Body fluids + membrane transport

• Body Fluid Compartments

  • Mostly water = total body water (TBW)
  • Most TBW is intracellular fluid (ICF) (within the cytosol of cells)
  • Rest of TBW is extracellular fluid (ECF) = interstitial fluid, plasma and transcellular
• Transcellular Fluids

  • Smallest portion of ECF
  • Contained in epithelial lines spaces
  • Fluid that has crosses cell layers
  • Examples: cerebrospinal fluid; urine; sweat; aqueous and vitreous humours and synovial fluids
• Blood plasma = slightly less [Na+] [K+] [Cl-] compared to interstitial fluid
• Blood plasma = slightly higher [protein] compared to interstitial fluid
• Intracellular fluid = higher [protein] compared to interstitial fluid and blood plasma
• Intracellular fluid = higher [K+] compared to interstitial fluid and blood plasma
• Intracellular fluid = lower [Cl-] compared to interstitial fluid and blood plasma
• Osmolarity (attraction of solution to water) is constant through all so not movementof force driving water
• • Blood plasma = 5L
  • Interstitial fluid = 13L
  • Intracellular = 25L
• • All contain different levels of electrolytes
  • Barriers between blood plasma, interstitial fluid, intracellular fluid and transcellular fluid:
  • Blood plasma ⇋ interstitial fluid = capillary endothelium
  • Interstitial fluid ⇋ intracellular fluid = plasma membrane
  • Transcellular fluid ⇋ interstitial fluid = epithelial cells
• ICF ⇋ ISF = plasma membrane
• ISF ⇋ plasma = endothelium
• transcellular fluid ⇋ ISF = epithelium
• Plasma Membrane

  • Highly selective permeability provided by proteins, channels, carriers and pumps.
  • H2O impermeable due to amphipathic properties of phospholipid bilayer
• Categories of Transport

  1. Simple diffusion (passive)
  2. Channel (passive)
  3. Carriers (passive)
  4. Pumps (active)
• Types of Carriers

  • facilitator/uniport = one ion species, one direction
  • cotransporter/symporter = electroneutral, net movement of change across membraneis the same
  • exchanger/antiporter = swaps ions of same charge
• 1. Functions of Transport Proteins

  • Uptake of: nutrients, substrates, cofactors. Export waste products
  • Regulation of intracellular ions, pH, cell volume (ie. Na-K ATPase pump)
  • Glucose = uniporter
  • Amino acids + Na+ = cotransporter
  • Urea = uniporter
  • H+ + lactate- = exchanger
• Regulation of intracellular ions, pH and cell volume

  • Pumps require ATP to move ions across their concentration gradient, for exampleNa-K ATPase pump
  • pH regulated by acid extrusion and base extrusion.
  • acid extrusion = Na+ into cell, H+ out
  • Base extrusion = Cl- into cell, HCO3- out
  • Regulatory volume increase = Na+, Cl-, K+ into cell
  • Regulatory volume decrease = K+ and Cl- out of cell
• Distribution of H2O and solutes

  *Negatively charged organic molecules and proteins cannot pass the barrier due to selective permeability.

  
• Asymmetric distribution of K+ generates a membrane potential

  • Inside of the cell has -ve charge due to potassium ions (120mM in, 5mM out)
  • Potential difference of -70mV
  • 3 Na+ OUT
  • 2 K+ IN
  • With K+ and -ve organic molecules are trapped within the cell = -ve internal charge.
• Water distribution by osmosis

  • Water moves passively across a membrane via osmosis
  • Water distributes itself so that osmotic pressures (πi and πo) are equal
  • No net movement of water when things are in balance
• Capillary endothelium

  • Thin layer of cells lining blood vessels
  • Highly permeable in some organs (ie kidneys)
  • Important role in regulation of interstitial fluid
• Distribution of solutes across the capillary endothelium

  • Beginning of capillary network = absorption of nutrients and ions for cells
  • End of capillary network = filtration of waste products
  • Beginning of capillary network = higher pressure in plasma than in ISF

  
• Water distribution is decided by hydrostatic and osmotic forces

  • Proteins cannot get out → movement of water
  • Hydrostatic Pressure achieved via contraction of heart pumping
  • High pressure → fluids out
  • Low pressure – H2O with nutrient in
• Epithelia

  • Layers of cells covering internal and external surfaces of organs and tissues
  • protective/barrier function
  • Important in absorption and secretion