Partial Pressure of Oxygen and Carbon Dioxide

What Is Partial Pressure of Gases?

The partial pressure of a gas is the pressure that one gas in a mixture would exert if it alone filled the whole space, and the total pressure of the mixture is the sum of all the partial pressures. This rule is Dalton's law of partial pressures, and it is the starting point for understanding how the lungs load and unload gases.

Dalton's law in air

To find a partial pressure, multiply the fraction of the gas by the total pressure. At sea level, air presses with about 760 mmHg, and oxygen makes up about 20.9% of dry air, so the partial pressure of oxygen (PO2) is about 0.209 × 760 ≈ 159 mmHg, usually rounded to 160. Nitrogen, at about 78%, contributes most of the rest, while carbon dioxide is only a trace of about 0.3 mmHg. The table below lists each gas.

Partial pressure in a liquid

Gases also dissolve in liquids such as blood plasma, and partial pressure describes how strongly a dissolved gas tends to escape. By Henry's law, the amount of gas that dissolves equals its partial pressure multiplied by its solubility in that liquid. When blood and alveolar air reach balance, the gas has the same partial pressure in both, even though the amounts are very different, which is why partial pressures in air and blood can be compared directly.

Explained simply

Imagine a crowded room where everyone is pushing on the walls. The total push is the air pressure, and the share of the push that comes from one group of people, say everyone wearing red, is that group's partial pressure. Oxygen makes up about a fifth of the crowd, so it supplies about a fifth of the push, and like people leaving a packed room, gas always drifts toward where its own crowd is thinner.

What Is Partial Pressure of Oxygen?

The partial pressure of oxygen (PO2) is the pressure produced by oxygen alone in air or in a liquid; in the arteries it is written PaO2 and is normally about 75 to 100 mmHg in a healthy adult at sea level. It measures the oxygen dissolved in the plasma, which is the oxygen that drives loading of hemoglobin and diffusion into the tissues.

Reading the symbols

Physiology uses a short code for where a value is measured. A capital P means pressure; a capital A means alveolar air, as in PAO2; a small a means arterial blood, as in PaO2; and a small v means venous blood, as in PvO2. A capital I means inspired air, as in PIO2. Mixing up PAO2 and PaO2 is one of the most common mistakes, because they differ by just one letter.

The alveolar gas equation

Alveolar PO2 is lower than the PO2 of outside air, and the alveolar gas equation shows why. Inhaled air is first humidified, and water vapor at body temperature has a pressure of about 47 mmHg, so inspired PO2 becomes 0.21 × (760 - 47) ≈ 150 mmHg. Carbon dioxide arriving from the blood then takes up part of the alveolar space, so PAO2 ≈ 150 - PaCO2 ÷ 0.8 = 150 - 40 ÷ 0.8 = 100 mmHg, where 0.8 is the typical ratio of carbon dioxide made to oxygen used.

Normal values and age

Arterial PO2 is a little lower than alveolar PO2, by about 5 to 10 mmHg in young adults, because a small amount of blood bypasses ventilated alveoli and because ventilation and blood flow are never perfectly matched. This gap, the alveolar-arterial (A-a) gradient, widens with age, so normal PaO2 drifts downward in older adults. Mixed venous blood returning to the lungs has a PO2 of about 40 mmHg.

What Is Partial Pressure of Carbon Dioxide?

The partial pressure of carbon dioxide (PCO2) is the pressure produced by carbon dioxide alone; in arterial blood it is written PaCO2 and is normally about 35 to 45 mmHg, with about 40 mmHg as the textbook value. Blood returning from the body has a PCO2 of about 45 to 46 mmHg, and alveolar air holds about 40 mmHg.

A mirror of ventilation

PaCO2 is set almost entirely by how much air reaches the alveoli. It is inversely proportional to alveolar ventilation: if alveolar ventilation doubles while carbon dioxide production stays the same, PaCO2 roughly halves, and if ventilation halves, PaCO2 roughly doubles. A high PaCO2, called hypercapnia, means breathing is not keeping up, while a low PaCO2, called hypocapnia, means the person is breathing more than needed, as described on hyperventilation and hypoventilation.

