text: The symbol for pressure is usually  or  which may use a subscript to identify the pressure, and gas species are also referred to by subscript. When combined, these subscripts are applied recursively.
  Examples:
  * or  = pressure at time 1
  * or  = partial pressure of hydrogen
  * or  or PO = arterial partial pressure of oxygen
  * or  or PO = venous partial pressure of oxygen