases, people can maintain an appropriate fluid balance by drinking water and eating foods that promote water retention in the digestive tract. However, this technically only impacts the body’s water balance, an important component of fluid balance, but not the sole factor affecting it. Electrolytes must also be present in appropriate proportions and can become depleted by heavy exercise, diarrhea, vomiting, and through certain metabolic processes. People with imbalanced electrolyte levels must take in the missing compounds from mineral-rich foods and fluids, such as coconut water, milk, watermelon water, fruit juices, sports drinks, electrolyte-infused water, or water-soluble electrolyte tablets, among others.

At a more technical level, healthcare professionals consider three components when assessing fluid balance in clinical settings: the relationship between total body water and total effective solute, the control and measurement of sodium balance in relation to extracellular volume, and extracellular volume changes in patients with severe illness. The first concept, total body water versus total effective solute, is expressed in terms of cell tonicity, which governs whether cells expand or contract in response to osmotic pressure. When severely disturbed, cell tonicity can change the volume of cells, potentially impeding brain cell function and thus threatening a person’s health and survival. Sodium balance and extracellular volume, the second concept, applies in certain cases involving certain acute and chronic life-threatening medical conditions and illnesses. It impacts critical bodily functions such as cardiac function and blood vessel capacity, and in such situations, fluid balance interventions aim to prevent negative health outcomes associated with imbalanced relationships between sodium levels and extracellular volume. Extracellular volume changes, the third concept, are often a consequence of sodium and extracellular volume imbalances. Acute and chronic illnesses affect extracellular volumes differently, and some illnesses demand the application of different measurement techniques. Researchers have thus established it as a separate concept to reflect growing scientific understanding of how various illnesses and conditions impact extracellular volume and its associated bodily processes and functions.

In clinical settings, healthcare professionals track a patient’s fluid balance with a form of documentation known as a fluid balance chart. These charts track the patient’s fluid intake and output over a specified period—typically 24 hours. Caregivers monitor all fluid input and output in exact levels across the entire time period, recording the precise amount and type of fluid given to a patient for intake, along with the specific amount of fluid excreted in urine, vomit, or bowel movements. These charts yield three possible results: positive fluid balance, proper fluid balance, and negative fluid balance. Positive fluid balance (hypervolaemia) means that the patient’s fluid input is higher than their fluid output. Proper fluid balance means that input and output are equal. Negative fluid balance (hypovolaemia) signifies that the patient is excreting more fluid than they are inputting. Hypervolaemia and hypovolaemia both indicate that interventions may be necessary to restore the correct balance.

Staying hydrated has many health benefits, including proper kidney function, cardiovascular efficiency, and healthy skin. Maintaining proper fluid levels can help prevent illness or enhance recovery from sicknesses. Water should always be the drink of choice, but may be supplemented with beverages such as naturally flavored sparkling water and milk. Sugared drinks should generally be avoided. Many beverages that advertise themselves as sports drinks contain high amounts of sugar. Water is generally enough to keep an athlete hydrated. If the person is exercising for more than an hour, an electrolyte tablet or packet can help maintain homeostasis without added, unnecessary sugars. Fruits and vegetables are also very good for maintaining proper fluid levels.

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