Lungs - Describe the Structure of the Specimen Including Its Major Cell and Tissue Types That Contribute to Its Function

LUNGS

Describe the structure of the specimen including its major cell and tissue types that contribute to its function. 

The lungs constitute a major part of the respiratory system, and have a specialized structure suited to extract oxygen from atmospheric air and exchange it with carbon dioxide in the bloodstream. There are two lungs, the left and the right with two and three lobes respectively. The right lung consists of the superior, middle and inferior lobes – the horizontal fissure separates the superior and middle lobes and the oblique fissure separates the middle and inferior lobes. The left lung contains only a superior and inferior lobe which are divided by the oblique fissure. Both lungs are surrounded by two pleural membranes – a visceral pleura, which is closely associated with the lung surface and the parietal pleura which is attached to the wall of the thoracic cavity. The two pleura are serous membranes which secrete pleural fluid in order to lubricate the intrapleural space and facilitate the ease with which the lungs expand. The parietal and visceral pleurae come together at the hilum of both the lungs. The hilum is a region through which major arteries, veins, nerves and bronchi enter the lungs. In order to carry out gas exchange, the inspired air must first enter the lungs through the trachea, then the bronchi and later the bronchioles, which eventually lead to the alveoli. The trachea is supported by 16-20 C-shaped rings of hyaline cartilage (on its posterior wall) as well as smooth muscle that bridges the gaps between the free ends of the cartilage rings. Seromucous glands that secrete mucous are found in the layer surrounding the cartilage. At the most inferior part of the trachea is a cartilaginous ridge called the carina, that marks the point at which the primary bronchi branch from the trachea. There are two primary bronchi – one for each lung. Primary bronchi give rise to lobar bronchi, which then result in segmental bronchi in a phenomenon known as dichotomous branching, in which one passageway divides equally into two. Bronchial walls are essentially similar to tracheal walls in terms of their constituent tissues, however unlike the trachea, the hyaline cartilage is not arranged in C shaped rings in the bronchi but instead forms plates that support all sides of the walls. Large conducting airways such as the trachea and bronchi have pseudostratified ciliated columnar cells and goblet cells which together create a mucocilliary escalator that traps foreign particles and delivers back up the trachea. In addition, the mucus secreted by the exocrine goblet cells warms and humidifies passing air. This is aided by the secretions of the seromucous glands in the submucosa. The segmental bronchi give rise to bronchioles which divide to from terminal and then respiratory bronchioles, also through dichotomous branching. The terminal bronchioles are the end of the conducting zone of the respiratory system, and the respiratory bronchioles indicate the start of the respiratory zone, at which gas exchange can occur. Bronchioles differ from bronchi because they lack the cartilaginous support and are collapsible other than being physically supported by elastic fibres that are attached to surrounding lung tissue. They do however, also contain airway smooth muscle which controls the diameter of the lumen, thus controlling airflow. The epithelium in bronchioles consists of simple ciliated columnar cells, which become cuboidal as the bronchioles decrease in size. From the level of terminal bronchioles, there are no goblet cells present however Clara cells begin to be evident and they secrete proteins involves in detoxification and cell repair. Respiratory bronchioles eventually turn into alveolar ducts and then alveoli, the major site of gas exchange. Alveoli are thin walled balloon-like structure that are found in sacs that have a high surface area supported by collagen and elastin. The inter-alveolar walls contain many capillaries and the alveolar epithelium predominantly two cell types: Type I and Type II alveolar cells. Type I epithelial cells are squamous cells with flattened nuclei and long cytoplasmic extensions that occupy the majority of the alveolar surface area and participate in gas exchange – as such, they are closely associated with capillaries. Type II cells are cuboidal and have rounded nuclei as well as microvilli. They occupy 10% of the surface area and contain lamellar bodies which are specialized vesicles that secrete surfactant in order to reduce surface tension in the alveoli, easing their expansion as well as preventing their collapse.