Replacing fluids in the body is based on body fluid needs. Water comprises approximately 60% of the adult body. Approximately 40% is intracellular fluid and 20% is extracellular fluid, of which 15% is interstitial (tissues) and 5% is intravascular. The walls separating these compartments are porous, allowing water to move freely between them. Small particles such as sodium and chloride can pass through the walls, but larger molecules such as proteins and starches usually are unable to pass through the walls.
Hydrostatic and osmotic pressures are forces that move water and regulate the body’s water. Intravenous fluid manipulates these two pressures. Hydrostatic pressure reflects the weight and volume of water. The greater the volume, the higher the blood pressure.
Effects of Osmotic Pressure: Osmosis is the diffusion of water across a semipermeable membrane from an area of high concentration to an area of low concentration (water moves into the compartment of higher concentration of particles, or solute). This is similar to the action of a sponge soaking up water. This pull is referred to as osmotic pressure. It is the number of particles in each compartment that keeps water where it is supposed to be. By administering fluids with more (or fewer) particles than blood plasma, fluid is pulled into the compartment where it is needed the most.
How do we know where the water is needed? To assess water balance, measure the osmolality of blood plasma (number of particles [osmoles] in a kilogram of fluid). Osmolarity is the number of particles in a liter of fluid. Normal serum osmolality is approximately 300 milliosmoles (mOsm) per liter.
Crystalloids are made of substances that form crystals (e.g., sodium chloride) and are small, so easy movement between compartments is possible. Crystalloids are categorized by their tonicity (a synonym for osmolality). An isotonic solution has the same number of particles (osmolality) as plasma and will not promote a shift of fluids into or out of cells. Examples of isotonic crystalloid solutions are 0.9% sodium chloride and lactated Ringer’s solution. Dextrose 5% in water is another isotonic crystalloid. However, it is quickly metabolized, and the fluid quickly becomes hypotonic. Hypotonic solutions (e.g., D5W, 0.45% sodium chloride) are a good source of free water, causing a shift out of the vascular bed and into cells by way of osmosis. Hypotonic solutions are given to correct cellular dehydration and hypernatremia. Hypertonic solutions have more particles than body water and pull water back into the circulation, which can shrink cells.
• Extracellular fluid replacement when chloride loss is greater than or equal to sodium loss
• Treatment of metabolic alkalosis in the presence of fluid loss; chloride ions cause a compensatory decrease of bicarbonate ions
• Sodium depletion, extracellular fluid volume deficit with sodium deficit
• Initiation and termination of blood transfusion, preventing hemolysis of RBCs (occurs with dextrose in water solutions)
• Acidosis: 0.9% sodium chloride contains one-third more chloride ions than is present in extracellular fluid; excess chloride ions cause loss of bicarbonate, resulting in acidosis
• Hypokalemia: Increased potassium excretion at the same time extracellular fluid is increasing, which further decreases potassium concentration in extracellular fluid
Note: Once infused, dextrose is rapidly metabolized to water and carbon dioxide, becoming hypotonic rather than isotonic.
| Solution | Comments |
| Dextrose 5% in water (D5W) | Supplies approximately 170 cal/L and free water to aid in renal excretion of solutes
Avoid excessive volumes in pts with increased antidiuretic hormone activity or to replace fluids in hypovolemic pts |
| 0.9% Sodium chloride (0.9% NaCl) | Isotonic fluid commonly used to expand extracellular fluid in presence of hypovolemia
Can be used to treat mild metabolic alkalosis |
| 0.45% Sodium chloride (0.45% NaCl) | Hypotonic solution that provides sodium, chloride, and free water; sodium and chloride allow kidneys to select and retain needed amounts
Free water is desirable as aid to kidneys in elimination of solutes |
| 3% Sodium chloride | Used only to treat severe hyponatremia |
| Lactated Ringer’s solution | Isotonic solution that contains sodium, potassium, calcium, and chloride in approximately the same concentrations as found in plasma
Used to treat hypovolemia, burns, and fluid loss as bile or diarrhea |