The most superior portion of the pharynx is the:

A. Carotid artery and aorta contain the carotid bodies and aortic bodies, which are peripheral chemoreceptors. These structures detect changes in blood oxygen, carbon dioxide, and pH levels, sending signals to the respiratory centers to adjust breathing accordingly.
B. The cerebellum and pons are parts of the brain involved in motor control and respiratory rhythm, respectively, but do not contain peripheral chemoreceptors.
C. The coronary sinus and alveoli are not involved in detecting chemical changes in the blood for breathing regulation. The coronary sinus drains blood from the heart muscle, and the alveoli are sites of gas exchange, not chemoreception.
D. The brachiocephalic vein and superior vena cava are large veins that return deoxygenated blood to the heart but do not house chemoreceptors involved in respiratory regulation.

A. Capillaries are small blood vessels that are involved in the exchange of gases but are not part of the bronchial tree.
B. The bronchiole is the smallest division of the bronchial tree. It is a narrow airway that connects the larger bronchi to the alveoli, where gas exchange occurs.
C. Arterioles are small blood vessels that are part of the circulatory system, not the respiratory system.
D. The alveoli are the small air sacs at the end of the bronchioles where gas exchange occurs, but they are not the smallest subdivision of the bronchial tree—this is the bronchiole itself.