Blood gas tension refers to the
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
of
gases in
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells.
Blood is com ...
.
There are several significant purposes for measuring gas tension.
The most common gas tensions measured are
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
tension (P
xO
2),
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
tension (P
xCO
2) and
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
tension (P
xCO).
The subscript ''x'' in each symbol represents the source of the gas being measured: "''a''" meaning
arterial
An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...
, "''A''" being
alveolar, "''v''" being
venous, and "''c''" being
capillary
A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the inn ...
.
Blood gas tests (such as
arterial blood gas tests) measure these partial pressures.
Oxygen tension
;Arterial blood oxygen tension (normal)
P
aO
2 – Partial pressure of oxygen at sea level ( in the atmosphere, 21% of the standard atmospheric pressure of ) in
arterial blood
Arterial blood is the oxygenated blood in the circulatory system found in the pulmonary vein, the left chambers of the heart, and in the artery, arteries. It is bright red in color, while venous blood is dark red in color (but looks purple through ...
is between .
;Venous blood oxygen tension (normal)
P
vO
2 – Oxygen tension in
venous blood at sea level is between .
Carbon dioxide tension
Carbon dioxide is a by-product of food metabolism and in high amounts has toxic effects including:
dyspnea,
acidosis and
altered consciousness.
;Arterial blood carbon dioxide tension
PaCO2 – Partial pressure of carbon dioxide at sea level in arterial blood is between .
;Venous blood carbon dioxide tension
P
vCO
2 – Partial pressure of carbon dioxide at sea level in venous blood is between .
Carbon monoxide tension
;Arterial carbon monoxide tension (normal)
P
aCO – Partial pressure of CO at sea level in arterial blood is approximately . It can be slightly higher in smokers and people living in dense urban areas.
Significance
The partial pressure of gas in blood is significant because it is directly related to
gas exchange, as the driving force of
diffusion across the
blood gas barrier and thus blood
oxygenation.
When used alongside the
pH balance of the blood, the P
aCO
2 and
(and
lactate) suggest to the health care practitioner which interventions, if any, should be made.
Equations
Oxygen content
:
The constant, 1.36, is the amount of oxygen (ml at 1 atmosphere) bound per gram of
hemoglobin
Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...
. The exact value of this constant varies from 1.34 to 1.39, depending on the reference and the way it is derived. S
aO
2 refers to the percent of arterial hemoglobin that is saturated with oxygen. The constant 0.0031 represents the amount of oxygen dissolved in plasma per mm Hg of partial pressure. The dissolved-oxygen term is generally small relative to the term for hemoglobin-bound oxygen, but becomes significant at very high P
aO
2 (as in a
hyperbaric chamber) or in severe
anemia
Anemia (also spelt anaemia in British English) is a blood disorder in which the blood has a reduced ability to carry oxygen. This can be due to a lower than normal number of red blood cells, a reduction in the amount of hemoglobin availabl ...
.
Oxygen saturation
:
This is an estimation and does not account for differences in temperature, pH and concentrations of 2,3 DPG.
See also
*
Alveolar air equation
*
Fick's laws of diffusion
*
Fraction of inspired oxygen
References
{{reflist
Hematology
Respiratory therapy
Pulmonology