During respiration, O2 is taken into the blood and CO is released through the blood. The oxygen tension or oxygen partial pressure is the proportion of oxygen in the blood gas mixture. The physician usually determines all blood gases for clinical diagnosis and thus gathers evidence of respiratory insufficiency, for example.
What is oxygen tension?
By oxygen tension, physicians mean the partial pressure of oxygen in the blood. This value is known as pO2 and, together with the partial pressure of carbon dioxide, forms the blood gas levels. The human lungs are primarily responsible for respiration. Gas exchange takes place in the alveoli of the lungs. CO is released. Oxygen is taken up from the air we breathe and transferred to all regions and tissues of the body via the blood as a transport medium. The oxygen supply to organs and tissues is vital. If the oxygen supply fails, the body’s tissues suffer damage in a very short time. Cells cannot maintain their metabolic processes without oxygen. For this reason, they die after a certain time if the blood no longer carries oxygen. In addition to transporting oxygen in dissolved form, the blood is also responsible for transporting bound oxygen. For this purpose, O2 binds to the hemoglobin of the blood. By oxygen tension, physicians mean the partial pressure of oxygen in the blood. This value is known as pO2 and, together with the partial pressure of carbon dioxide, forms the blood gas values. Accordingly, the pO2 is the proportion of oxygen in the total pressure of the blood gas mixture. According to Dalton’s law, the partial pressures of individual gases in the blood add up to the total pressure.
Function and task
As a respiratory gas, oxygen is one of the most important blood gases. In addition to oxygen, the blood also transports carbon dioxide as a waste product of respiration. In addition to oxygen and carbon dioxide, blood gases include base excess, pH, and bicarbonate. Each of these parameters plays a role in respiration. For example, pH affects the binding affinity of hemoglobin to oxygen, which is indispensable for transport. The oxygen content and oxygen saturation of the blood are equally important parameters. In normal breathing air, the oxygen content is about 21 percent. At sea level, the total air pressure is around 101 kPa. This results in an oxygen partial pressure of around 21 kPa. In arterial blood, the partial pressure of oxygen is lower and can range from 9.5 to 13.3 kPa based on age physiology. The partial pressure correlates with the respective concentration of the gas according to the formula c=α times P. Here, α corresponds to Bunsen’s solubility coefficient, c is the concentration and P corresponds to the partial pressure. The lower the partial pressure, the lower the proportion of oxygen in the blood. The substance-specific constant α influences the solubility. For carbon dioxide, this constant is much higher than for oxygen. Thus, the partial pressure of oxygen is significant for solubility and for the transport of O2 in the blood. If the partial pressure values for oxygen fall too low, the oxygen supply to the body tissues is impaired. In addition, if the body cannot exhale enough carbon dioxide, the carbon dioxide builds up and the blood becomes acidic (pH). The more acidic the blood, the less binding affinity there is between oxygen and hemoglobin. Carbon dioxide has a much greater binding affinity to hemoglobin than oxygen. Therefore, when it is present in the blood at elevated concentrations, it can displace oxygen from hemoglobin. On the other hand, increased exhalation of CO makes the blood basic. Determining the partial pressure of oxygen, the partial pressure of carbon, and the pH therefore provides crucial information about lung health. The blood gas values are closely interconnected. Thus, a changed partial pressure for one of the gases always changes the value for the other gas.
Diseases and ailments
Blood gas testing occurs almost exclusively in clinical settings and intensive care settings. As a rule, the determination is only necessary for seriously ill patients, such as for monitoring patients on a ventilator. Because of the close relationship between the individual blood gas values, the physician usually considers the parameters together in clinical diagnostics and thus determines, for example, the severity of respiratory or metabolic disorders. A typical disease with altered blood gas values is respiratory insufficiency.This is the term used to describe a disturbance in pulmonary gas exchange. Respiratory partial insufficiency or pulmonary insufficiency corresponds to isolated arterial hypoxemia. Thus, there is a lack of oxygen in the arterial blood, which results in a reduced supply to the body tissues. This phenomenon causes the partial pressure of oxygen to fall below the limit of 70 mmHg. Carbon dioxide is either normal or also decreased. In respiratory global insufficiency, so-called hypercapnia occurs in addition to hypoxemia. The partial pressure of carbon dioxide increases pathologically to more than 45 mmHg, causing the partial pressure of oxygen to fall to a greater or lesser extent. The most important symptoms of respiratory insufficiency are dyspnea, cyanosis, inner restlessness, confusion and palpitations. Depending on the cause, these symptoms may be associated with other symptoms. In addition to respiratory insufficiency, partial pressure of oxygen also plays a role in tachypnea. This is an increased respiratory rate, as occurs with increased oxygen demand. The depth of breathing is either decreased, constant, or exaggerated. Tachypnea is symptomatic of many diseases, as the phenomenon occurs, for example, in the context of febrile reactions. Tachypnea is more specific for heart and lung diseases. The organism tries to compensate for the reduced oxygen supply by increasing the work of breathing. As a rule, tachypnea is manifested by pathologically altered blood gas values. However, the phenomenon can theoretically occur even with physiological blood gases, for example, as a hyperventilation syndrome during mental excitement.
