Histology (fine reconstruction)
A pulmonary alveoli is a honeycomb-like bulge of the bronchial system. Pulmonary alveoli have a very thin wall. This thin wall is necessary for the optimal conditions of rapid gas exchange between blood and respiratory air.
The wall of the pulmonary alveoli is formed by various cells. Pneumocytes type I make up the main part with 90%. These large and thin cells are very similar to the endothelium and line the pulmonary alveoli.
These type I pneumocytes are no longer capable of dividing. They are responsible for gas exchange and belong to the blood-air barrier. About 7% of the cells are pneumocytes type II.
These cells are higher in comparison to pneumocytes type I and are not as flat. Pneumocytes type II are responsible for the formation of surfactant. Surfactant is a surface-active substance consisting of phospholipids and surfactant proteins.
This substance additionally lines the pulmonary alveoli and reduces the surface tension of the lungs. It thus ensures that the pulmonary alveoli do not collapse, i.e. would not collapse. Pneumocytes type II are also capable of dividing and can replace pneumocytes type I that have been lost due to defect coverage.
Alveolar macrophages may also be present as additional cells in the pulmonary alveoli. These cells belong to the immune system, i.e. the defense system of the lung. Alveolar macrophages can phagocytize pathogens that have entered the alveoli and thus keep the lungs and alveoli clean. The alveoli are separated from each other by these walls. In these walls, however, there are small pores, so-called “Kohn” pores, through which the alveoli are in contact with each other.
Function
The pulmonary alveoli are used for gas exchange between the air inhaled and passed on through the bronchial system and the blood in the capillaries. The gas exchange takes place via the membrane that separates the alveoli from the capillaries. This is the so-called blood-air barrier, i.e. the path that the oxygen has to take to pass from the air we breathe into the blood.
The blood-air-barrier consists of the following parts: the cell extensions of the pneumocytes type I, a thin basal lamina and the cell extensions of the endothelial cells.The endothelial cells belong to the wall structure of the capillaries. This blood-air barrier is only 0.2 to 0.6 μm thick. This short distance that the gas has to travel and the density of the capillary network around the alveoli ensure a fast and efficient gas exchange.
This is because the time in which the blood in the capillaries is available for gas exchange is only very short, about 0.75 seconds. The gas exchange now means that oxygen in the inhaled air reaches the alveoli via the bronchial system. Here the gaseous oxygen molecules can cross the blood-air barrier and enter the blood.
In return, carbon dioxide is released from the blood, which is carried out of the body in the exhaled air. Good perfusion and ventilation is necessary for efficient gas exchange. Perfusion means that the pulmonary alveoli are sufficiently supplied with blood via the capillary, i.e. that enough blood flows along the alveoli. Ventilation means that the lungs and thus the alveoli are sufficiently ventilated, i.e. sufficient air flows in and out of the lungs.