Carbon dioxide and blood pH

Carbon dioxide reacts with water to form carbonic acid, so PaCO2 directly affects the acidity of the blood. A rise in PaCO2 lowers pH, and a fall raises it. Most carbon dioxide travels in the blood as bicarbonate, as explained on our carbon dioxide transport page.

The main signal for breathing

Because PaCO2 tracks ventilation so closely, it is the main chemical signal the brain uses to set breathing. Central chemoreceptors in the medulla respond to the acidity that carbon dioxide creates in the fluid around the brain, and even a small rise in PaCO2 increases breathing. Oxygen plays a smaller role at rest: the peripheral chemoreceptors of the carotid and aortic bodies respond strongly only when PaO2 falls below about 60 mmHg. The whole system is described on control of breathing.

How Does the Partial Pressure of Oxygen Change From Air to Cells?

The partial pressure of oxygen falls step by step from about 160 mmHg in outside air to only a few mmHg inside the mitochondria of working cells, a downhill path called the oxygen cascade. Each drop has a physical cause, and the slope of the cascade is what keeps oxygen moving toward the cells, as shown in the diagram at the top of the page.

1. Humidification: air warmed and moistened in the nose and trachea gains water vapor, which dilutes oxygen from about 160 to about 150 mmHg.
2. Mixing in the alveoli: fresh air mixes with the air left in the lungs and with carbon dioxide arriving from the blood, so PO2 falls to about 100 mmHg in the alveoli.
3. Into arterial blood: blood leaving the lungs is nearly in balance with alveolar air, but a little blood that bypassed the alveoli mixes in, so PaO2 is about 95 to 100 mmHg.
4. Into the tissues: in the systemic capillaries, oxygen diffuses into the cells, and blood leaves with a PO2 of about 40 mmHg.
5. Inside the cells: mitochondria use oxygen continuously in cellular respiration, so the PO2 there is only a few mmHg.

Carbon dioxide follows the reverse path. It is made in the cells, where its partial pressure is highest, diffuses into the blood at about 45 to 46 mmHg, crosses into alveolar air at about 40 mmHg and is breathed out into air that holds almost none.

Why Do Gases Move From High to Low Partial Pressure?

Gases move from high to low partial pressure because their molecules are in constant random motion, and more molecules cross from the crowded side to the less crowded side than the other way, until the partial pressures are equal. This net movement is diffusion, and it needs no energy from the cells.

In the lungs, the oxygen gradient between alveolar air (about 100 mmHg) and the blood arriving from the body (about 40 mmHg) is about 60 mmHg. The carbon dioxide gradient is much smaller, about 5 mmHg, but carbon dioxide still crosses easily because it dissolves in water about 20 times more readily than oxygen. The full process across the respiratory membrane is described on our gas exchange page, and the matching exchange in the tissues on external and internal respiration.

Why so little oxygen is dissolved

Oxygen dissolves poorly in plasma: at a PaO2 of 100 mmHg, only about 0.3 mL of oxygen dissolves in each 100 mL of blood. Hemoglobin solves the problem by binding oxygen inside red blood cells, so the same blood carries about 20 mL of oxygen per 100 mL, roughly 65 times more. The dissolved oxygen is small in amount but sets the PaO2, and the PaO2 in turn decides how full the hemoglobin becomes, as described on our oxygen transport page.

Partial Pressure of Oxygen vs Oxygen Saturation

Partial pressure of oxygen (PaO2) measures the pressure of oxygen dissolved in arterial plasma, in mmHg, while oxygen saturation (SaO2, or SpO2 when measured with a pulse oximeter) measures the percentage of hemoglobin binding sites that carry oxygen. PaO2 is the cause and saturation is the result: the higher the PaO2, the more hemoglobin is loaded, up to nearly 100%.

The S-shaped link

The two values are linked by the oxygen-hemoglobin dissociation curve, which is S-shaped rather than straight. Typical points on the curve for normal adult blood are:

• PaO2 about 100 mmHg: saturation about 97 to 98%.
• PaO2 about 60 mmHg: saturation still about 90%.
• PaO2 about 40 mmHg: saturation about 75%, typical of venous blood at rest.
• PaO2 about 27 mmHg: saturation about 50%, a point called the P50.

