The factors that determine the values for
alveolar pO
2 and
pCO2 are:
*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
Partial pressures
The partial pressures (in
torr
The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be the same as one " millimeter of merc ...
) for a human at rest:
Partial pressure of oxygen (at sea level)
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.
* The rest of the difference is due to the continual uptake of oxygen by the
pulmonary
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...
capillaries
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
, and the continual diffusion of CO
2 out of the capillaries into the alveoli.
The alveolar pO
2 is not routinely measured but is calculated from blood gas measurements by the
alveolar gas equation.
Partial pressure of carbon dioxide
Pathology
The partial pressure of carbon dioxide, along with the
pH, can be used to differentiate between
metabolic acidosis
Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys ...
,
metabolic alkalosis,
respiratory acidosis, and
respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35–7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four primary dis ...
.
Hypoventilation
Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapni ...
exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values – greater than 45mmHg. If pH is also less than 7.35 this is
respiratory acidosis.
Hyperventilation exists when the same ratio decreases – less than 35mmHg. If the pH is also greater than 7.45 this is
respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35–7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four primary dis ...
.
Leticia Godoy Dias Sanderson. Gasometria arterial - Artigo de revisão. Fevereiro 2012.
See also
* Alveolar-arterial gradient
* Diffusing capacity
Diffusing capacity of the lung (DL) (also known as Transfer factor is another expression for the formerly used diffusing capacity.) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. It is part of a c ...
* Pulmonary alveolus
References
{{Respiratory physiology
Respiratory physiology