THE HISTOLOGICAL EFFECTS OF COVID-19 ON THE LOWER RESPIRATORY SYSTEM

Both histologically and anatomically, the respiratory system is a compound system having diverse functions. It fulfills the process of respiration that is regarded as gas exchange through conduction and filtration of air. The respiratory system is functionally divided into conducting and respiratory parts.Anatomically, the respiratory tract is categorized into the upper and lower respiratory tract. The lower respiratory tract represents the organs within the thorax such as the trachea, bronchi, bronchioles, alveolar duct, and alveoli which make up the lungs.The paired lungs are parts of the lower respiratory tract which are regularly presented microorganisms.Although COVID-19 affects different organs simultaneously it seriously affects the lower respiratory system, produces respiratory distress to affected individuals, and mostly produces histological changes in the respiratory tract. In terms of COVID-19 effects on the lower respiratory tract, the lungs are affected in most cases, and the review is largely focused on the lungs. The histological effects of COVID-19 on the lungs are different and reveal a broad spectrum of abnormalities.Diffuse alveolar damage (DAD) remained the principal lung histopathological finding in patients with COVID-19. DAD results from damage of the covering cells of the alveoli and endothelium ending with disarrangement of the blood-air barrier. Concerning the event of pulmonary injury, DAD is classified into three histopathological stages that are acute (exudative), subacute (organizing), and chronic (fibrotic) stage.Since there is no adequate information about the histological effects of COVID-19 on the lower respiratory system, this article aimed to review what is known about the histological effects of COVID-19 on the lower respiratory system. It summarizes the previous literature of published articles that examined the histological effects of COVID-19 on the lower respiratory system to display the present understanding of histological changes of the lower respiratory system associated with COVID-19. Further data about the histological effects of COVID-19 on the lower respiratory system is needed to adequately define the histopathological changes of the lower respiratory system associated with COVID-19.