The main physiological function of the lungs is gas exchange, which involves the uptake of oxygen from outside the body through the lung tissues and the excretion of carbon dioxide from the body after metabolism through the lung tissues. The transport of gases within the body depends on the blood cycle, while tissue cells take in oxygen and expel carbon dioxide from the blood or tissue fluid environment. The whole process of respiration consists of three interrelated parts: ① External respiration, which refers to the exchange of gases between the external environment and blood in the lungs. It includes two processes: pulmonary ventilation (gas exchange between the lungs and the outside world) and pulmonary ventilation (gas exchange between the alveoli and the blood). ②The transport of gases in the blood. (iii) Internal respiration, which refers to the exchange of gases between blood or tissue fluid and tissues. The mechanism involved in respiratory failure is mainly external respiration, which includes pulmonary ventilation and pulmonary air exchange, and is described separately below. Pulmonary ventilation dysfunction Pulmonary gas exchange refers to the exchange of gases in the alveoli with those in the blood of the alveolar capillaries, mainly between oxygen and carbon dioxide. Pulmonary gas exchange is mainly determined by the ventilation/perfusion ratio (V/Q) and diffusion function. The main pathogenesis of type I respiratory failure is ventilatory dysfunction, mainly ventilation/blood flow dysfunction and diffusion dysfunction. Ventilation/blood flow disorders: Effective lung gas exchange requires not only adequate ventilation and blood flow, but also an appropriate ratio of the two. In the resting state, the alveolar ventilation of healthy people is about 4L/min, the pulmonary blood flow is about 5L/min, and the average V/Q of the whole lung is about 0.8. When the ventilation volume is larger than the pulmonary blood flow, V/Q>0.8, the gas entering the alveoli cannot fully contact with the blood in the alveolar capillaries, thus not getting sufficient gas exchange, that is, too much gas exchange in the alveoli without sufficient blood flow. exchange, resulting in ineffective cavity ventilation. Examples include emphysema, pulmonary alveoli and pulmonary embolism, which are common in clinical practice. When pulmonary blood flow increases compared to pulmonary ventilation, V/Q < 0.8, then venous blood flow returns to the left heart via poorly ventilated alveolar capillaries without adequate oxygenation, forming an intra-arterial venous blood adulteration, called functional arteriovenous blood shunt, for example, functional shunt exists in patients with severe COPD. In pulmonary atelectasis, when there is little or no gas in the lungs and blood flow continues, V/Q=0. The blood flowing through the lungs at this time is not exchanged for gas at all and is adulterated with arterial blood, similar to anatomical shunts, also called true shunts, or pathological arteriovenous shunts. V/Q disorders mainly cause hypoxemia, and are the most common mechanism causing hypoxemia, with little effect on PaCO2. The reasons for this are: ①The partial pressure difference between arterial and venous carbon dioxide is only 6 mmHg, while the partial pressure difference between arterial and venous blood is about 60 mmHg. When V/Q<0.8 mixed venous blood is added to arterial blood, the effect on PaO2 is significantly greater than PaCO2. ②When V/Q>0.8 or V/Q<0.8, both can show a compensatory increase in alveolar ventilation with normal V/Q, while the CO diffusion rate is about 21 times higher than that of oxygen, and the dissociation curve of CO2 is linear, so that more CO2 can be expelled as long as normal alveolar ventilation increases. The result is a decrease in PaO2 without an increase in PaCO2. 2, diffusion dysfunction: gas diffusion refers to the process of gas molecules moving from the high concentration area to the low concentration area. Diffusion is a passive moving process, so it does not need to consume energy. The mechanism of diffusion is the random movement of gas molecules, and the result of diffusion is that molecules of different concentrations eventually reach equilibrium. The exchange of gases (mainly oxygen and carbon dioxide) between the alveoli and the blood in the capillaries of the alveolar wall takes place by diffusion. Pulmonary diffusion capacity is influenced not only by the alveolar capillary membrane, but also by pulmonary capillary blood flow. The pulmonary diffusion volume (DL) in healthy adults is approximately 35 ml O2/(mmHg-min). Any factors that can affect the alveolar capillary membrane area, alveolar capillary bed volume, diffusion membrane thickness, and gas binding to hemoglobin can affect diffusion function. In clinical practice, diffusion dysfunction is rarely the only pathologic factor, and diffusion dysfunction tends to always coexist with ventilation/blood flow dysregulation during the course of the disease. This is because thickened or reduced alveolar membranes often lead to an imbalance in the ventilation/blood flow ratio. Since carbon dioxide diffuses through the alveolar capillary membrane at a rate approximately 21 times that of oxygen, diffusion dysfunction primarily affects oxygen exchange. Hypoxemia due to diffusion dysfunction can be corrected by inhaling high concentrations of oxygen, as increased partial pressure of alveolar oxygen can overcome the increased diffusion resistance. Oxygen inhalation is often used clinically to correct hypoxemia, and it is also used to identify whether hypoxemia is due to diffusion dysfunction or arterial-venous shunt hypoxemia by whether oxygen inhalation corrects hypoxemia.