Body Fluids and Composition

• Body fluids are vital to normal body functioning

• Total body fluid (TBW) accounts for about 60% of total body weight

• Total body fluid (TBW) is divided into intracellular and extracellular fluids.

• Water is essential for proper organ system functioning and survival

• People can survive weeks without food, but only a few days without water

• Water is essential for cell life, interferes in chemical and metabolic reactions, facilitates nutrients absorption and transport, regulates body temperature, and assists in waste elimination

• Muscle mass is rich in water, but adipose tissue has a lower percentage of water content

• Overweight or obese individuals typically have a lower percentage of water compared to someone who is lean and muscular

• Women usually have a lower percentage of total body water than men because of a higher percentage of body fat

• Older adults tend to have a lower concentration of water overall due to an age-related decrease in muscle mass

• Children tend to have a higher percentage of water, as much as 70-80% in a full-term neonate

• Blood consists of 92% water, Bones consist of 22% water and Muscles and Brain consist of 75% water

Intracellular fluids

• Account for 2/3 of total body water (TBW)

• Account for 40% of total body weight

• Humans with an average weight of 70 kg have an intracellular compartment that contains about 28 liters of fluid

Extracellular Fluid

• Accounts for 1/3 of the TBW

• Accounts for 20% of total body weight

• Extracellular fluid is divided into interstitial and intravascular fluids

• Interstitial fluid accounts for 2/3 of exracellular fluid wherease intravascular, 1/3

Interstitial Compartment

• The interstitial compartment consists of the small, narrow space between tissues or parts of an organ

• The interstitial compartment is filled with interstitial fluid

• Edema develops when excessive fluid accumulates in the interstitial space

• The interstitial space in the average 70 kg male contains approximately 10.5 liters of fluid, or 15% of the TBW

• Intravascular fluid includes blood

• The average volume of blood in humans is approximately 70-75 ml/kg

• The microenvironment from water present in the body allows movement of ions, proteins, and nutrients across the cell barrier

• Daily input is the minimal obligatory daily intake and consists of ingested water, water content in food and water from oxidation in quantities of 500mL, 800mL and 300mL, respectively

• Minimal obligatory daily water output includes urine, evaporation from the skin, respiratory tract and stool

• These outputs occur in the following quantities; 500mL from urination, 500mL from evaporation, 400mL from respiration, and 200mL from stools

• An adult's input and output averages 30-35mL/kg/day (about 2.4L/day)

• Water requirements increases in patients with fever, sweating, burns, tachypnea, surgical drain, polyuria, and gastrointestinal losses

• Water requirements increase by 100 to 150 mL/day for each degree Celsius of body temperature elevation

Definitions

• Osmolality is the number of moles of a chemical compound that contributes to the solution's osmotic pressure expressed as mOsm/kg of water

• Osmolarity is the number of osmoles of solute particles per unit volume of solution (mosm/L)

• Osmotic pressure is the pressure exerted by osmotically active particles in the fluid and depends on the number of particles per unit volume

Plasma Osmolality

• Plasma osmolality is largely determined by sodium salts

• Normal plasma osmolality is 275-295 mosm/kg

• The calculation for plasma osmolality is Plasma osmolality = 2*Na + glucose/18 + BUN/2.8

• Effective plasma osmolality determines solutes in plasma that do not permeate the cell wall freely and act to hold water within ECF

• The formula for effective osmolality: Effective osmolality = 2*Na + glucose/18

Types of Intravenous Fluid (IVF) Therapy

• Deficit indicates that input did not meet the output and involves more inaccessible fluid

• Mild fluid deficit uses the percentage (2-3% of TBW)

• Moderate fluid defecit indicates dryness in the mucous membranes and axilla, and decreased urine output, and increased pulse pressure

• Severe fluid deficit can result in cardio vascular collapse (patient is in shock) and corresponds to 10-15% of TBW

• Any deficit is typically replaced and administered half in the first 8 hours, and half in the remaing 16 hours

Fluid Replacement

• IV fluid replacement maintenance dose is 4/2/1 ml/Ke/h or per 24 hours 100/50/20 is sometimes used in surgery
• Ongoing fluid losses indicate blood loss according to gauze
• Crystalloids provide 1:3 ratio of intravascular to interstitial fluid
• Colloids provide 1:1 ratio of intravascular to interstitial fluid
• Normal IV fluid deficit is calculated and given from the time of admission and not the time of insult as burns

Maintenance Regard

• Regarding maintenances a patient needs a certain amount of sodium and potassium per day
• 2-3 meq/kg 140 meq can be obtained using normal saline at 1L of the 3L for example
• The rest are given dextrose or isotonic solutions to avoid gluconeogenesis and breakdown of protein in muscle

Third Spacing Calculation

• Minimal surgeries: 3-4 ml/kg/h
• Moderate surgeries: 5-6 ml/kg/h
• Severe surgeries, 8-10 ml/kg/h
• Also be considered like in the floor like diarrhea or vomiting, and be calculated as much as possible with the same solution (similar PH, same electrolytes content, etc....

Commonly Used Fluids

• 5% Dextrose

• Has a 5% dextrose composition with 50 gms of glucose

  • It corrects dehydration and supplies energy at 170Kcal/L

• 0.9% Normal Saline: Indications would be used to correct dehydration, correct Na level

  • The composition consists of Na 154 mEq & Cl 154 mEq
  • Its a pharamcological way to provide major EC electrolytes
    • Corrects both water and electrolyte deficit
    • Increase the intravascular volume substantially

• Ringer Lactate: The composition contains Na, k, cl, lactate, and ca

  • Is the most physiological fluid - Lactate rapidly expands the IV volume and is metabolized in liver to bicarbonate, providing buffering capacity

• Colloid: Large molecular weight substances that largely remains in the intravascular compartment thereby, generating oncotic pressure

• Hydroxyethyl starch: Composed of more than 90% esterified amylopectine, with osmolality of 310 mosm/L, leads longer plasma expansion and promotes higher colloidal pressure

• Advantage: Crystalloids are initial choice of resuscitation in patients from Hemorrhagic schok: and recommend initial fluid of choice and not evidence from RCT that resuscitation with colloid Reduces the risk of death, compared with crystalloid