Fluid, Electrolyte, and Acid-Base Balance Notes

Fluid, Electrolyte, and Acid-Base Balance

Body Fluids

• At least half of body weight is water, separated into two major compartments:
  • Intracellular fluid compartment: fluid inside cells (~40% body weight)
  • Extracellular fluid compartment: fluid outside cells (~20% body weight)
    • Interstitial fluid between cells.
    • Plasma of the blood.
    • Lymph in lymphatic vessels.
    • CSF of brain and spinal cord.
    • Synovial fluid within joints.
• Each compartment is composed of water, electrolytes, and other solutes with specific distribution in each compartment.
• The concentration of solutes is expressed as osmolarity which is the number of solute particles in a particular volume of solution.
  • Adding water dilutes the concentration.
  • Losing water would concentrate the solution.

Intracellular Fluid vs Extracellular Fluid composition

• While total osmotic pressure is similar between fluid compartments, intracellular fluid composition is very different from extracellular fluid due to:
  • Selectively permeable plasma membrane
  • Transport proteins in plasma membrane use ATP to maintain uneven distribution of molecules, ions.
  • Large molecules synthesized by cell cannot leave

Influence of Osmotic Pressure on Fluid Movement

• The two main forces that regulate fluid movement into and out of the blood are hydrostatic pressure and osmotic pressure.
• When osmotic pressures on each side of a plasma membrane are equal, there is no net movement of water molecules.
• If osmotic pressure is higher in the interstitial fluid than that in the blood, there is a net movement of water by osmosis out of the blood.
• If osmotic pressure of the blood is higher than that of the interstitial fluid, there is net movement of water by osmosis into the blood.

Fluid Input and Output

• Volume of water entering the body must equal the volume exiting the body each day (~1.5 to 3.0 L of water per day).
  • Fluid input - primary sources:
    • Food and beverages (90%)
    • Cellular respiration (10%)
  • Fluid output - routes of water loss:
    • Kidneys – 61% of fluid loss as urine – regulatable
    • Evaporation – 35% through the skin and respiratory passageways.
    • Feces – 4% loss from the digestive tract
• Changes in total water volume alter solute concentration of body fluids, blood pressure, and interstitial fluid pressure.

Regulation of Fluid Balance

• Thirst is the sensation that induces an urge to drink liquid.
• Mechanisms that increase thirst:
  1. Hypothalamic osmoreceptors detecting increased osmolarity
  2. Baroreceptors detecting decreased BP (including juxtaglomerular baroreceptors, via RAA mechanism producing angiotensin II)
  3. Dryness of the mouth
• The thirst sensation decreases when water content in the body is adequate (absence of the stimuli above).

Fluid Imbalances

• Dehydration occurs when the body fluid volume decreases, increasing osmolarity of extracellular fluid.
  • Water moves by osmosis out of cells into the extracellular fluid.
• Hyperhydration (water intoxication or hypotonic hydration) occurs when too much water is ingested and the extracellular fluid becomes hypotonic.
  • Water can move into cells by osmosis, causing them to swell.
  • Dangerous to infants whose formulas have been diluted

Electrolyte Balance

• Electrolytes are formed when molecules dissociate into ions in water. They can be cations or anions. (E.g. Inorganic salts like NaCl, also acids and bases)
• Nonelectrolytes do not dissociate into ions in water (Lipids, urea, glucose).
• Electrolytes, especially sodium, are the component of body fluids that contribute the most to their osmolality.
• The regulation of electrolytes involves the coordinated participation of several organ systems.

Regulation of Sodium

• Function:
  • Sodium ions are the dominant extracellular cations; they exert substantial osmotic pressure (90-95%)*.
    • *Includes effect of chloride ions, whose regulation/movement is tied to $$"Na^+