Next, let's discuss the parts and structure of the lower respiratory tract.
The lower respiratory system, or lower respiratory tract, consists of the trachea, the bronchi and
bronchioles, and the alveoli, which make up the lungs.
Like the upper respiratory tract, the lower tract also functions to conduct air into and out of the lungs.
In the lower respiratory tract, the air flows from the trachea through the bronchial tree of the lungs to
the alveoli.
These pull in air from the upper respiratory system, absorb the oxygen, and release carbon
dioxide in exchange. Other structures, namely the thoracic cage (or rib cage) and the
diaphragm, protect and support these functions.
Pulling in air from the URT (Upper Respiratory Tract): The Air travels from the nose
or mouth down to the lungs through the lower tract.
Absorbing oxygen: The Oxygen from the air enters the alveoli and passes into the
bloodstream.
Releasing carbon dioxide: The Carbon dioxide from the blood is expelled out through
the same airway pathway.
LUNGS
The left lung has two lobes and is slightly smaller than the right. It curves in at the
cardiac notch to accommodate the heart. The right lung has three lobes. It is slightly
shorter because the diaphragm muscle sits higher below it to accommodate the liver.
During inhalation, air enters the lungs through the bronchi and bronchioles. Oxygen
passes from the alveoli into surrounding capillaries, while carbon dioxide diffuses from
the capillaries into the alveoli. This deoxygenated air is then expelled during exhalation.
TRACHEA
(1) The trachea is a tube less than an inch in diameter, covered by cartilaginous rings. It
extends from the bottom of the larynx down behind the sternum, until it branches into
smaller tubes, the bronchi.
(1) During inhalation, air filtered and warmed by the upper respiratory system passes
from the pharynx and larynx into the trachea, then down to the bronchi and into the
lungs. Deoxygenated air from the lungs passes back up through the trachea during
exhalation. The cartilaginous rings support the tube of the trachea and prevent it from
over-expanding or from collapsing, like when you suck on a straw too hard. They are C-
shaped, with a gap on the posterior side.
(3) The left image shows the trachea with the bronchial tree. The right image shows a
transverse cross-section of the trachea and esophagus.
(3) The epithelium that lines the lumen, or hollow inside of the trachea, produces mucus
and contains cilia. Mucus functions to trap foreign particles and pathogens, while the
tiny, hairlike projections of cilia move in a coordinated way to sweep mucus upward
toward the pharynx where it can be swallowed or expelled. This cleaning process is
known as mucociliary clearance.
BRONCHI
(1) During exhalation, deoxygenated air (now rich with carbon dioxide) leaves the lungs
by the reverse route. When we exercise, relaxation of smooth muscle in the bronchioles
causes them to dilate.
(2) Bronchi are categorized into 3 types:
1. Main bronchi result from the bifurcation of the distal portion of the trachea into two
passageways. These passageways are the right and left main, or primary, bronchi. Each
main bronchus penetrates the medial surface of the lung in a region called the hilum
prior to branching into lobar bronchi.
2. Lobar bronchi branch from the main bronchi and penetrate the lobes of each lung.
Lobar bronchi are also known as secondary bronchi. There are three lobar bronchi on
the right that penetrate the three lobes of the right lung. Two lobar bronchi on the left
penetrate the two lobes of the left lung.
3. Segmental bronchi branch from the lobar bronchi and penetrate into individual
segments of the lung lobes. Segmental bronchi are also known as tertiary bronchi.
