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 is able to 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, meaning that 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 climb too high or fall too low beyond these baseline norms, as both dehydration and hyperhydration can have serious health impacts.

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 preserve the correct fluid balance by primarily or exclusively drinking water and eating foods that release water into the digestive tract. However, this technically only impacts the body’s water balance, which is 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 fluid sources, such as coconut water, milk, watermelon water, fruit juices, sports drinks, electrolyte-infused water, or water-soluble electrolyte tablets, among others.

On a more technical level, healthcare professionals consider three component concepts when assessing fluid balance in clinical settings: the relationship between total body water and total effective solute, the control and measurement of sodium balance as it relates to extracellular volume, and extracellular volume changes in patients suffering from severe illness. The first concept, total body water vs. total effective solute, is expressed in terms of cell tonicity, which relates to cells expanding or contracting in size 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, is 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 input and output levels over a specific period of time (usually 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 by such outputs as 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 his or her 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 he or she is inputting. Hypervolaemia and hypovolaemia both indicate that interventions may be necessary to restore the correct balance.

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