respiratory tract
Meaning of respiratory tract: Most of its length does no gas exchange at all, which is why a shallow breath moves air without refreshing much of it.
Definition of respiratory tract
The respiratory tract is the continuous passage air travels between the outside world and the gas-exchanging surface of the lungs. It begins at the nostrils and mouth and runs through the throat, voice box, windpipe and a branching tree of airways, ending in millions of thin-walled air sacs. Its upper part conditions and cleans incoming air; only its deepest part exchanges gas with blood.
Where is the line between upper and lower?
At the voice box. Everything above and including the larynx counts as the upper tract: the nose, the sinuses draining into it, and the throat behind. Everything below is the lower tract: the trachea, the branching airways beyond it, and the lungs. The split is not arbitrary. Illness above the line tends to be brief and to settle on its own, while illness below reaches tissue involved in gas exchange and carries more weight. The two regions also host different resident organisms and are reached by different treatments, which is why the distinction appears in almost every description of a chest or throat infection.
Which parts carry air and which exchange gas?
| Zone | Structures | What happens there |
|---|---|---|
| Conducting zone | Nose, throat, larynx, windpipe, bronchi, bronchioles | Air is warmed, moistened, filtered and channeled |
| Transitional | Respiratory bronchioles | Both, as the first air sacs begin to appear |
| Respiratory zone | Alveolar ducts and air sacs | Oxygen and carbon dioxide cross to and from blood |
The conducting zone holds a volume of air that never reaches a gas-exchanging surface at all, known as the dead space. It explains why a shallow breath moves air without refreshing much of it, and why breathing through a long tube becomes impossible past a certain length.
Is it the same as the respiratory system?
Not quite, although the names get swapped freely. The tract is the passage itself. The respiratory system takes in the machinery that moves air along it as well: the diaphragm and the muscles between the ribs, the rib cage, the slippery double membrane wrapping each lung, and the nerve pathways setting the rate and depth of breathing. Someone can have a perfectly normal tract and still fail to breathe if those muscles are weak. The lining defends itself too. A blanket of mucus traps particles, and hair-like cilia beneath it sweep that blanket steadily upward to the throat to be swallowed. Smoke stalls the cilia, which is why a lasting cough and repeated chest infections follow.
Also known as
Used in a sentence
The infection appeared confined to the upper respiratory tract.
Good to know
The respiratory tract names the air passages themselves, while the respiratory system also covers the breathing muscles, the chest wall and the membranes around the lungs, so breathing can fail while the airways remain entirely normal.
Word origin
Where “respiratory tract” comes from
Respiratory comes from the Latin respirare, built from re-, meaning again, and spirare, to breathe. Tract comes from tractus, a Latin word for a drawn-out course or stretch.