The partial pressure of CO2 in Earth's atmosphere is 0.3 millimeters of mercury. Partial pressures depend upon the particular mixtures of gases involved. Inside human blood vessels, the partial pressure of CO2 or carbon dioxide is much higher because the mixture of gases inside human blood is different.
The partial pressure of a gas is the amount of pressure it exerts if it is the only gas in the mixture. Thus, partial pressure is relative to the amount of gas in the mixture. In air, carbon dioxide exists at low levels, so it has very low partial pressure. Inside human lungs and arteries, the partial pressure of carbon dioxide is between 35 and 40 millimeters of mercury. This is because carbon dioxide is naturally produced by the body as a waste product of respiration. Some of the waste gas is retained within the body during gas exchanges between the lungs and the air outside.
High carbon dioxide levels produce a medical problem called hypercapnia. This problem makes it difficult to exhale and inhale, causing waste gases to build up within the body.