Structure of the lungs

In the lung, the bronchi undergo a total of over 20 divisions: First, three lobes of the lung are distinguished on the right and two on the left, which can be further subdivided. The walls of the bronchial tubes contain cartilage rods and smooth muscles (bronchial musculature). The stock of cartilage rods decreases continuously with increasing distance from the mouth.

The cartilage braces have the task of preventing the bronchi from collapsing during inhalation (negative pressure in the lung tissue!). As they pass through the lung tissue, the bronchi are accompanied by the pulmonary arteries with the oxygen-depleted blood from the right heart. In contrast, the veins with the oxygen-rich blood run in the boundaries between the individual lung segments.

This is important because the surgeon has easy orientation within the lung tissue and can remove individual segments without loss of function of the remaining tissue if necessary (partial lung resection). The final stage of the airway branches are the alveoli (air sacs). Although they are very small (diameter well below 1 mm), they are so numerous (estimated 300 million) that their total surface area is as large as a tennis court.

If the total surface area of the alveoli, which is important for the exchange of gases (oxygen pure carbon dioxide out), is reduced, this is referred to as a restrictive ventilation disorder. Signs (symptoms) of this disease are shortness of breath and accelerated respiratory rate, as not enough oxygen can be absorbed per breath due to lack of surface area. The individual pulmonary alveoli are grouped like grapes around the smallest extensions of the bronchi.

Since they do not conduct air but exchange it, they have a special wall construction.The cells are particularly thin and no longer have the cilia typical of the respiratory tract. There are other special cells in the wall of the alveoli. Their task is to form surfactant.

This surfactant is a mixture of fat and protein and is responsible for reducing the surface tension in the alveoli. The surface tension is the force that prevails at the air-liquid interface between the alveolar wall with its layer of mucus on the one hand and the air space inside the alveoli on the other. The surface tension gives the alveoli a tendency to contract.

This tendency is favored by numerous elastic fibers in the lung tissue, which stretch during inhalation and are the driving force for exhalation. In the walls of the pulmonary alveoli and also in the smallest bronchial tubes there are tiny blood vessels (capillaries), but no lymphatic vessels. This makes it more difficult for the body to carry out the task of the lymph channels (removal of fluid).

Therefore, an accumulation of fluid in this area (pulmonary edema) leads to a significant functional impairment. The blood vessels transport the used blood to the lungs and release the end product of the metabolism (carbon dioxide; CO2) in the pulmonary alveoli. At the same time, they take up fresh oxygen and enter the large circulation via the left heart.

This gas exchange takes place in a contact time between blood cells and alveolar wall of only 0.3 seconds! If you follow the path of the air once again, you will notice that all airways have a direct connection with the environment; there is no barrier between the lips and the inner lining of the alveoli. Since 500 ml of air is inhaled per breath (approximately 12 times per minute), one can imagine that the lungs are intensively exposed to viruses, bacteria and fungi from the environment.

Furthermore, the lung tissue with its mucus layer provides excellent growth conditions for pathogens of all kinds. In all sections of the respiratory tract, one therefore finds cells of the body’s own defense system (immune system), which try to ward off this danger partly directly themselves, partly by means of secreted products. If this is not successful, an inflammation of the air-conducting systems (bronchitis) or worse, pneumonia itself occurs.

Right lung
Trachea (windpipe)
Tracheal bifurcation (carina)
Left lung