Follow one breath inward
The airway is the whole connected pipe that air travels through, and it is helpful to walk it once from top to bottom. Air enters through the nasal cavity, where it is warmed, moistened, and filtered. It passes into the pharynx (the throat, shared with food), then through the larynx (the voice box, which guards the airway and lets you speak). From there it enters the trachea (the windpipe), which splits into the left and right main bronchi, one for each lung.
Inside each lung the bronchi branch again and again, like an upside-down tree, into ever smaller tubes. At the very tips sit the alveoli — hundreds of millions of tiny air sacs, each wrapped in blood vessels. This is where the air finally meets the blood and the real work of breathing happens.
The dividing line
Doctors draw a line through this pipe and call the parts above it the upper respiratory tract and the parts below it the lower respiratory tract. The usual boundary is the larynx: the nose, sinuses, pharynx, and larynx are upper; the trachea, bronchi, smaller tubes, and alveoli are lower. It is a simple split, but it organizes a great deal of medicine.
Why does the line matter so much? Because illness behaves differently on each side. The common cold, a sore throat, and most sinus trouble live upstairs in the upper tract. Pneumonia, asthma, and bronchitis live downstairs in the lower tract. When you hear someone describe “an upper respiratory infection,” they are telling you, in one phrase, roughly where the problem sits and how serious it is likely to be.
Two jobs along the way
The airway is not just plumbing; it does work on the air as it passes. The upper tract conditions every breath — warming, humidifying, and filtering it so the delicate deep lung receives clean, body-temperature air. The lower tract guides that conditioned air down to the alveoli and keeps the smallest tubes open and clear.