Understanding Body Fluids and Compartments

Body Fluids

• Body fluids are essential and are absorbed in the intestines and distributed throughout the body
• Water eliminated as urine via the bladder
• Water is reabsorbed, filtered, and eliminated via the kidney
• Rehydration essential for maintaining balance

Learning Outcomes

• Body fluid compartments need identification
• Describes the distribution of water and ionic composition of the body fluid compartments
• Osmosis and osmotic pressure (Osmolality) and Oncotic pressure needs describing
• Water particles movement across cell membrane and the capillary bed require explanation
• The effects of solutions on cell volume from Osmosis and tonicity principles have to be applied.

Fluid Compartments

• Include intracellular and extracellular fluids

Body Composition

• On average, females are 45% solids and 55% fluids
• On average, males are 40% solids and 60% fluids
• Interstitial fluid includes lymph plasma, CSF, synovial fluid, aqueous and vitreous humor, pleural, peritoneal, and pericardial fluids
• Water continuously exchanges between fluid compartments to help maintain osmotic balance between them.

Fluid Body Compartments and Volumes

• Intracellular Fluid (ICF) is the fluid within cells; also known as cytoplasm
  • The fluid averages about 28 litres in a 70 kg person
  • It accounts for about 67% of total body water
• Extracellular Fluid (ECF) is all fluid outside cells
  • It averages about 14 litres in a 70 kg person
  • Plasma: the fluid in the vascular space with about 3 litres of volume
  • Interstitial fluid: fluid between cells of tissues, including lymph with about 10.2 litres of volume
• Transcellular fluid: Fluid in the CSF, joints, ocular fluid, and G-I tract with about 0.8 litres
• The balance between these compartments is crucial for tissue function
• The kidney plays an essential role

Body Water Summary in 70Kg Male

• 40% Solids and 60% Fluids
• 2/3 Intracellular fluid (ICF)
• 1/3 Extracellular fluid (ECF)
• 80% Interstitial fluid
• 20% Blood plasma

Ion Distribution in Body Fluid Compartments

• Osmolality (osmotic pressure) is equivalent in all body fluid compartments
• Concentration of solutes must be the same within the extracellular and intracellular spaces; net movement of water must be prevented between these compartments
• Interstitial fluid and extracellular fluid (plasma and ISF) contain the following elements (mM): Na+, K+, Ca2+, Mg2+, Cl-, HCO3
• The body compartments are in a state of chemical disequilibrium
• The cell membrane is a selectively permeable barrier between the ECF and ICF.

processes affect the total volume of each body fluid compartment:

• Filtration affects the volume of body fluid compartments
• Reabsorption affects the volume of body fluid compartments
• Diffusion affects the volume of body fluid compartments
• Osmosis affects the volume of body fluid compartments
• There is a continual exchange of water and solutes between compartments
• Osmosis is the largest water movement source between intracellular and interstitial fluid
• Movement direction is determined via solute concentrations in fluid

Osmosis and Osmotic Pressure

• Osmosis is the movement of water molecules from a low solute concentration (dilute) to a high solute concentration (concentrated) across a semi-permeable membrane
• Two chambers of different concentrations of a solute
• Water crosses the membrane to equalize solute concentrations in both chambers
• The amount of water that moves depends on the difference of impermeable solute concentrations Movement of water could be counter-balanced by physical pressure
• Difference in concentration of the impermeable solute exerts an osmotic pressure

Osmotic Pressure Measurements and Solutions

• There are two ways to express the number of particles in solution: osmolarity and osmolality
• Osmolarity is the total number of dissolved particles (ions, molecules or mix of both) per liter of solution

  • It involves the molarity x the number of particles per molecule once the particle dissolves

  • For example:

    • In the case of an undissociated molecule (e.g. glucose) 1 mole = 1 osmole
    • In the case of a solute that fully dissociates (e.g. NaCl) 1 mole = 2 osmoles
    • In the case of a protein (e.g. albumin) 1 mole = 1 osmole

  • It is measures in osmoles/ liter of solution (osmol/L or OsM)

• Osmolality is the total number of dissolved particles (ions, molecules or mix of both) per Kg of water Generally, easy to measure the volume of a solution generated; osmolarity used.
• Most clinical samples taken are dilute, therefore, a 1% difference.
• Osmolarity is the osmoles/liter of solution
• Osmolality is the osmoles / kg solvent

Terms and Conditons of Osmosis in action

• Properties of the membrane determine which solutes are osmotically active; important:
  • Sodium (Na+) and Chloride (Cl-) are found in the Extracellular fluid (EFC) and are usually osmotically active
  • The Membrane is nearly impermeable to Sodium ( Na+)

Water Movement during Osmosis

• Water is lost from extracellular space during dehydration
  • Results in the increase in extracellular osmolality
  • Water flows into the intracellular fluid which causes cell to shrink and lose function
• The kidney returns osmolality to a normal level

Osmosis Problems

• One example is increase in extravascular osmolality, such as during inflammation
• Capillary Wall can exacerbate or counter-act these conditions:
  • Is freely Sodium (Na+) permeable, therefore, not osmotically active to the cell
  • Impermeable to plasma proteins
• Accumulate extravascular proteins that exert osmotic pressure
• An alteration of osmotic forces is due to changes in large molecule (e.g., protein) distribution; referred to as changes to colloid osmotic (or oncotic) pressures

Tonicity

• Interest in solutions that won't cause cells to swell or shrink, isotonic solutions
• Hypotonic solutions cause cellular swelling to increase
• Hypertonic solutions cause cellular shrinkage to reduce
• Isotonic solutions example is 0.9% NaCl solution. Also known as saline solution

Solution Tonicity Effects on Cell Volume

• Isotonic solutions have the same osmolality as plasma (= 290 mosmoles/kg H₂O)
  • Water movement tends to maintain intracellular volume
• Hypertonic solutions have a greater osmolality than plasma (>290 mosmoles/kg H₂O)
  • Water moves out of the cell, tends to reduce intracellular volume
• Hypotonic solutions have a lower osmolality than plasma (<290 mosmoles/kg H₂O)
  • Water moves into cell, tends to increase intracellular volume.

Body Fluid Intake & Output (Young Adult)

• Water is gained from food at about 2L per day
• Water is gained from metabolism at about 0.4L per day
• Water is lost via skin and insensible loss at about 0.8L/day
• Water loose via metabolic lung process
• Water is lost via urine at about 1.5L/day
• Water is lost via feces at about 0.1L/day
• Fluid gained at a total of 2.4L/day and lost at output of 2.4L/day to balance fluids

Lecture Content Week 1:

• A lecture bite to cover homeostasis
• Feedback requires positive input
• Applications of osmosis to physiology
• Application of all the week 1 content requires checking