Body Fluids and Compartments

• The goals are to describe body fluid compartments and isotonic, hypotonic, and hypertonic solutions

Water in the Body

• Water is the most abundant substance in the human body
• Maintaining constant volume and stable fluid composition is essential for homeostasis
• Water makes up differing percentages of total body weight based on age group:
  • Infants: 70%
  • Adults: 60%
  • Elderly: 50-55%
• Factors such as age, gender, and body fat percentage affect the fraction of total body weight contributed by water.
• Females have 50% fluids and 50% solids
• Males have 60% fluids and 40% solids

Body Fluid Compartments

• The internal environment constitutes fluid surrounding cells known as extracellular fluid (ECF)
• A 70 kg man has approximately 42 liters of total body water
  • 28 liters are intracellular fluid (ICF)
  • 14 liters are extracellular fluid (ECF), comprised of 3 liters of plasma and 11 liters of interstitial fluid (ISF)

Fluid Volumes and Distribution

• Total Body Water (TBW) is 0.6 times body weight, totaling 42 liters in a 70 kg man
• Extracellular Fluid Volume (ECF) is one third of TBW, which is 14 Liters
• Intracellular Fluid Volume (ICF) is two thirds of TBW, about 28 Liters
• Interstitial Fluid constitutes three fourths of ECF, approximately 10.5 Liters
• Plasma constitutes one fourth of ECF, about 3 Liters
• Transcellular Fluid is approximately 0.5 Liters

Water Intake and Output

• Water intake equals water elimination to maintain a steady state
• The body adds water from two major sources: ingested liquids/food and synthesized water during carbohydrate oxidation
• Ingested water adds about 2100 ml/day to body fluids
• Synthesized water adds about 200 ml/day for a total intake of about 2300 ml/day

Daily Water Loss

• Insensible water loss is a continuous evaporation of water from the respiratory tract and diffusion through the skin
• Insensible water loss accounts for about 700 ml/day
• Cold weather increases water loss from the respiratory system
• The cholesterol-filled cornified layer of the skin acts as a barrier to prevent excessive diffusion loss
• Fluid loss occurs through sweat, typically around 100 ml/day
• A small amount of water, around 100 ml/day, is lost in feces
• The remaining water loss occurs through urine excreted by the kidneys
• Urine volume varies from 0.5 L/day in dehydrated individuals to 20 L/day in those with high water intake

Transcellular Fluid

• Transcellular fluid includes fluid in synovial, peritoneal, pericardial, intraocular, and cerebrospinal spaces
• It is a specialized type of extracellular fluid and makes up about 0.5 - 1 L.

Composition of Body Fluids

• The composition of transcellular fluids differs significantly from plasma and from each other
• Plasma and interstitial fluid compartments of the ECF have a similar composition
• Plasma contains significantly more protein than other body fluids

Osmosis and Osmotic Pressure

• Osmosis is the net diffusion of water across a selectively permeable membrane from an area of high water concentration to an area of lower water concentration.
• The total number of particles in a solution is measured in osmoles.
• Osmotic pressure prevents osmosis
• Osmotic pressure measures water and solute concentrations within a solution.
• Higher osmotic pressure indicates lower water concentration and higher solute concentration
• Osmotic pressure of a solution is directly proportional to the concentration of osmotically active particles, regardless of solute size
• Van't Hoff's Law defines osmotic pressure (π) via the formula: π = C * RT, where C is concentration, R is the ideal gas constant, and T is absolute temperature (Kelvin); use 19.3

Osmolarity and Solutions

• Osmolarity of body fluids is 290 ± 10 mOsm/L
• Osmotic pressure = C x 19.3
• 19.3 mmHg = 1 mosm/L x 19.3
• 5597 mmHg = 290 mosm/L x 19.3
• Hypotonic, isotonic, and hypertonic solutions are defined by their effect on cell volume:
• Isotonic environments don't cause volume changes

Fluid Balance Summary

• Body fluid volume represents a balance between water gain from ingestion and metabolism, and water loss through renal/GI elimination, sweat, and insensible water losses.
• Water movement between intracellular and extracellular compartments is controlled by the osmotic gradient that created by electrolyte concentration differences