9.4: Overview of Fluid And Electrolyte Balance

Water Distribution and Composition

As previously discussed, water is an important vehicle helps move a diverse array of molecules throughout the body. In the human body, water is distributed into two compartments: inside cells, called intracellular fluid or ICF, and outside cells, called extracellular fluid or ECF. Extracellular fluid includes both the fluid component of the blood, known as plasma, and the interstitial fluid or IF that surrounds all cells not in the blood (Figure \(\PageIndex{1}\)).

Although water makes up the largest percentage of body volume, it is not actually pure water but rather a mixture of dissolved substances, collectively called solutes, that are critical for osmoregulation and water balance. These solutes include electrolytes, substances that dissociate into charged ions when dissolved in water. For example, table salt or sodium chloride contains electrolytes in the form of sodium (Na⁺) and chloride (Cl⁻) in water. In extracellular fluid, sodium is the major positively-charged electrolyte, or cation, and chloride (Cl⁻) is the major negatively-charged electrolyte, or anion. Inside cells in the intracellular fluid, potassium (K⁺) is the major cation. Together, these electrolytes are involved in many body functions, including water balance and acid-base balance, and they assist in transmitting electrical impulses along cell membranes in nerves and muscles.

Figure \(\PageIndex{1}\): Fluid compartments in the human body. The intracellular fluid (ICF) is the fluid within cells. The extracellular fluid (ECF) includes both the blood plasma and the interstitial fluid (IF) between the cells. Source: “Fluid Compartments in the Human Body” from Anatomy & Physiology, 2e by OpenStax is licensed under CC BY 4.0.

Fluid and Electrolyte Balance

One of the essential homeostatic functions of the body is to maintain fluid and electrolyte balance within cells and their surrounding environment. Cell membranes are selectively permeable: Water can move freely through the cell membrane, while other substances, such as electrolytes, require special transport proteins, channels, and often energy. The movement of water between the intracellular and extracellular fluid happens by osmosis, which is simply the movement of water through a selectively permeable membrane from an area where solutes are less concentrated to an area where solutes are more concentrated (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Osmosis is the diffusion of water through a semipermeable membrane towards higher solute concentration. If a membrane is permeable to water but not a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is more concentrated with solutes; therefore, water diffuses to the right side of the beaker to equalize its concentration. Source: “Osmosis” by OpenStax is licensed under CC BY 4.0.

To maintain water and electrolyte balance, cells control the movement of electrolytes across their membranes, and water follows the electrolytes by osmosis. The health of the cell depends on proper fluid and electrolyte balance. If the body’s fluid and electrolyte levels change too rapidly, cells can struggle to correct the imbalance quickly enough. For example, consider a person exercising strenuously, losing water and electrolytes in the form of sweat, and drinking excessive amounts of water. The excess water dilutes the sodium in the blood, leading to hyponatremia, or low blood sodium concentrations. Sodium levels within the cells are now more concentrated, leading water to enter the cells by osmosis. As a result, the cells swell with water and can burst if the imbalance is severe and prolonged.

In contrast, the opposite situation can occur in a person exercising strenuously for a long duration with inadequate fluid intake. This can lead to dehydration and hypernatremia, or elevated blood sodium levels. The high concentration of sodium in the extracellular fluid causes water to leave cells by osmosis, making them shrink (Figure \(\PageIndex{3}\)). This scenario can also occur anytime a person is dehydrated because of significant fluid loss, such as from diarrhea and/or vomiting caused by illness.

Recall that the thirst response is triggered when solutes like sodium become too concentrated in the blood during dehydration. Sensory receptors in the thirst center in the hypothalamus monitor the concentration of solutes in the blood. If blood solutes (like sodium) increase above ideal levels, the hypothalamus transmits signals that result in a conscious awareness of thirst. The hypothalamus also communicates to the kidneys to decrease water output through the urine.

Figure \(\PageIndex{3}\): Effect of fluid imbalance on cells. With dehydration, the concentration of electrolytes becomes greater outside of cells, leading to water leaving cells and making them shrink. In fluid balance, electrolyte concentrations are balanced inside and outside cells, so water is in balance, too. During overhydration, electrolyte concentrations are low outside the cell relative to inside the cell (as in hyponatremia), so water moves into the cells, making them swell. Source: “Fluid Balance Effects on Cells” by Tamberly Powell is licensed under CC BY 4.0; edited from “Osmotic pressure on blood cells diagram” by LadyofHats is in the Public Domain.

Attributions

9.3: Vitamins and Minerals Involved In Fluid And Electrolyte Balance is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Alice Callahan, Heather Leonard, & Tamberly Powell (OpenOregon) .