Medical Lecture Notes

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Monday, 15 January 2018

Body fluids

Body fluids 

compose of :

  1. water 
  2. dissolved substances (solutes)

Water :

  • Provide form of body structure
  • Protective cushion:  acts as shock absorber e.g amniotic fluid and CSF
  • Transport:  act as a medium for delivery of nutrients and removal metabolic waste
  • Provide medium for metabolic reactions within the cell
  • Act as a solvent:dissolve ions e.g NaCl and protein and solutes within the cell
  • Act as a lubricants e.g synovial fluid
  • Temperature regulation


  • Electrolytes:  Charged particles
  • Cations  à positively charged ions e.g Na+, K+, Ca++ and H+
  • Anions  negatively charged ions e.g Cl-,     HCO3-, and PO4-
  • Non electrolytes: Uncharged particles e.g  urea, glucose, O2 and CO2
  • Electrolytes  are  molecules  that  dissociate  in      water  to  their  cation  and  anion  equivelants.
  •  Why  don’t  they  reassociate  ?? because  of  the  net  charge  on  water  molecules.

Electrolytes :

  • Electrolytes are the charged particles (ions) that are dissolved in body fluids
  • Electrolytes are major component of body fluids enter in food and drink
  • The major cation and anion in body fluids are :
  1. Major Positive Ions: Cations

  • Sodium ion, Na+
  • Potassium ion, K+
  • Calcium ion, Ca2+
  • Magnesium ion, Mg2+

  1. Major Negative Ions: Anions

  • Chloride ion, Cl-
  • Bicarbonate ion, HCO3-
  • Phosphate ions, H2PO4- & HPO42-
  • Sulfate, SO42-
  • Organic Acids
  • Proteins

  • Electrolytes must be maintained in a narrow concentration range in order for cells to function properly. 
  • Even small deviation can have serious consequences . 
  • Each fluid compartment need  just the right types and levels of electrolytes for proper functioning  of neuron, muscle cells and other cells in the body. 
  • The electrolytes composition of ECF and ICF have significant differences Proteins and phosphate  are the major intracellular ionsOther negative ions found in the intracellular fluid are bicarbonate, chloride, organic acids,  and sulfate 
  • The composition of interstitial fluid is almost identical to that of blood plasma, except for one negative electrolyte - proteinTo summarize, the major positive ion of the extracellular fluid is sodium and the major negative ion is chloride. 
  • The major positive ion of the intracellular fluid is potassium and the major negative ions are protein and phosphates


  • The difference between plasma and interstitial fluid that plasma contain protein

Function of electrolytes

  1. Co factor for enzymes: co factors acts together with enzymes to speed up reactions in the body eg Ca, Mg.
  2. Action potentials in neuron and muscle cells
  3. Secretion and action of hormones and neurotransmitters
  4. Muscle contraction
  5. Maintain acid - base balance
  6. Secondary active transport
  7. Control fluid movement  between fluid compartments through osmosis

Role of sodium

  • Nerve conduction and muscle conduction
  • Regulation of water movement à if Na+ level in the plasma change, those changes determine fluid levels in the other compartment
  • Hypernatremia  cell shrink  signs of CNS dysfunction such as confusion

Role of potassium

  • K+ is responsible for intracellular fluid volume(Osmosis)
  • Key role in maintaining resting membrane potential à nerve muscle conduction, muscle contraction and maintenance of cardiac rhythm
  • Acid – base balance. In acidosis H+ enter the cell in exchange for K+ .
  • In Alkasosis K+ enter the cells in exchange for H+

Role of calcium

  • Ca++ is crucial to normal body function, even small change in Ca++ concentration can be deadly.
  • Normally, total Ca++ levels in the plasma vary between 9-11mg/dl
  • Hypercalcaemia à dysrhythmia , Fatigue, confusion and heart arrest (the heart can stop if the Ca++ level gets too high)
  • Hypocalcaemia à muscle spasms can occur
  • If ca++ level is very low a person can go into tetanus and breathing will stop

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