Biomedical Engineering Reference
In-Depth Information
other hand, the intracellular pH, which varies from one part to another, is about 7.0 on
average [142]. Certain body organs require specifi c low pH, e.g.
pH 2 in the stom-
ach, or high pH, e.g.
pH 8 in the small intestine environments, for normal function.
Abnormalities in pH are assumed to be of primary physiological signifi cance [143].
The two main organs regulating the pH disturbance (acid-base balance) are the
lungs and kidneys. The body is able to respond to changes in the pH either immedi-
ately through respiratory control of O
2
and CO
2
gases, or by slowly adapting to the pH
changes through the kidneys through regulation of the dissolved CO
2
or bicarbonate in
the plasma [143].
Moreover, several buffer systems exist in the body, such as proteins, phosphates,
and bicarbonates. Proteins are the most important buffers in the body. Protein mol-
ecules contain multiple acidic and basic groups that make protein solution a buffer that
covers a wide pH range. Phosphate buffers (HPO
2
4
/H
2
PO
4
) are mainly intracellular.
The pK of this system is 6.8 so that it is moderately effi cient at a physiological pH of
7.4. The concentration of phosphate is low in the extracellular fl uid but the phosphate
buffer system is an important urinary buffer. Bicarbonate (H
2
CO
3
/HCO
3
) is also
involved in pH control but it is not an important buffer system because normal blood
pH 7.4 is too far from its pK 6.1 [144].
Acid-base balance involves chemical and physiological processes responsible for
the maintenance of the pH of body fl uids at levels that allow optimal function of the
whole individual. The ability for the body to regulate pH is critically important in
maintaining the operation of many cellular enzymes and the function of vital organs,
such as the brain and the heart [143].
The importance of
in-vivo
pH measurement has been demonstrated in many clinical
studies. Some examples of these studies, which involved the applications of pH meas-
urements in several body organs, are discussed in this section.
10.5.2 Measurement of pH in blood
Our human health depends on a balanced and buffered blood pH. For instance, human
blood is slightly basic with a pH between 7.3 and 7.5. If the blood pH drops below
7.3, acidosis occurs. If the blood pH rises above 7.5, alkalosis occurs. Adverse events
including death will occur if a signifi cant change in the blood pH goes below 6.8 or
above 7.8. pH levels in blood are frequently determined in clinical laboratories [145].
pH, pO
2
, pCO
2
, and bicarbonate [HCO
3
], known as blood gases, are four impor-
tant parameters in blood chemistries. Clinical problems of pH are all related to the pH
of the plasma of whole blood. pH in extracellular fl uid is always close to that of blood.
pH inside cells differs from that of blood but it is not recognized as being an important
clinical problem apart from blood pH changes [144]. Changes in plasma pH refl ect pH
changes in other compartments, such as extracellular fl uid and intracellular fl uid.
The
in-vivo
pH of red blood cell hemolysates was measured in 20 normal subjects
and it was determined that the mean arterial red cell was a pH of 7.19 (7.15 to 7.22). The
mean difference in pH between plasma and cells was determined to be 0.21, ranging
from 0.15 to 0.23 [146].
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