Siemens RAPIDLAB 248 Operator's Manual page 129

Operating Principles
pO
Figure G-4. pO2 sensor (cutaway view)
Measuring pH and Blood Gases
pH
pH expresses the hydrogen ion activity in a solution as the negative
logarithm of the hydrogen ion concentration:
where cH
The hydrogen ion is the determinant of the acidity of blood or plasma.
Normal cellular metabolism requires an exacting environment where
hydrogen ion concentration must be maintained within narrow limits.
Hydrogen ion activity reflects the acid-base balance within blood. Acids
donate hydrogen ions; bases remove hydrogen ions. The lungs, kidneys
and blood all work to maintain the acid-base status within the strict
limits necessary.
The Henderson-Hasselbalch equation describes how pH expresses the
interaction of acid and base in blood:
where K is the dissociation constant, which describes the ability of a
solution to release hydrogen ions. Since K, and therefore pK, is a
constant, this equation can be used to demonstrate that pH is
proportional to the acid-base concentrations in blood.
pH is clinically significant as a means to determine acid-base
disturbances. Acid-base disorders can result in several pathological
conditions. An acid-base disorder resulting initially from ventilatory
dysfunction is called a primary respiratory acidosis or alkalosis, while
one due to renal or gastrointestinal inadequacy is referred to as
metabolic acidosis or alkalosis. Using acceptable therapeutic ranges, a
pH less than 7.3 indicates acidosis, and a pH greater than 7.5 indicates
alkalosis
Sensor
2
pH = –logcH +
is the molar concentration of hydrogen ions.
+
base
pH = pK + log
acid
15
.
Sensor Contacts
Sample Path
O-Ring
Page G-5