Respiratory bronchioles are the final branches of the airways entering the lungs that end with the alveoli, the bunched spherical sacs inside of which the exchange of oxygen and carbon dioxide takes place. If these airways were compared to a bunch of cauliflower, the bronchus would be the large stem, the bronchioles would be the smaller thick limbs branching off the stem, the terminal bronchioles would be even smaller divisions of these limbs, the respiratory bronchioles would be the final, smallest limbs, and the alveoli would be the cauliflower heads. All bronchioles conduct inhaled air toward the alveoli, and the respiratory bronchioles are the final passageway for that air. They also are involved in the exchange of gases between this final airspace and the blood, which penetrates the alveoli via tiny capillary beds.
In the lungs, blood pumped in by the heart via the pulmonary arteries receives oxygen from inhaled air. This oxygenated blood is next cycled back to the heart by the pulmonary veins and then pumped out to the body to distributed oxygen and other nutrients. Once the body’s tissues have received the oxygen and released carbon dioxide as a metabolic byproduct, the deoxygenated blood is returned to the heart, when the cycle begins again. Additionally, the carbon dioxide is eliminated from the blood while in the lungs and released from the body in exhaled air, completing the gaseous exchange.
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Inhaled air travels from the nasal passages or mouth down the pharynx or throat, through the larynx or voicebox, and into the trachea or windpipe. Between the two lungs the trachea divides into two airways—these are the bronchi. Almost entirely contained within the lungs themselves, the bronchi extend a short distance into the organ before beginning their subdivisions. The bronchioles are the first branches, and they are responsible for conducting air into individual units within the lungs known as pulmonary lobules. Each bronchiole splits into multiple terminal bronchioles, which further conduct inhaled air and which end or terminate in the respiratory bronchioles, the entrances to the alveoli.
Respiratory bronchioles, though very small, are made up several layers of tissue in their walls. The innermost epithelial layer is made up of two kinds of cells: ciliated cells, which filter the air, and Clara cells, which secrete substances called glycosaminoglycans, as well as specific proteins that protect the epithelium or inner lining and fight disease. Beneath this layer is the lamina propria, a layer of connective tissue that adheres the epithelium to the wall of smooth muscle beneath, muscle that propels the air onward. To the outside of the smooth muscle is the adventitia, another layer of connective tissue that is exposed to the lumen, the space within the lungs.