You do not need much chemistry to understand basic physiological mechanisms but you do need
 to know what atoms, molecules and ions are……

 An atom is a very small object indeed. There are millions of atoms in the ink that makes up this
 full stop                    .

 Elements are pure substances containing only one type of atom. Elements have their own
 shorthand symbol eg. carbon = C, Nitrogen = N etc. Here are a few common elements….

hydrogen nitrogen carbon oxygen sodium phosphorus sulphur chlorine iron calcium
   H          N          C        O        Na            P          S         Cl      Fe       Ca

 A molecule is two or more atoms joined together by a chemical bond. For example, water is
 made up of two hydrogen and one oxygen atom and is written H2O.

             O                        O         O                    O         C          O
        H          H
                                 an oxygen gas molecule              a carbon dioxide gas
             water                   is two atoms of              molecule is two atoms of
                                       oxygen - O2                 oxygen and one carbon
                                                                          atom - CO2
 Some atoms and molecules carry a charge. These are called ions. Ions are very common in
 physiological systems, in fact you are unlikely to find some atoms and molecules in non-ionic
 form. Ions are very important in physiology and many processes such as nervous conduction
 would not work without them.

 Here are some common ions:

  sodium ion    Na+              hydrogen ion    H+           phosphate ion          PO4 3-
  potassium ion K+               chloride ion    Cl -         hydrogen carbonate ion HCO3-
  calcium ion   Ca2+             sulphate ion    SO42-

 Like magnet poles, opposites attract and some oppositely charged ions bond together. An
 example of this is salt, sodium chloride formed by one sodium ion and one chloride ion.

                                    in water they
            Na+         Cl-        dissociate into                 Na+              Cl-
                                    separate ions

 Positively charged ions are called cations. Negatively charged ions are called anions. Ions are
 soluble in water and all of the above cations and anions are found as electrolytes in body fluids.

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