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The cells of the respiratory tract can help in removing clots in pulmonary blood vessels.
They also activate hormones and either remove or add to the substances circulating in blood.
They can make incoming air warm and moist, in order to protect the delicate cells of the
inner respiratory passages.
Finally, epithelial cells of the lung also produce surfactant that makes the process of
inhalation and exhalation easier. In fact, adequate production of surfactant by fetal lung cells
is an important prerequisite for viability in pre-term births.
Organs of the Respiratory System
In humans and most mammals, there are three parts to the respiratory system. The first is the
series of conducting tubes that carry air from the atmosphere towards the lungs. The second
part consists of the muscles of respiration – the diaphragm and intercostal muscles in the
ribs. The lungs form the third part.
The diaphragm is a dome-shaped muscle that curves upwards towards the lungs. When it
contracts, it becomes flattened and therefore increases the volume of the thoracic cavity.
Similarly, contraction of the external intercostal muscles moves the ribs upwards and
outwards. This increase in volume leads to a drop in pressure within the lungs, allowing air
to flow passively into the airways. Gas exchange occurs at alveoli till these muscles relax,
reversing the process.
Figure 5.2 (muscle)
The airways can be divided into the conducting and respiratory zones. The conducting zone
begins at the nose and ends at smaller bronchioles, and these passages carry air towards the
inner recesses of the lungs. The respiratory zone contains the terminal bronchioles and the
alveoli – the sites where gas exchange occurs.
The nose and the mouth form the main external openings and mark the beginning of the
conducting zone of the airway or respiratory tract. The nasal cavity situated behind the nose
contains hair and filters and humidifies air. Most large environmental pollutants are trapped
in the mucus secreted by the cells of the nose and nasal cavity. The mouth is unable to
reproduce all the functions of the nasal cavity and acts as a secondary opening when the nose
is either blocked, or there is an immediate need for large quantities of air. This is why we