Fluid & Electrolytes - Chapter 42

• Chapter 42 covers Fluid & Electrolytes, but excludes Acid-Based balance.

Functions of Water in the Body

• Transports nutrients to cells and wastes from cells.
• Transports hormones, enzymes, blood platelets, red and white blood cells.
• Facilitates cellular metabolism and proper cellular chemical functioning.
• Acts as a solvent for electrolytes and nonelectrolytes.
• Maintains normal body temperature.
• Facilitates digestion and promotes elimination.
• Acts as a tissue lubricant.
• Fever can lead to dehydration.

Total Body Fluid

• 50%-60% of body weight in a normal adult is fluid.
• Intracellular fluid comprises 35%-40% of body weight.
• Extracellular fluid is 15%-20% of body weight.
• Plasma makes up 5% of body weight.
• Interstitial fluid accounts for 10%-15% of body weight.

Characteristics of Body Fluids

• Fluid is water containing dissolved or suspended substances such as glucose, mineral salts, and proteins.
• Fluid amount is volume.
• Fluid concentration is osmolality.
• Fluid composition is electrolyte concentration.
• Degree of acidity is pH.

Location and Movement of Water and Electrolytes

• Intracellular Fluid (ICF) are fluids within cells, making up ~2/3 of total body water.
• Extracellular Fluid (ECF) is fluid outside of cells, making up ~1/3 of total body water.
• Interstitial fluid is the fluid between cells and outside the blood vessels, including lymph.
• Intravascular fluid is blood plasma found in the vascular system.
• Transcellular fluid is secreted by epithelial cells, including cerebrospinal, pleural, peritoneal, and synovial fluids.
• Filtration is the movement from intravascular space to interstitial space
• Osmosis is the movement from intracellular to interstitial space

Electrolytes and Ions

• Electrolytes (mineral salts) are compounds that separate into ions (charged particles) when dissolved in water.
• Ions are charged particles.
• Cations are positively charged ions such as sodium, potassium, calcium, and magnesium.
• Anions are negatively charged ions such as chloride, bicarbonate, and sulfate.
• Osmolality of a fluid is its concentration.
• Osmolality is measured by the number of particles per kilogram of water.
• Electrolytes are dissolved in plasma.
• The liquid in which a solute is dissolved is called a solvent.

Movement of Water and Electrolytes

• Active transport is the movement of ions against osmotic pressure to an area of higher pressure, requiring energy.
• Diffusion is the passive movement of electrolytes or other particles down the concentration gradient (from higher to lower concentration).
• Osmosis is the movement of water (or other solute) from an area of lesser to one of greater concentration.
• Filtration is the movement of fluid into and out of capillaries (between the vascular and interstitial compartments) from higher to lower pressure.

Active Transport

• Active transport involves the movement of ions against osmotic pressure.
• Requires energy in the form of adenosine triphosphate (ATP).
• Moves electrolytes across cell membranes against the concentration gradient, from lower to higher concentration.
• Carrier molecules within a cell bind to incoming molecules.
• Example: sodium-potassium pump, which moves Na+ out of a cell and K+ into it.
• This keeps ICF lower in Na+ and higher in K+ than ECF.

Diffusion

• Diffusion involves the passive movement of electrolytes or particles down the concentration gradient.
• Moves from higher to lower concentration, and does not require energy.
• Affected by molecular size, concentration, and temperature of a solution.
• Electrolytes diffuse easily until the concentration is the same in all areas.
• Fluids and electrolytes shift through compartments separated by cell walls and capillary membranes.

Osmosis

• Osmosis is the movement of water or solute against the concentration gradient.
• Moves from lower concentration to higher concentration through a semi-permeable membrane.
• Water crosses easily across cell membranes, separating interstitial fluid from ICF.
• Membranes are not permeable to all particles & electrolytes.
• Water moves into the compartment with a higher particle concentration until the concentration is equal on both sides of the membrane.

Isotonic, Hypotonic & Hypertonic Solutions

• Particles that cannot cross cell membranes determine the concentration of a fluid.
• Isotonic solutions have the same concentration as the cell, with no fluid shift or change in cells.
• Hypotonic solutions are less concentrated than the cell, causing fluid to shift into the cell and to swell.
• Hypertonic solutions are more concentrated than the cell, causing fluid to shift out of the cell, resulting in cell shrinkage.

