Pulmonary gas pressures
Encyclopedia
The factors that determine the values for alveolar PO2 and PCO2 are:
Following is a list of average partial pressure
s (in torr
) for a human at rest:
The alveolar oxygen partial pressure
is lower than the atmospheric O2 partial pressure for two reasons.
The alveolar pO2 is not routinely measured but is calculated from blood gas measurements by the Alveolar gas equation
.
The pCO2, along with the pH, can be used to distinguish among metabolic acidosis
, metabolic alkalosis
, respiratory acidosis
, and respiratory alkalosis
.
exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values.
Hyperventilation
exists when the same ratio decreases.
- The pressure of outside air
- The partial pressures of inspired oxygen and carbon dioxide
- The rates of total body oxygen consumption and carbon dioxide production
- The rates of alveolar ventilation and perfusion
Following is a list of average partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
s (in torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
) for a human at rest:
Partial pressure of oxygen
| Location | pO2 (mmHg) >- | Ambient air |
AO2) >- | Arterial blood |
aO2) >- | Venous blood |
>- | 20-40 |
|---|
The alveolar oxygen partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
is lower than the atmospheric O2 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.
- The rest of the difference is due to the continual uptake of oxygen by the pulmonary capillaries, and the continual diffusion of CO2 out of the capillaries into the alveoli.
The alveolar pO2 is not routinely measured but is calculated from blood gas measurements by the Alveolar gas equation
Alveolar gas equation
The partial pressure of oxygen in the pulmonary alveoli is required to calculate both the alveolar-arterial gradient of oxygen and the amount of right-to-left cardiac shunt, which are both clinically useful quantities. However it is not practical to take a sample of gas from the alveoli in order...
.
Partial pressure of carbon dioxide
| Location | pCO2 (Torr) Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... >- | Outside air - dry air at sea level |
>- | >- | >- | >- | 50 |
|---|
The pCO2, along with the pH, can be used to distinguish among metabolic acidosis
Metabolic acidosis
In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, i.e., blood pH is low due to increased production of hydrogen by the body or the...
, metabolic alkalosis
Metabolic alkalosis
Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range . This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct result of increased bicarbonate concentrations.-Terminology:*Alkalosis...
, respiratory acidosis
Respiratory acidosis
Respiratory acidosis is a medical condition in which decreased ventilation causes increased blood carbon dioxide concentration and decreased pH ....
, and respiratory alkalosis
Respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH...
.
Pathology
HypoventilationHypoventilation
In medicine, hypoventilation occurs when ventilation is inadequate to perform needed gas exchange...
exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values.
Hyperventilation
Hyperventilation
Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
exists when the same ratio decreases.