Blood contact time is the time during which blood circulates in the small blood vessels of the lungs and during which diffusion of respiratory gases occurs. Therefore, blood contact time has an important effect on blood oxygen levels.
What is blood contact time?
Blood contact time refers to the amount of time that blood spends in the alveoli (air sacs) of the lungs. Gas exchange takes place in the alveoli of the lungs. Blood contact time refers to the period of time that the blood spends in the alveoli (alveoli) of the lungs. Gas exchange takes place in the alveoli of the lungs, which means that carbon dioxide is released and oxygen is taken up by the red blood cells (erythrocytes). The air rich in carbon dioxide is then exhaled. Blood contact time depends on several factors, such as cardiac output, blood pressure, and flow resistance.
Function and purpose
Gas exchange in the alveoli is influenced both by the ventilation (aeration) of the lungs and by their blood flow. Two considerations must be taken into account. One is hemodynamics and the other is gas exchange between the alveoli and the small blood vessels surrounding the alveoli (alveolar capillaries). There are several interrelationships between hemodynamics, diffusion, and gas exchange that affect blood contact time. An important role in hemodynamics is played by cardiac output, vascular resistance to flow, and blood pressure. The size of the contact area between the alveoli and the blood is significant for the diffusion of respiratory gases. The flow velocity of the blood in the capillaries also has a positive or negative effect and is influenced, among other things, by vasodilatation and vasoconstriction. For the balance of respiratory gases, i.e. the exchange of carbon dioxide for oxygen, the blood contact time is of decisive importance, because only here does the blood have contact with the air in the alveoli and transmission is possible. Roughton, a physician, found that a normal blood contact time is about 0.7 to 0.8 seconds. Up to a blood contact time of 0.35 seconds, there can still be an approximate physiological balance between the air in the alveoli and the blood. However, the required blood contact time also depends on the oxygen saturation of the venous blood. If the blood is very low in oxygen and high in carbon dioxide, it may not be possible for sufficient oxygen equalization to occur within the normal blood contact time. Since blood passes through the capillaries of the lungs more quickly when the cardiac output increases, there should actually be an undersupply of oxygen. However, the body is very adaptable, so that the cardiac output can even be increased tenfold during exertion without oxygen undersaturation occurring. The reason for this is presumably so-called reserve capillaries, which can be opened when the oxygen demand increases. According to some authors, only 60 to 75 percent of all alveoli are ventilated and supplied with blood at all at rest. This also contributes to the fact that blood pressure in the lungs does not increase dramatically during exercise. The blood contact time thus remains approximately constant.
Diseases and ailments
Both too long and too short a blood contact time can cause discomfort. A blood contact time that is too long occurs when there is blood congestion in the lungs. This can be caused by several factors. One possible cause is left heart failure. In left heart failure, the left heart is no longer able to distribute the blood that reaches the left heart from the right heart via the lungs throughout the body’s circulation. This results in a backflow into the lungs. This causes fluid from the blood to leak into the alveoli. This is also referred to as pulmonary edema. As a result, gas exchange is no longer possible in the affected sections of the lungs. Shortening of the blood contact time usually results from lung diseases. In emphysema, the air content in the lungs is abnormally increased. This increase is accompanied by destruction of the lung tissue. The blood vessels of the lungs are also damaged. As a result, the blood contact time decreases. The most common cause of emphysema is smoking. Passive smoking also poses a risk. Typical symptoms of emphysema include shortness of breath and a blue discoloration of the skin (cyanosis). In addition, a fassthorax may develop. Blood contact time is also shortened in the setting of fibrosis.Pulmonary fibrosis is a chronic lung disease in which the functional tissue of the lung remodels into connective tissue. These remodeling processes are initiated by chronic inflammatory processes in the lungs. Possible causes include infections, particulate matter, gases, vapors, hairspray, smoking, pharmaceuticals, herbicides, and systemic diseases such as sarcoidosis or chronic polyarthritis. Due to the shortened blood contact time, shortness of breath occurs in fibrosis just as in emphysema. In the early stages, this occurs only during exertion. Later, however, those affected suffer from it even at rest. Breathing is rapid and shallow. A dry, irritating cough may also occur. In the advanced stage, the symptoms of oxygen deficiency dominate. These include cyanosis, drumstick fingers and clock glass nails. Due to the connective tissue remodeling, blood congestion occurs in some cases. This puts a strain on the heart. This is also referred to as cor pulmonale. In the advanced stage, respiratory insufficiency develops. The heart can also cause a shortened blood contact time. The cause is then right heart failure. In right heart failure, the right heart can no longer deliver sufficient blood to the pulmonary vessels. The cardiac output decreases. Right heart failure can result from heart valve defects, for example. Typically, cyanosis occurs due to the lack of oxygen. Other symptoms of congestion include edema, ascites, and enlargement of the liver (hepatomegaly) and spleen (splenomegaly). In addition, there may be increased nocturnal urination and enlargement of the heart.