Filtration

• Capillary filtration moves fluid between vascular and interstitial compartments from higher pressure to lower pressure.
• Affected by four forces: hydrostatic pressure and colloid osmotic pressure.
• Hydrostatic pressure is the force of fluid pressing outward against a surface.
• Capillary hydrostatic pressure pushes fluid from capillaries into the interstitial area.
• Interstitial fluid hydrostatic pressure pushes fluid from the interstitial area back into capillaries.
• Colloid osmotic pressure is exerted by large particles (proteins).
• Blood colloid osmotic pressure (oncotic pressure) pulls fluid from the interstitial area back into capillaries.
• Interstitial fluid colloid osmotic pressure is a very small opposing force, pulling fluid from capillaries into the interstitial area.
• Arterial capillary hydrostatic pressure is strongest, moving fluid from the capillary into the interstitial area.
• Venous capillary hydrostatic pressure is weaker, & blood colloid osmotic pressure is stronger, fluid moves into the capillary.
• Edema is the accumulation of excess fluid in the interstitial space.
• Heart failure can cause venous congestion, increasing capillary hydrostatic pressure.
• Inflammation can increase capillary blood flow, allowing capillaries to leak colloids which increases capillary hydrostatic pressure.

Fluid Balance

• Fluid intake is regulated by thirst.
• Thirst regulation results in fluid intake of ~2300 mL/day.
• Foods contain water and food metabolism creates water.
• Fluid can be administered through intravenous routes or irrigation of body cavities.
• Fluid distribution involves the movement of fluid among compartments.
• Extracellular and intracellular movement occurs through osmosis.
• Vascular and interstitial movement occurs through filtration.
• Fluid output occurs through kidneys, skin, lungs, and the GI tract.
• Insensible loss is not visible and occurs via skin and lungs continuously.
• Sensible loss is visible and occurs via urine, feces, and sweat.

Fluid Intake & Losses

• Fluid intake should roughly equal fluid output.
• Daily weights can record fluid balances.
• Daily weights are most reliable as a first thing in the morning.
• Fluid output normally occurs through skin, lungs, GI tract, and kidneys.
• Abnormal fluid output includes vomiting, wound drainage, and hemorrhage.
• Kidneys regulate fluid and respond to hormones that influence urine production.
• Increased fluid intake results in increased urine production.
• Decreased fluid intake results in decreased urine production.

Fluid Imbalances

• Volume imbalances involve extracellular fluid volume imbalances.
• ECV deficit is decreased volume in ECF but normal osmolality.
• Example of ECV deficit: hypovolemia.
• ECV excess is increased volume in ECF but normal osmolality.
• Osmolality imbalances involve disturbances of concentration of body fluids.
• Fluids become hypertonic or hypotonic, causing osmotic shifts of water across cell membranes.
• Hypernatremia is "water deficit," hypertonic (body fluids too concentrated, causes cells to shrivel).
• Hyponatremia is "water excess," hypotonic (body fluids too dilute, causes cells to swell).
• Clinical dehydration combines ECV deficit and hypernatremia at the same time.

Electrolyte Disorders Signs and Symptoms

• Sodium (Na): Normal range is 136 – 145 mEq/L.
  • Hypernatremia (above 145) presents as decreased level of consciousness.
  • Hyponatremia (below 136) is also decreased level of of consciousness.
• Potassium (K): Normal range is 3.5 – 5 mEq/L.
  • Hyperkalemia (above 5) is bilateral muscle weakness, abdominal cramps, diarrhea, cardiac dysrhythmias.
  • Hypokalemia (below 3.5) is bilateral muscle weakness, abdominal distention, decreased bowel sounds.
• Calcium (Ca): Normal range is 9.0 – 10.5 mg/dL.
  • Hypercalcemia (above 10.5) presents as diminished reflexes, lethargy, cardiac arrest.
  • Hypocalcemia (below 9.0) presents as numbness/tingling, hyperactive reflexes, muscle twitching and cramping.
• Magnesium (Mg): Normal range is 1.3 – 2.1 mEq/L.
  • Hypermagnesemia (above 2.1) is hypoactive deep tendon reflexes (DTRs), lethargy.
  • Hypomagnesemia (below 1.3) is hyperactive DTRs, muscle cramps and twitching.
• Positive Chvostek's sign and Positive Trousseau's sign indicate hypocalcemia.

Gerontologic Considerations

• Kidneys have a decreased ability to conserve water.
• Hormonal changes result in an inability to concentrate urine and conserve water.
• Loss of subcutaneous tissue leads to increased loss of moisture.
• Reduced thirst mechanism results in decreased fluid intake.
• Nurses must assess for these changes and implement treatment accordingly.

Types of Intravenous Fluids

• Crystalloids are simple solutions of small solutes, they are clear and transparent.
  • Isotonic solutions: 5% Dextrose in Water (D5W), 0.9% NaCl (Normal saline), Lactated Ringer's solution (LR).
  • Hypotonic solutions: 0.33% NaCl (1/3 strength normal saline), 0.45% NaCl (1/2 strength normal saline).
  • Hypertonic solutions: 5% dextrose in 0.45% NaCl (D51/2NS), 10% dextrose in water (D10W), 5% dextrose in 0.9% NaCl (D5NS).
• Colloids are suspensions of macromolecules, or cells.
  • Colloids include blood and plasma/albumin.