fluids and electrolytes

MEDAVIE Health Education: Fluids and Electrolytes

Module: 04 Section: 04

Overview of Fluids and Electrolytes

  • This module covers the essential elements of fluids and electrolytes critical for primary care paramedicine.

Key Electrolytes

  • Common electrolytes include:
    • Na (Sodium)
    • K (Potassium)
    • Cl (Chloride)
    • Co2 (Total CO₂)
    • Ca (Calcium)

Total Body Water (TBW)

  • Normal values expressed in percentage of total body weight:
    • 80% TBW in newborns
    • 65% - 70% TBW in children
    • 50% - 60% TBW in adults
    • 45% TBW in the elderly
  • The total body water percentage varies by:
    • Age
    • Sex
    • Fat content

Body Fluid Compartments

  • Total body mass distribution:

    • For females:
    • 45% Solids
    • 55% Fluids
      • Breakdown: 2/3 Intracellular fluid (ICF), 1/3 Extracellular fluid (ECF)
    • For males:
    • 40% Solids
    • 60% Fluids
      • Breakdown: 2/3 ICF, 1/3 ECF

  • Extracellular Fluid (ECF):

    • Includes plasma and interstitial fluid.
    • Other components include lymph, cerebrospinal fluid (CSF), joint fluids, and humors of the eye (transcellular fluid).

  • Intracellular Fluid (ICF):

    • Water inside the cells.

Fluid Composition and Homeostasis

  • A stable balance between fluid and electrolytes is necessary for homeostasis.
  • Types of substances:
    • Electrolytes:
    • Salt substances that dissociate into charged components when dissolved in water (i.e. Na+, K+).
    • Non-electrolytes:
    • Substances that do not carry an electrical charge in water (e.g., glucose, urea).

Distribution of Electrolytes

  • Key electrolytes in different fluid compartments (in mEq/liter):
    • Sodium (Na⁺):
    • Blood plasma: 142
    • Intracellular fluid: 10
    • Potassium (K⁺):
    • Blood plasma: 4
    • Intracellular fluid: 150
    • Calcium (Ca²⁺):
    • Blood plasma: 1
    • Intracellular fluid: 20
    • Chloride (Cl⁻):
    • Blood plasma: 100
    • Intracellular fluid: 1

Electrolyte Types

Positively Charged Electrolytes (Cations)

  • Intracellular Cations:

    • Potassium (K+)
    • Calcium (Ca++)
    • Magnesium (Mg++)

  • Extracellular Cation:

    • Sodium (Na+)

Negatively Charged Electrolytes (Anions)

  • Intracellular Anion:

    • Phosphate (PO43-)

  • Extracellular Anions:

    • Chloride (Cl-)
    • Bicarbonate (HCO3-)

Regulation of Electrolytes

  • Aldosterone: The primary regulator of electrolyte concentrations, promoting:
    • Reabsorption of Na+
    • Excretion of K+

Avenues of Water Movement

  • Water Enters the Body:

    • Digestive system
    • Cellular metabolism

  • Water Exits the Body:

    • Kidneys
    • Lungs
    • Sweat
    • Feces

  • Input = Output:

    • Balance between water intake and output is vital for homeostasis.

Factors Affecting Fluid Movement

Starling’s Law of the Capillaries

  • Describes how fluid movement due to filtration across capillary walls is dependent on:
    • Hydrostatic pressure gradient
    • Oncotic pressure gradient
  • Forces that affect fluid movement include:
    • Hydrostatic forces
    • Osmotic pressures
    • Tonicity and membrane permeability

Hydrostatic Pressure

  • Definition: The pressure of any fluid enclosed in a space.
  • Main hydrostatic forces:
    • Blood hydrostatic pressure (BHP): The force exerted by blood in the capillaries, driving fluid out into tissues.
    • Interstitial fluid hydrostatic pressure (IFHP): The opposing pressure in the interstitial space as fluid is pushed out.

Blood Hydrostatic Pressure (BHP)

  • Exerted by blood against the capillary walls, promoting filtration into the surrounding tissues.
  • As fluid exits a capillary, interstitial fluid hydrostatic pressure increases.

Interstitial Fluid Hydrostatic Pressure (IFHP)

  • Represents the mechanical force of water within the interstitium against cell membranes.
  • Works with plasma osmotic pressure to regulate filtration in and out of the capillaries.

Osmotic Pressure

  • Definition: The net pressure driving reabsorption of fluids from interstitial spaces back into capillaries, also termed oncotic pressure.
  • Major osmotic forces:
    • Blood colloid osmotic pressure (BCOP): The pressure exerted by proteins in the intravascular fluid, influencing water reabsorption.
    • Interstitial fluid colloid osmotic pressure (IFCOP): Causes suction of fluid into the interstitium, helping maintain fluid balance.

