Significance of Blood PO and PCO2 Measurements

Since blood PO2 measurements are not directly affected by the oxygen in red blood cells, the PO2 does not provide a measurement of the total oxygen content of whole blood. It does, however, provide a good index of lung function. If the inspired air had a normal PO2 but the arterial PO2 was below normal, for example, you could conclude that gas exchange in the lungs was impaired. Measurements of arterial PO2 thus provide valuable information in treating people with pulmonary diseases, in performing surgery (when breathing may be depressed by anesthesia), and in caring for premature babies with respiratory distress syndrome.

When the lungs are functioning properly, the PO2 of systemic arterial blood is only 5 mmHg less than the PO2 of alveolar air. At a normal PO2 of about 100 mmHg, hemoglobin is almost completely loaded with oxygen. Thus an increase in blood PO2—produced, for example, by breathing 100% oxygen from a gas tank—cannot significantly increase the amount of oxygen contained in the red blood cells. It can, however, significantly increase the amount of oxygen dissolved in the plasma (because the amount dissolved is directly determined by the PO2). If the PO2 doubles, the amount of oxygen dissolved in the plasma also doubles, but the total oxygen content of whole blood increases only slightly. This is because the plasma contains relatively little oxygen compared to the red blood cells.

Since the oxygen carried by red blood cells must first dissolve in plasma before it can diffuse to the tissue cells, however, a doubling of the blood PO2 means that the rate of oxygen diffusion to the tissues would double under these conditions. For this reason, breathing from a tank of 100% oxygen (with a PO2 of 760 mmHg) would significantly increase oxygen delivery to the tissues, although it would have little effect on the total oxygen content of blood.

An electrode that produces a current in response to dissolved carbon dioxide is also used, so that the PCO>2 of blood can be measured together with its PO2. Blood in the systemic veins, which is delivered to the lungs by the pulmonary arteries, usually has a PO2 of 40 mmHg and a PCO>2 of 46 mmHg. After gas exchange in the alveoli of the lungs, blood in the pulmonary veins and systemic arteries has a PO2 of about 100 mmHg and a

Respiratory Physiology 497

Right atrium and ventricle

From pulmonary artery

Alveoli po2= 105 pco2 = 40

To pulmonary

Left atrium and ventricle

To pulmonary

Systemic veins po2 = 40

pco2= 46

Body cells

Body cells

Left atrium and ventricle


Systemic arteries


,= 100 Pco2 = 40 A


■ Figure 16.23 Partial pressures of gases in blood. The PO2 and PCO2 values of blood are a result of gas exchange in the lung alveoli and gas exchange between systemic capillaries and body cells.

Pco2 of 40 mmHg (fig. 16.23). The values in arterial blood are relatively constant and clinically significant because they reflect lung function. Blood gas measurements of venous blood are not as useful because these values are far more variable. Venous PO2 is much lower and Pco2 much higher after exercise, for example, than at rest, whereas arterial values are not significantly affected by moderate physical activity.

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  • ferdinand
    Why po2 is lower in systemic arterial blood?
    8 years ago
  • celio
    What is the pco2 for pulmonary arteries?
    8 years ago
  • olga sabbatini
    What is the importance of pco2 and p02 in the blood?
    8 years ago
    What is the significance of the blood po2 measurements?
    8 years ago
  • ralph
    How much mmHg PCO2 in right atrium?
    7 years ago
  • cedivar
    What is the significance of elevated pO2?
    7 years ago
  • xavier
    Why is PO2 lungs 100 mmHg?
    7 years ago
  • Mikaela
    What is signifance of a hight po2?
    7 years ago
  • heather
    What are the normal measurements for PO2, PCO2 and pH in systemic arterial blood?
    7 years ago
  • IOLO
    Why measure PO2 and PCO2?
    6 years ago
    What is PO2and PCO2 of systemic blood?
    4 years ago
  • simone
    What is is PO2 and PCO2of systemic blood?
    4 years ago
    How blood pco2 and blood po2 measured?
    4 years ago
  • anita
    What is pco2 measurement of in anatomy?
    3 years ago
  • daniel
    What i s po2 measure normal evel?
    1 year ago
  • lowell
    How would po2 in the systemic arteries be impacted?
    12 months ago
  • juliane
    Why is PCO2 and PO2 so important?
    10 months ago
  • bellina
    What is the clinical significance of a high Po2?
    8 months ago
  • agostino
    What does a Paco2 blood level measure?
    8 months ago
  • ilaria
    What is significance of pH 7.560 and low pCO2?
    7 months ago
  • george
    What doesa po2, arterial of 204mm hg indicate?
    6 months ago
  • jan stewart
    What is the Po2 of oxygebated blood from lungs?
    6 months ago
  • Bertoldo
    What does po2 over 104 mean?
    6 months ago
    What does a venous pco2 of 58 mean?
    6 months ago
  • james
    What is the optimal pCO2 level in pulmonary hypertension?
    5 months ago
  • anastasio
    How does po2 and pco2 affect gas exchange?
    5 months ago
  • Christina
    What is the po2 of the blood in the pulmonary vein?
    5 months ago
  • J
    What is significnce of pO2 in ABGS?
    3 months ago
    What proportion of air to blood produces a significant change in po2 and pco2 values?
    3 months ago
  • JENS
    What is normal pO2 of blood in body tissues?
    3 months ago
  • Helena
    What cuases the change of po2 and pco2 levels from the arteries to the pulmonary arteries?
    2 months ago
  • patricia
    How is paco2 measured?
    27 days ago
  • James
    Is PC02 blood oxygen level?
    23 days ago

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