Alveolus
Definition
Pulmonary alveoli are the smallest structural unit of the lung and belong to the respiratory tract. This means that the pulmonary alveoli serve to exchange gases between inhaled air and blood. Each lung contains about 300 – 400 million air sacs.
The lung can generally be divided into two large lobes, the left and right lobes. In these lobes, the bronchial tree branches out into ever smaller sections. The bronchial tree is the entirety of the air-carrying system.
It begins after the trachea with the two main bronchi. Then each main bronchus splits into two (left side) or three (right side) lobe bronchi. These correspond in number to the pulmonary lobes.
The lobe bronchi are further divided into the segmental bronchi and the lobular bronchi. The bronchioli follow. While the Bronchioli terminales still belong to the air-bearing part of the bronchial tree, the Bronchioli respiratorii that come after are already part of the respiratory part of the bronchial tree.
The difference between air-ducting and respiratory sections is that the wall structure of the respiratory sections allows gas exchange between blood and respiratory air, while the air-ducting section, as the name suggests, can only conduct respiratory air to the alveoli and back again. Individual alveoli are already attached to the walls of the bronchioli respiratorii and serve for gas exchange. The alveolar ducts (Ductus alveolares) and the alveolar sacs (Sacculi alveolares) hanging from them then depart from the bronchioli respiratorii.
These only consist of many individual alveoli and also serve as a gas exchange. All pulmonary alveoli that belong to a terminal bronchioli are combined to form an acinar as a unit. This corresponds to the smallest lung unit.
Depending on the filling, the pulmonary alveoli have a diameter of up to 250μm. The pulmonary alveoli are surrounded by a fine capillary network. This capillary network is fed by the pulmonary arteries (arteriae pulmonales). These lead the oxygen-poor blood from the heart to the lungs to be enriched with oxygen. After the capillary network, the pulmonary veins (Venae pulmonales) lead the now oxygen-rich blood back to the heart.
