The atmosphere of Earth is the layer of gases, commonly known as air, retained by Earth’s gravity, surrounding the planet Earth and forming its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth’s surface, absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air composition, temperature, and atmospheric pressure vary with altitude, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth’s troposphere and in artificial atmospheres.

The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 75 miles. Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The body breathes the air into the lungs where the oxygen passes through very thin blood vessels and through the process of diffusion attaches to a hemoglobin molecule (found on a red blood cell). The oxidation of this hemoglobin molecule is what gives the blood it’s red color. Air contains about 21% oxygen. Exhaled air (from a human) contains about 16% oxygen.

An SaO2 (arterial oxygen saturation, as determined by an arterial blood gas test) value below 90% indicates hypoxemia (which can also be caused by anemia). Hypoxemia due to low SaO2 is indicated by cyanosis. Oxygen saturation can be measured in different tissues: Venous oxygen saturation (SvO2) is the percentage of oxygenated hemoglobin returning to the right side of the heart. It can be measured to see if oxygen delivery meets the tissues’ demands. SvO2 typically varies between 60% and 80%. A lower value indicates that the body is in lack of oxygen, and ischemic diseases occur. This measurement is often used under treatment with a heart lung machine (extracorporeal circulation), and can give the perfusionist an idea of how much flow the patient needs to stay healthy. Tissue oxygen saturation (StO2) can be measured by near infrared spectroscopy. Although the measurements are still widely discussed, they give an idea of tissue oxygenation in various conditions.

Peripheral oxygen saturation (SpO2) is an estimation of the oxygen saturation level usually measured with a pulse oximeter device. It can be calculated with pulse oximetry according to the formula.