Blood and Lymphatic Supply
The upper part of the trachea receives and drains blood through the inferior thyroid arteries and veins;[2] the lower trachea receives blood from bronchial arteries.[3] Arteries that supply the trachea do so via small branches that supply the trachea from the sides. As these branches approach the wall of the trachea, they split into inferior and superior artery branches that join with artery branches above and below; these then split into artery branches that supply the anterior and posterior parts of the trachea.[3] The inferior thyroid arteries arise just below the isthmus of the thyroid, which sits atop the trachea. These arteries join (anastomoses) with ascending branches of the bronchial arteries, which are direct branches from the aorta, to supply blood to the trachea.[2] The lymphatic vessels of the trachea drain into the pretracheal nodes that lie in front of the trachea and paratracheal lymph nodes that lie beside it.[2]
BRONCHUS
At the end of the trachea, the carina divides into two primary bronchi; the right goes into the right lung, and the left into the left lung. This junction begins at the level of the fifth thoracic verterbra. The bronchi contain cartilage, smooth muscles, and mucous membranes. Cartilage keeps the bronchi from collapsing during inhalation and exhalation. As the bronchi subdivide into smaller bronchi, the amount of cartilage decreases, and the number of smooth muscles increases.
The bronchus is the passageway that conducts air into the lungs. It forms many branches from the primary bronchi, which are wide at the top and narrow as they go from the mainstem, lobar, to lobar. Bronchioles are narrow branches without any cartilage.
Like the trachea, these bronchioles have respiratory epithelium linings, classified as ciliated pseudostratified columnar epithelium.
The epithelium in the main bronchi contains glandular goblet cells and modified simple columnar epithelial cells that produce mucins, the main component of mucus. No gas exchanges take place in the bronchi. Their purpose is to deliver air into the lungs. The bronchi vessels divide into bronchioles (internal airways of the lungs), terminal bronchioles, respiratory bronchioles, alveolar sacs, and alveoli, where the gas exchange occurs.
ALVEOLUS & ALVEOLAR DUCTS
Alveoli in the respiratory bronchioles are scattered out-pockets extending from their lumens. The respiratory bronchioles, which run at considerable lengths, become increasingly alveolated with side branches of alveolar ducts that become deeply lined with alveoli. There are between two and eleven alveolar ducts on each bronchiole. Each duct opens into five or six alveolar sacs into which clusters of alveoli open.
At the end of the bronchioles, millions of tiny Alveoli air sacks take up 90% of the total lung volume. They contain collagen and elastic fibers, surrounded by a network of capillaries. Alveoli are where the exchange of oxygen and carbon dioxide takes place. Alveoli move oxygen and carbon dioxide (CO2) molecules into and out of your bloodstream. They are the endpoint of the respiratory system. From there, oxygen molecules pass through the single-cell layer in a capillary to enter the blood system. There are three types of alveolar cells. Types 1 and 2 are in the alveolar wall. Type 3, large phagocytic cells (alveolar macrophage), move about in the lumen of the alveoli and the connective tissues between them. They continue to develop until age 3. But, the number and size of the alveoli increase until the development of the lungs finishes at approximately age 8. This is another example of why diet plays a big part in childhood development, starting with prenatal care.
Type 1 cells are involved in the gas exchange between the alveoli and blood. Their thin lining enables a fast diffusion of gas between the air in the alveoli and the blood in the surrounding capillaries. Its nucleus and organelle clusters occupy a large area. Its cytoplasm may play a role in removing small particulate contaminants from the outer surface. Type 1 cells are unable to replicate and are susceptible to toxic insults.
Type 2 cells are cuboidal and much smaller than type 1 cells. However, they are capable of cellular division and increase when type 1 cells are damaged. Hence, they are the most numerous in the alveoli. Type 2 cells secret a fatty-pulmonary surfactant made from phospholipids that reduce alveolar surface tension in the thin alveoli lining. Without this coating, the alveoli would collapse.
The alveolar macrophages and pulmonary macrophages or dust cells reside on the internal luminal surfaces of the alveoli, the alveoli, the alveoli ducts, and the bronchioles. They are mobile scavengers that engulf foreign particles in the lungs or blood cells from injuries.
CO2 is a byproduct of the process in cells that uses oxygen to make energy. As oxygen moves out of the alveolus, CO2 molecules pass into it. Then, they are exhaled through your nose or mouth. The surfactant fluid lining the alveoli maintains the shape of each air sac and helps to keep it open so oxygen and CO2 can pass through.
Breath sounds heard during auscultation of the lungs can help diagnose lung disease.