Fluids, Electrolytes & Nutrition: Fluid Physiology

Fluid Volumes

Volumes

  • Total Body Water:
    • Total Body Water (Males) = Weight (Kg) x 60%
    • Total Body Water (Females) = Weight (Kg) x 50%
  • Total Body Water Composition:
    • 2/3 Intracellular
    • 1/3 Extracellular
  • Extracellular Water Composition:
    • 2/3 Extravascular (Interstitial)
    • 1/3 Intravascular (Plasma)
  • Total Blood Volume:
    • Adult: 5 L (7-8% Total Body Weight)
    • Peds: 80 cc/kg

Fluid Shift

  • Osmotic Pressure (Intracellular & Extracellular)
    • Mediator: Small Ions (Mostly Sodium)
    • Effect: Pulls Fluid In
    • “Osmole”: A Unit of Osmotic Pressure Equivalent to the Amount of a Solute that Dissociates in a Solution to Form One Mole of Particles
  • Oncotic Pressure (Colloid Osmotic Pressure)
    • Mediator: Proteins/Large Molecules (Mostly Albumin)
    • Effect: Pulls Fluid In
  • Hydrostatic Pressure
    • Mediator: Fluid Itself
    • Effect: Pushes Fluid Out

Osmolarity

  • Definition: Number of Osmoles in a Liter of Solution
    • Less Precise than Osmolality (Solution Volume Varies by Temperature)
    • Terms Essentially Interchangeable Under Normal Physiologic Conditions
  • Both Based on Number of Molecules (Not Size)
  • Osmolarity = (2 x Na) + (BUN/2.8) + (Glucose/18) Mn
    • If Alcohol is Present: Add + (Ethanol/4.6)

Osmolality

  • Definition: Number of Osmoles in a kg of Water
  • More Precise than Osmolarity

GI Secretions

GI Secretions (Per Day)

  • Exogenous Oral Intake: 2.0 L
  • Saliva: 1.5 L
  • Stomach: 1.0-2.0 L
  • Biliary: 0.5-1.0 L
  • Pancreas: 1.0-1.5 L
  • Small Intestine: 1.5 L

GI Reabsorption (Per Day)

  • Small Intestine: 8.5 L
  • Large Intestine: 0.5 L

Secretion Contents

  • High Bicarbonate Secretion:
    • Pancreas – Highest
    • Bile
  • Colon – Highest Potassium (K)
mEq/L Sodium Potassium Chloride Bicarbonate
Saliva 10-12 10-25 10-12 30
Stomach 60-120 10-15 100-130 0
Bile 135-145 5-10 90-110 30-40
Pancreas 135-145 5-10 70-90 95-115
Small Intestine 110-140 5-10 90-120 30-40 (Only in Ileum)
Large Intestine 60 30 40 0

Fluid Losses

Sensible Loss

  • Definition: Fluid Loss that Can Be Perceived by the Senses & Measured
  • Sources:
    • Urine
    • Defecation

Insensible Loss

  • Definition: Fluid Loss that Cannot Be Perceived by the Senses & Cannot Be Measured
  • Sources:
    • Skin (75%)
    • Respiratory System (25%)
  • Average Insensible Loss: 10 cc/kg/Day
  • Causes of Increased Insensible Loss:
    • Open Abdomen – Lose 1 L/Hour
    • Mechanical Ventilation
    • Burn Wounds
    • Fever – Cutaneous Losses Increase 10% for Every Degree Over 37°C

Fluid Sequestration

  • Definition: Loss of Fluid into Interstitial Spaces, Withdrawn from the Circulating Volume
  • Perioperative Fluid Sequestration: 4-8 cc/kg/hr
    • Extends into the Postoperative Period
    • Postoperative Losses Higher After Open Abdominal than Laparoscopic Procedures

Recurrent Emesis

  • Result: Hypochloremic Hypokalemic Metabolic Alkalosis
  • Hypochloremia:
    • Emesis Causes a Loss of HCl
  • Hypokalemia:
    • Low Volume Causes Secondary Hyperaldosteronism
    • Aldosterone Causes Retention of Sodium & Excretion of Potassium in Urine
      • Hypernatremia is Also Seen
  • Metabolic Alkalosis:
    • “Paradoxic Aciduria”: Low Potassium Causes Renal Tubules to Retain Sodium & Exchange Hydrogen Instead
      • Normally Would Exchange Potassium for Sodium
    • Emesis Also Causes Some Loss of HCl
      • Effect is to a Lesser Extent Than Paradoxic Aciduria

Mnemonic

Dirty Method to Calculate Osmolarity

  • Osm = Na x 2 + 10
  • Good Unless Other Abnormalities in BUN, Glucose or ETOH