Blood Gas Measurements

Measurement of the oxygen content of blood (in ml O2 per 100 ml blood) is a laborious procedure. Fortunately, an oxygen electrode that produces an electric current in proportion to the concentration of dissolved oxygen has been developed. If this electrode is placed in a fluid while oxygen is artificially bubbled into it, the current produced by the oxygen electrode will in

Table 16.5

Effect of Altitude on Partial Oxygen Pressure (Po2)

Altitude (Feet

Atmospheric

PO2 in Air

PO2 in Alveoli

Po2 in Arterial

Above Sea Level)

Pressure (mmHg)

(mmHg)

(mmHg)

Blood (mmHg)

0

760

159

105

100

2,000

707

148

97

92

4,000

656

137

90

85

6,000

609

127

84

79

8,000

564

118

79

74

10,000

523

109

74

69

20,000

349

73

40

35

30,000

226

47

21

Terminal Bronchioles

Bronchiole

Terminal, bronchiole

Respiratory bronchiole

Alveoli

Terminal, bronchiole

Respiratory bronchiole

Alveoli

■ Figure 16.21 The relationship between alveoli and blood vessels. The extensive area of contact between the pulmonary capillaries and the alveoli allows for rapid exchange of gases between the air and blood.

crease up to a maximum value. At this maximum value, the fluid is saturated with oxygen—that is, all of the oxygen that can be dissolved at that temperature and PO2 is dissolved. At a constant temperature, the amount dissolved, and thus the electric current, depend only on the PO2 of the gas.

As a matter of convenience, it can now be said that the fluid has the same PO2 as the gas. If it is known that the gas has a PO2 of 152 mmHg, for example, the deflection of a needle by the oxygen electrode can be calibrated on a scale at 152 mmHg (fig. 16.22). The actual amount of dissolved oxygen under these circumstances is not particularly important (it can be looked up in solubility tables, if desired); it is simply a linear function of the PO2. A lower PO2 indicates that less oxygen is dissolved; a higher PO2 indicates that more oxygen is dissolved.

If the oxygen electrode is next inserted into an unknown sample of blood, the PO2 of that sample can be read directly from the previously calibrated scale. Suppose, as illustrated in figure 16.22, the blood sample has a PO2 of 100 mmHg. Since alveolar air has a PO2 of about 105 mmHg, this reading indicates that the blood is almost in complete equilibrium with the alveolar air.

First step: Calibration of O2 electrode

Second step: Measurement of arterial Po2

Certified gas cylinder

O2 electrode

Certified gas cylinder

O2 electrode

Gas Cylinder

Total pressure = 760 mmHg % O2 = 20% Pq2 = 152 mmHg

Calibrate to PO2 of gas

Total pressure = 760 mmHg % O2 = 20% Pq2 = 152 mmHg

Calibrate to PO2 of gas

Dissolved O2

Dissolved O2

Arterial Blood Sample
Arterial blood sample
Human Blood Gases

■ Figure 16.22 Blood gas measurements using the PO2 electrode. (a) The electrical current generated by the oxygen electrode is calibrated so that the needle of the blood gas machine points to the PO2 of the gas with which the fluid is in equilibrium. (b) Once standardized in this way, the electrode can be inserted into a fluid such as blood, and the PO2 of this solution can be measured.

The oxygen electrode responds only to oxygen dissolved in water or plasma; it cannot respond to oxygen that is bound to hemoglobin in red blood cells. Most of the oxygen in blood, however, is located in the red blood cells attached to hemoglobin. The oxygen content of whole blood thus depends on both its Po2 and its red blood cell and hemoglobin content. At a Po2 of about 100 mmHg, whole blood normally contains almost 20 ml O2 per 100 ml blood; of this amount, only 0.3 ml of O2 is dissolved in the plasma and 19.7 ml of O2 is found within the red blood cells (see fig. 16.32). Since only the 0.3 ml of O2 affects the PO2 measurement, this measurement would be unchanged if the red blood cells were removed from the sample.

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Responses

  • luis
    Where are the terminal bronchioli?
    7 years ago
  • Leila
    Where is the terminal bronchiole?
    7 years ago

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