Most textbooks describe the larynx as part of the upper
respiratory tract, it is also shown in the diagram above to make
clear the position of the structures described here relative to those
of the upper respiratory tract.
The trachea (also known as the windpipe)
extends from below the larynx towards the lungs. It is reinforced by
a column of C-shaped rings of hyaline cartilage
that support
the
trachea
in the
open position
when
thoracic
pressure falls. The trachea is also coated with mucous membranes and
cilia (ciliated epithelium) that trap minute dust particles and sweep
them upwards. The C-shaped cartilage merges with the submucosa.
Outer Structures of the Lower Respiratory Tract:
The external intercostal
muscles contract to raise the rib cage upwards and outwards.
For more about the function of these muscles, see the page about external
respiration.
Two continuous layers of epithelium called "pleurae" cover
the lungs, chest wall, and mediastinum*. The inner pleura (covering
the lung) is
called the visceral pleura and the outer pleura
(covering the chest wall) is called the parietal pleura. These
two pleurae are separated by a thin layer of liquid called the pleural
effusion this fluid occupies the space between the pleurae (which
is called the pleural cavity). The pleural effusion
acts as a lubricant, allowing the surfaces of the two pleurae to
slip over each other during breathing.
The pleural membranes are also described on the page
about components
of the respiratory system.
Inner Structures of the Lower Respiratory Tract:
Inside the pleural membranes are the structures that perform the functions
of the lungs - their main functions being the gaseous exchange
that oxygenates blood. This involves 3 main groups of structures:
- Structures delivering deoxygenated blood to the lungs:
The pulmonary
artery conveys blood to the lungs after it has passed around the
body delivering oxygen and nutrients to tissues. This blood is therefore
low in oxygen but high in carbon dioxide (CO2) when it is
returned to the lungs. The pulmonary artery divides into many arterioles that
carry blood further into the structures of the lung. These continue
to sub-divide until they eventually form capillaries.
- Structures that re-oxygenate the blood
by removing carbon dioxide
(CO2) from it and replacing the CO2 removed with
oxygen (O2) :
These structures perform the specialised functions that re-oxygenate
blood.
Only some of the bronchial structures are illustrated above
- to show their position relative to other structures of the lower
respiratory tract. The diagram on this page shows the main bronchus,
an example of a terminal bronchiole (there are many in each
lung), and some alveoli (sing. = "alveolus"),
in each lung. These structures are all part of the main bronchial structure
of each lung, which is called the tracheobronchial tree.
To show this in more detail, the tracheobronchial
tree is illustrated
on it's own page.
- Structures that return the re-oxygenated blood to the heart:
The pulmonary
vein and the network of venules that
lead to it convey newly oxygenated blood from the capillaries of the
lung to the heart (from
which that oxygenated blood is pumped around the body for supply to
the rest of the body, via the blood
vessels of the systemic
circulation system).
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