The normal range of blood carbon dioxide concentration is 23-31 mmol/L, with an average of 27 mmol/L. It is expressed in the form of carbon dioxide binding capacity, which is the amount of plasma carbon dioxide measured in a specimen of venous blood, separated from the plasma at room temperature and equilibrated with normal human alveolar gas. It indicates the total amount of carbon dioxide from bicarbonate and carbonic acid, so it is influenced by both metabolic and respiratory factors. Therefore, it is not possible to determine whether acidosis or alkalosis is due to carbon dioxide binding capacity alone, but mainly refers to the carbon dioxide in the plasma in a bound state, so it reflects the amount of alkaline reserves in the body. The decrease of carbon dioxide binding capacity is common in metabolic acidosis and respiratory alkalosis, while the increase of carbon dioxide binding capacity can be seen in respiratory acidosis and metabolic alkalosis.