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The values of pH and HCO3 of venous blood are close enough to arterial blood for direct comparison. The pCO2 of venous blood is less reliably compared to arterial blood but may be used in some cases. The PO2 level of venous blood is always significantly lower that arterial and should be reported, labeled and interpreted as venous PO2. [21]
Arterial blood oxygen tension (normal) P a O 2 – Partial pressure of oxygen at sea level (160 mmHg (21.3 kPa) in the atmosphere, 21% of the standard atmospheric pressure of 760 mmHg (101 kPa)) in arterial blood is between 75 and 100 mmHg (10.0 and 13.3 kPa).
The alveolar oxygen partial pressure is lower than the atmospheric O 2 partial pressure for two reasons.. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.
Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide, or both cannot be kept at normal levels. A drop in the oxygen carried in the blood is known as hypoxemia ; a rise in arterial carbon dioxide levels is called hypercapnia .
A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg. Normally, the A–a gradient increases with age. For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg. A conservative estimate of normal A–a gradient is [age in years + 10]/ 4.
Normal values for humans are in the range 35–45 mmHg. Values less than this may indicate hyperventilation and (if blood pH is greater than 7.45) respiratory alkalosis. Values greater than 45 mmHg may indicate hypoventilation, and (if blood pH is less than 7.35) respiratory acidosis. [2] [3]
The alveolar gas equation is the method for calculating partial pressure of alveolar oxygen (p A O 2).The equation is used in assessing if the lungs are properly transferring oxygen into the blood.
A blood gas test or blood gas analysis tests blood to measure blood gas tension values and blood pH.It also measures the level and base excess of bicarbonate.The source of the blood is reflected in the name of each test; arterial blood gases come from arteries, venous blood gases come from veins and capillary blood gases come from capillaries. [1]