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 in the cir ...

. There are several significant purposes for measuring gas tension. The most common gas tensions measured are

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

tension (P_xO₂),

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

tension (P_xCO₂) and

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

tension (P_xCO). The subscript ''x'' in each symbol represents the source of the gas being measured: "''a''" meaning

arterial An artery (plural arteries) () is a blood vessel in humans and most animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pu ...

, "''A''" being alveolar, "''v''" being

venous Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...

, and "''c''" being

capillary 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 bod ...

. Blood gas tests (such as arterial blood gas tests) measure these partial pressures.

Oxygen tension

;Arterial blood oxygen tension (normal) P_aO₂ – Partial pressure of oxygen at sea level (160

mmHg A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg. Although not an ...

in the atmosphere, 21% of standard atmospheric pressure of 760 mmHg) in arterial blood is between 75 mmHg and 100 mmHg. ;Venous blood oxygen tension (normal) P_vO₂ – Oxygen tension in

venous blood Venous blood is deoxygenated blood which travels from the peripheral blood vessels, through the venous system into the right atrium of the heart. Deoxygenated blood is then pumped by the right ventricle to the lungs via the pulmonary artery wh ...

at sea level is between 30 mmHg and 40 mmHg.

Carbon dioxide tension

Carbon dioxide is a by-product of food metabolism and in high amounts has toxic effects including:

dyspnea Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing di ...

acidosis Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma. The term ''acidemia'' describe ...

and altered consciousness. ;Arterial blood carbon dioxide tension P_aCO₂ – Partial pressure of carbon dioxide at sea level in arterial blood is between 35 mmHg and 45 mmHg. ;Venous blood carbon dioxide tension P_vCO₂ – Partial pressure of carbon dioxide at sea level in venous blood is between 40 mmHg and 50 mmHg.

Carbon monoxide tension

;Arterial carbon monoxide tension (normal) P_aCO – Partial pressure of CO at sea level in arterial blood is approximately 0.02. 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_aCO₂ and (and

lactate Lactate may refer to: * Lactation, the secretion of milk from the mammary glands * Lactate, the conjugate base of lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with w ...

) suggest to the health care practitioner which interventions, if any, should be made.

Equations

Oxygen content

C_a \ce = 1.36 \cdot \ce \cdot \frac + 0.0031 \cdot P_a \ce

The constant, 1.36, is the amount of oxygen (ml at 1 atmosphere) bound per gram of

hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...

. The exact value of this constant varies from 1.34 to 1.39, depending on the reference and the way it is derived. S_aO₂ 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_aO₂ (as in a

hyperbaric chamber A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of ...

) or in severe

anemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, t ...

Oxygen saturation

\ce = \left(\frac +1\right)^

This is an estimation and does not account for differences in temperature, pH and concentrations of 2,3 DPG.

References

{{reflist Hematology Respiratory therapy Pulmonology