The most superior portion of the pharynx is the:

A. Rhythmic variation in breathing refers to breathing patterns such as Cheyne-Stokes respiration, not orthopnea.
B. Orthopnea is defined as difficulty breathing when lying flat, which is relieved by sitting or standing upright. It is often seen in patients with heart failure or other conditions causing fluid buildup in the lungs.
C. Difficult or labored breathing is called dyspnea, not orthopnea. While orthopnea is a type of dyspnea, the key feature of orthopnea is its positional nature.
D. Cessation of breathing is known as apnea, not orthopnea.

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.