Lesson 1: Fluid Compartments

Fluid Compartments

Total Body Water (TBW)

  • Total Body Water in an adult is 42 L.
  • TBW can be divided into:
    • Intracellular Volume
    • Represents 40% of total body weight or 28 L.
    • Major ions include K+, Mg2+, and PO42-.
    • Extracellular Volume
    • Represents 20% of total body weight or 14 L.
    • Major ions include Na+, Ca2+, Cl-, and HCO3-.
    • Further divided into:
      • Interstitial Fluid: 16% of total body weight or 11 L.
      • Plasma Fluid: 4% of total body weight or 3 L.
  • Population Differences:
    • Neonates have the highest TBW % by weight.
    • Females, the elderly, and the obese have the lowest TBW % by weight.

Starling Forces and Glycocalyx

  • Starling forces dictate the passive exchange of water between the capillaries and interstitial fluid.
  • Endothelial Glycocalyx:
    • Forms a protective layer on the interior wall of the blood vessel.
    • Disruption can contribute to capillary leak (e.g., sepsis).

Fluid Exchange Dynamics

  • Starling Forces: Determinants of fluid movement between capillary and interstitial spaces.
    1. Forces Moving Fluid from Capillary to Interstitial Space:
    • Pc = Capillary hydrostatic pressure (pushes fluid out of capillary)
    • πif = Interstitial oncotic pressure (pulls fluid out of capillary)
    1. Forces Moving Fluid from Interstitial Space to Capillary:
    • Pif = Interstitial hydrostatic pressure (pushes fluid into capillary)
    • πC = Capillary oncotic pressure (pulls fluid into capillary)
Net Filtration Pressure (NFP)
  • Given by the formula:
    NFP=(P<em>c−P</em>if)−(π−πif)NFP = (P<em>c - P</em>{if}) - (π - π_{if})
  • Situational Outcomes:
    • NFP > 0: Filtration (fluid exits capillary)
    • NFP < 0: Reabsorption (fluid pulled into capillary)

Plasma Volume

  • Plasma: The non-cellular fraction of circulating blood volume.
  • Blood volume comprises 60% plasma and 40% blood cell volume.
  • Hematocrit (Hct): Fraction of blood volume occupied by erythrocytes.
    • Increased by:
    • Increased number of RBCs (polycythemia) or
    • Decreased plasma volume (hypovolemia).
    • Decreased by:
    • Decreased number of RBCs (anemia) or
    • Increased plasma volume (hemodilution).

Interstitial Fluid

  • The interstitium: Space between cells that contains predominantly a gel of fluid and proteoglycan filaments.
  • Less than 1% of interstitial volume is free-floating fluid.
  • Fluid movement relies on diffusion rather than free flow.

Lymphatic System

  • Function: Serves as a fluid scavenger, removing fluid, protein, bacteria, and debris from the interstitium.
  • Mechanism: Employs a pumping system to propel lymph through a vessel network lined with one-way valves, creating net negative pressure in the interstitial space.
  • Edema: Occurs when the rate of fluid accumulation in interstitium exceeds removal by lymphatic system.
Lymph Flow & Anatomy
  • Lymph returns to venous circulation via the thoracic duct at the junction of internal jugular and subclavian vein.
    • Thoracic Duct Risks: Injury during venous cannulation, especially larger on the left side, increasing the risk of chylothorax.
  • There’s a smaller thoracic duct adjacent to the right internal jugular and subclavian veins, draining the right upper extremity, thorax, and side of the head and neck.
  • A potential space for fluid accumulation is termed as an effusion.