Net Filtration Pressure (NFP)

  • Formula:
    NFP = BHP - IFHP - (BCOP - IFCOP)
  • Implications:
    • Hydrostatic forces dominate when fluid moves from the bloodstream into the interstitium.
    • Oncotic forces dominate when fluid moves from the interstitium into the bloodstream.

Tonicity

  • Definitions:
    • Isotonic: Equal solute concentration to the fluid being compared.
    • Hypotonic: Lesser solute concentration than the fluid being compared.
    • Hypertonic: Higher solute concentration than the fluid being compared.

Membrane Permeability

  • Only a small portion of plasma proteins can cross capillary membranes easily.
  • Fluid movement depends on the tonicity of either side of the membrane.

Transport Mechanisms

  • Osmosis: Diffusion of water through a selectively permeable membrane.
  • Diffusion: Movement of atoms, ions, or molecules from high to low concentration regions.
  • Active Transport: Movement against a concentration gradient (uphill).
  • Facilitated Diffusion: Movement down the gradient facilitated by carrier proteins.

Water Balance Regulation

  • Regulated mainly by Antidiuretic Hormone (ADH) and the perception of thirst.
  • ADH Release Triggers:
    • Increased plasma osmolality
    • Decreased circulating blood volume
    • Lowered venous and arterial pressures
  • Post ADH release, water is reabsorbed from renal tubules and collecting ducts of the kidneys.

Fluid Volume Dynamics

  • Normal ECF Volume: Maintains stability.
  • Changes in fluid volume:
    • Increased ECF volume: Result of increased blood volume.
    • Decreased ECF volume: Caused by factors such as fasting, resulting in decreased urine volume and arterial blood pressure.

Renin-Angiotensin-Aldosterone System (RAAS)

  • Key processes in managing blood pressure and volume:
    • Triggered by hypovolemia (dehydration) or decrease in blood volume.
    • Involves the production of renin by juxtaglomerular cells in the kidneys, leading to a series of events that increase blood pressure and volume.

Antidiuretic Hormone (ADH)

  • Also termed Vasopressin.
  • Produced in the hypothalamus; stored and released from the posterior pituitary in response to events that trigger its release such as drops in blood volume.

Atrial Natriuretic Hormone (ANH)

  • Released when the right atrium over stretches due to increased blood volume.
  • Effects of ANH include:
    • Decreased Na+ reabsorption
    • Increased glomerular filtration rate (GFR)
    • Peripheral vasodilation
    • Inhibition of ADH and Aldosterone release.

Fluid and Electrolyte Imbalances

Hypovolemia (Dehydration)

  • Can be isotonic, hyponatremic, or hypernatremic.

Hypervolemia (Overhydration)

  • Results in increased body water and decreased solute concentration.

Specific Imbalances

  • Isotonic: Caused by loss of equal amounts of Na+ and water (e.g., severe vomiting).

  • Hyponatremic:

    • Serum decrease in Na+ due to:
    • Salt-wasting diuretics
    • Excessive sweating
    • Increased water intake
    • Symptoms include:
    • Muscle cramps
    • Fatigue
    • Confusion
    • Potential seizures/coma in severe cases.

  • Hypernatremic:

    • Elevated serum Na+ due to water loss exceeding Na+ (causing cellular dehydration).
    • Causes include:
    • Diabetes insipidus
    • Insufficient water intake
    • Symptoms include:
    • Thirst
    • Lethargy

Overhydration

  • Defined by increased body water and decreased solute concentration.
    • Causes include excessive IV fluid administration and impaired renal function.

Roles of Electrolytes

  • Potassium (K+): Essential for nerve, muscle, and cardiac function.
  • Calcium (Ca++): Involved in neurotransmission, hormone secretion, and muscle contraction.
  • Magnesium (Mg++): Activates enzymes and influences neuromuscular functions.

Blood Electrolyte Imbalances

Normal Ranges (in mEq/liter)

  • Sodium (Na+): 136-148
  • Chloride (Cl-): 95-105
  • Potassium (K+): 3.5-5.0
  • Calcium (Ca++): Total = 9.0-10.5; Ionized = 4.5-5.5
  • Phosphate (HPO42-): 1.7-2.6
  • Magnesium (Mg++): 1.3-2.1

Deficiencies and Causes

  • Hyponatremia: Low sodium due to excess loss or excessive water intake.
  • Hypernatremia: High sodium largely due to dehydration or water deprivation.
  • Hypokalemia: Low potassium due to excessive loss or kidney disease.
  • Hyperkalemia: High potassium due to renal failure or excessive intake.
  • Hypocalcemia: Low calcium due to increased losses or hypoparathyroidism.
  • Hypercalcemia: High calcium due to hyperparathyroidism or excess vitamin D.
Symptoms of Imbalances
  • Symptoms vary based on deficiency or excess:
    • Muscular weakness
    • Dizziness
    • Tachycardia and arrhythmias
    • Confusion and seizures (in severe cases).