11.5: Overview of Fluid and Electrolyte Balance

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Water is made up of 2 hydrogen atoms and 1 oxygen atom. A human body is made up of mostly water. An adult consists of about 37 to 42 liters of water, or about eighty pounds. Fortunately, humans have compartmentalized tissues; otherwise we might just look like a water balloon!

An illustration of a water molecule: 1 oxygen molecule in red, 2 hydrogen molecules in white — Figure \(\PageIndex{1}\): The Water Molecule. “Water Molecule” by Chris Martin / Public Domain

Water Distribution and Composition

In the human body, water is distributed into two compartments: inside cells, called intracellular fluid (ICF), and outside cells, called extracellular fluid (ECF). Extracellular fluid includes both the fluid component of the blood (called plasma) and the interstitial fluid(IF) that surrounds all cells not in the blood.

This diagram shows a small blood vessel surrounded by several body cells. The fluid between the body cells is the interstitial fluid (IF), which is a type of extracellular fluid (ECF). The fluid in the blood vessel is also an example of extracellular fluid. The fluid in the cytoplasm of each body cell is intracellular fluid, or ICF. — 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.* “Fluid Compartments in the Human Body” by J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix, Anatomy and Physiology, OpenStax, licensed under CC BY 4.0

Although water makes up the largest percentage of body volume, it is not actually pure water, but rather a mixture of dissolved substances (solutes) that are critical to life. These solutes include electrolytes, substances that dissociate into charged ions when dissolved in water. For example, sodium chloride (the chemical name for table salt) dissociates into 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). Potassium (K⁺) is the major cation inside cells. Together, these electrolytes are involved in many body functions, including water balance, acid-base balance, and assisting in the transmission of electrical impulses along cell membranes in nerves and muscles.

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.

This image shows two different beakers that both contain a semipermeable membrane (illustrated by a dotted red line), water (illustrated in pink) and solutes (illustrated in purple). The beaker on the left has equal water levels on both sides of the semipermeable membrane, but a higher concentration of solutes to the right of the membrane. The beaker on the right has a higher water level to the right of the membrane, and therefore an equal concentration of solutes on both sides of the membrane. — 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. Figure 8.7. “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. 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.

When a person becomes dehydrated, and solutes like sodium become too concentrated in the blood, the thirst response is triggered. Sensory receptors in the thirst center in the hypothalamus monitor the concentration of solutes of 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.

Three different hydration states are shown with the cell. 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 equal inside and outside the cells, so water is in balance, too. During overhydration, electrolyte concentrations are low outside the cell relative to inside the cell (like in the situation of hyponatremia), so water moves into the cells, making them swell. — 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 in balance inside and outside of cells, so water is in balance too. During overhydration, electrolyte concentrations are low outside the cell relative to inside the cell (like in the situation of hyponatremia), so water moves into the cells, making them swell. “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.

The cell is able to control the movement of the two major cations, sodium and potassium, with a sodium-potassium pump (Na⁺/K⁺ pump). This pump transports sodium out of cells while moving potassium into cells.

This diagram shows many sodium potassium pumps embedded in the membrane. Potassium is pumped into the cytoplasm and sodium is pumped out of the cytoplasm. — Figure \(\PageIndex{4}\): The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell. “Sodium-Potassium Pump” by by J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix, Anatomy and Physiology, OpenStax, licensed under CC BY 4.0

Video \(\PageIndex{1}\): “Sodium Potassium Pump,” by McGraw Hill Animations, YouTube (June 4, 2017), 2:02 minutes.

The Na⁺/K⁺ pump is an important ion pump found in the membranes of many types of cells and is particularly abundant in nerve cells. When a nerve cell is stimulated (e.g., the touch of a hand), there is an influx of sodium ions into the nerve cell. Similar to how a current moves along a wire, a sodium current moves along a nerve cell.

Stimulating a muscle contraction also involves the movement of sodium ions. For a muscle to contract, a nerve impulse travels to a muscle. The movement of the sodium current in the nerve signals the muscle cell membrane to open and sodium rushes in, creating another current that travels along the muscle and eventually leading to muscle contraction. In both nerve and muscle cells, the sodium that went in during a stimulus now has to be moved out by the sodium-potassium pump in order for the nerve and muscle cell to be stimulated again.

References

Attributions

Alice Callahan, Heather Leonard, & Tamberly Powell, Instructors (Nutrition) at Lane Community College, Sourced from OpenOregon