The respiratory system includes the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and lungs. Respiratory organs also include the thorax, the endocrine and nervous system, pleural cavities, intercostal muscles, and the diaphragm. Blood vessels play an important role in all pulmonary processes. These organs have a different structure, though their one main function is to ensure gas exchange. However, this function is rather multifaceted and includes several steps, starting from air warming in nasal passages, air transmission along the tracheobronchial tree, and ending with gas exchange directly in the alveoli of the pulmonary parenchyma.
The blood-air barrier is the main site for gas exchange in the body; it is formed by capillaries and adjacent type I pneumocytes. The barrier also includes the surfactant (the film lining the alveolar surface).
The respiratory system is open to environmental impacts including different infections, dust, and harmful gases. This requires a corresponding defense system, which is also characterized by its own morphological structure. Thus, the respiratory system is characterized by structural variety, with various epithelia, drainage systems, immune and endocrine cells providing cellular and humoral immune protection of the lungs, as well as the neuroendocrine regulation of complex processes in the lungs. Impaired protection mechanisms in the bronchopulmonary system promote the development of pulmonary diseases.
Aside from respiratory functions, the lungs participate in the regulation of protein, water-salt, and carbohydrate metabolism. These are characterized by a high level of lipid metabolism, including the biogenesis of the surfactant, prostaglandins, leukotrienes. The lungs also have functions related to inactivation of the angiotensin-converting enzyme, epinephrine, norepinephrine, serotonin, histamine, bradykinin, prostaglandins, adenosine phosphokinase (APK) generation and inactivation. The respiratory organs participate in thermal regulation, blood storage, coagulation regulation, as well as cleansing the blood from microthrombi and metabolic products. Integrative body systems (nervous, endocrine, and immune system) participate in the regulation of various processes, so any disease developing in the respiratory organs has systemic effects.
The majority of these functions are handled by the pulmonary parenchyma, which is basically an enormous biologically active membrane (with a total area of 80 m2 upon expiration and 120 m2 upon inspiration). It serves for oxygen consumption and carbon dioxide removal from the body. Due to this, almost all pathological pulmonary processes (primarily inflammation, pneumosclerosis, and tumors) lead to respiratory hypoxia. With that, acute hypoxia may cause lung and heart failure, while chronic hypoxia leads to dystrophy, atrophy, and sclerosis in the pulmonary tissue itself and in many internal organs; however, the cardiovascular system also undergoes primary changes. Being a part of the whole body, the respiratory system possesses all general biologic and stereotypical reactions, yet, due to structural and functional features of different parts of this system, the pathological process in each of those parts may have its own peculiarities.