Why the difference matters

Because the top of the curve is flat, PaO2 can fall from 100 to about 60 mmHg while saturation drops only from about 98% to about 90%. Below that point the curve is steep, and small further drops in PaO2 cause large falls in saturation. Saturation also says nothing about how much hemoglobin there is, so a person with anemia can have a normal PaO2 and saturation yet carry less oxygen in total. Our oxygen saturation page explains pulse oximetry.

How Do Altitude and Diving Change Partial Pressure?

Altitude lowers the partial pressure of oxygen because the total air pressure falls while oxygen stays at about 21% of the air, and diving raises it because the pressure of the surrounding water adds to the pressure of the breathing gas. In both cases the percentage of oxygen does not change; only the total pressure does.

High altitude

At about 5,500 meters (18,000 feet), air pressure is about half of its sea-level value, so inspired PO2 is roughly halved too. On the summit of Mount Everest, barometric pressure is about 253 mmHg, which gives an inspired PO2 of only about 43 mmHg after humidification. The body responds by breathing faster and deeper, which lowers PaCO2 and lets alveolar PO2 rise a little, as explained on breathing at high altitude.

Diving

Under water, every 10 meters (about 33 feet) of seawater adds roughly one more atmosphere of pressure. At 10 meters a diver breathing compressed air has about twice the sea-level partial pressure of every gas in it. High partial pressures of nitrogen can cause nitrogen narcosis, and very high partial pressures of oxygen can be toxic to the nervous system and lungs, which is why divers' breathing gases are planned around partial pressure rather than percentage.

How Is Partial Pressure Measured in the Body?

The partial pressures of oxygen and carbon dioxide in blood are measured with an arterial blood gas (ABG) test, in which a small sample of blood, usually from the radial artery at the wrist, is run through an analyzer within minutes. The same test reports pH and bicarbonate, giving a picture of both breathing and acid-base balance.

How the analyzer works

Blood gas analyzers use electrodes that respond to each gas. An oxygen electrode measures PO2 from the tiny electrical current produced as oxygen is reduced at its surface, while a carbon dioxide electrode measures PCO2 from the change in pH it causes in a thin layer of solution behind a gas-permeable membrane. Results are reported in mmHg in the United States and in kPa in many other countries.

Other measurements

• End-tidal CO2 (capnography) measures carbon dioxide at the end of each exhaled breath; in healthy lungs it is normally about 35 to 45 mmHg, a few mmHg below PaCO2.
• Venous blood gases are easier to collect and give a useful pH and PCO2, but venous PO2 does not reflect arterial oxygen.
• Transcutaneous monitors estimate PO2 and PCO2 through warmed skin, mainly in newborns.
• Pulse oximeters estimate saturation, not partial pressure, from the color of blood under the skin.

Common Conditions That Affect Blood Partial Pressures

A low PaO2 (hypoxemia), a high PaCO2 (hypercapnia) or a low PaCO2 (hypocapnia) can result from problems with the air breathed, the breathing drive, the airways, the alveoli or the blood flow through the lungs. Our respiratory diseases section covers these conditions, and anyone with breathing symptoms should see a doctor.

Five physiological causes of low PaO2

• Low inspired oxygen: as at high altitude.
• Hypoventilation: too little air reaches the alveoli, so PaCO2 rises and PaO2 falls together, as with oversedation or breathing muscle weakness.
• Ventilation-perfusion mismatch: air and blood do not reach the same parts of the lung, the most common cause, seen in COPD, asthma and pneumonia; see ventilation-perfusion ratio.
• Diffusion limitation: a thickened respiratory membrane, as in pulmonary fibrosis, slows oxygen transfer, especially during exercise.
• Shunt: blood passes through lung that gets no air at all, such as fully collapsed or fluid-filled alveoli.

A high PaCO2 usually means alveolar ventilation is too low, as in severe COPD or depressed breathing, while a low PaCO2 means breathing is faster or deeper than needed. The effects of low oxygen on the body are covered on our hypoxia and hypoxemia page.