Transcript RESPIRATORY SYSTEM

RESPIRATORY SYSTEM
The respiratory system is formed of 2 functional
components:
– Conducting portion: for the transport of inspired and
expired air between the atmosphere and the circulatory
system.
– Respiratory portion: for the exchange of gases between
the atmospheric air and blood.
Conducting parts include:
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Nasal cavity
Pharynx
Larynx
Trachea
Bronchi (1ry & 2ry)
Bronchioles (terminal bronchioles)
Respiratory parts: include:
– Respiratory bronchioles
– Alveolar ducts
– Alveolar sacs (alveoli)
Extrapulmonary conducting portion
– This extends from nasal cavity to primary bronchi. It is
characterized by:
Ciliated pseudostratified columnar epithelium rich in goblet cells.
Submucous loose C.T. rich in mixed seromucous glands.
Function of the conducting passages:
– Trapping the inhaled particles and debris by the seromucous secretion which have suitable consistency to be
expelled out towards the nose.
Individuals who suffer from immobile cilia have chronic lung
infections.
The function of cilia is also lost in smokers thus inviting infections.
– The seromucous secretion has detoxifying action on the
soluble gases.
– The subepithelial C.T. (lamina propria) contains diffuse
lymphocytes, which produce secretory immunoglobulin A,
which kills the bacteria and viruses and prevents them from
penetrating the epithelium.
Anatomically the respiratory system is divided
into 2 parts:
– Upper respiratory tract.
– Lower respiratory tract.
The upper respiratory tract includes:
– Nasal cavity.
– Paranasal sinuses.
– Nasopharynx.
Its basic function is filtration, humidification and
adjusting the temperature of the inspired air.
The lower respiratory tract includes:
– Larynx which continues as the trachea, which divides and
re-divides almost about 20 times.
– At first, the trachea divides into 2 primary or main bronchi.
– Each primary bronchus gives rise to secondary or lobar
bronchi supplying the lobes of the lungs.
– These again divide into tertiary or segmental bronchi,
which supply segments of each lobe.
– The tertiary bronchi divide into smaller airways called
bronchioles. The smallest of such division is called
terminal bronchioles.
– The terminal bronchioles then divide into respiratory
bronchioles and alveolar ducts. These passages finally
terminate in dilated spaces called alveolar sacs, which
open into alveoli.
Each type of airway has its own features but there is a
gradual change in structure.
Respiratory mucosa
– Epithelium: pseudostratified columnar ciliated with
goblet cells.
– Lamina propria: loose C.T. containing:
Rich blood vessels for warming of air.
Lymphocytes for immune response.
Seromucous (branched tubuloalveolar glands of
Bowman) glands for secretion of mucous and absorption
and detoxification of gases.
Note: In some regions of nasal mucosa, there are veins
resembling erectile tissue termed swell bodies. These
are specialized veins which engorge periodically and
alternatively to close one side of the nasal cavity, thus
giving it time to recover from drying. This cyclic process
is controlled by autonomic nerves.
Olfactory mucosa:
– It is thicker than that of the respiratory mucosa and lacks the
Goblet cells.
– The glands in lamina propria are purely serous because the
fluid secretion dissolves the odoriferous substances and also
rapidly washed away to clear the receptors for new stimuli.
– The lamina propria is rich in vascular plexuses.
– The olfactory epithelium consists of 3 types of cells:
Supporting cells: have narrow bases and broad apices that carry
microvilli. The nucleus is just above the center of the cell. The
cytoplasm contains lipofuscin pigments that give the area its yellow
coloration.
Bipolar olfactory cells: which carry the receptors of smell (bipolar
neurons). The peripheral part is modified dendrite, which ends in a
bulb called olfactory knob, which gives long non-motile cilia lying flat
on the surface, and they carry the receptors for smell. The basal part
is the axon, which passes through the basement membrane, and
joins other axons forming the olfactory nerve that pass through the
cribriform plate to reach the brain.
Basal cells: short pyramidal with basal nuclei, they are
undifferentiated and can give other types of the above cells.
In cross section, the trachea and the main bronchi
are formed of 3 layers:
Mucosa:
– Epithelium: ciliated pseudostratified columnar epithelium
with goblet cells. The cells rest on thick basal lamina.
– Lamina propria: loose C.T. and elastic and reticular fibres
along with lymphocytes.
Submucosa:
– Loose C.T. with seromucous glands.
– Their long ducts project through the lumen.
– Satellite shaped myoepithelial cells surround the acini and
extend to ducts (mainly in the trachea).
– The submucosa may contain aggregates of lymphoid tissue.
Adventitia:
– Contains "U"-shaped hyaline cartilage and dense C.T.
containing blood vessels and nerves.
– In the cartilage free parts, there are smooth muscles
(trachealis).
Types of cells in lining of trachea and bronchi:
3 types of cells are seen by light microscope:
– Ciliated columnar cells.
– Goblet cells which produce mucoid secretion.
– Basal cells: these are undifferentiated cells which can divide
by mitosis and can give other types of cells.
2 types seen by electron microscope:
– Brush cells which contain glycogen granules and carry
microvilli. They may be either immature columnar cells or
degranuled goblet cells or sensory cells because their bases
make contact with nerve processes.
– Granular cells which are of 2 types according to function of
granules:
Catecholamine secreting cells (neurosecretory cells).
Protein hormone secreting cells (enteroendocrine like cells). They
secrete amines and amine precursors.
Secondary bronchi
The intrapulmonary secondary bronchi differs from
the extrapulmonary primary bronchi by the following,
otherwise the structure is the same:
– The mucosa is folded.
– The presence of muscularis mucosa which is composed of
smooth muscle sheet beneath the lamina propria, so the
mucosa is folded after fixation.
– Adventitia contains discontinuous cartilage plates and dense
C.T. The cartilage plates surround the bronchi leading to a
circular appearance and not flattened at one side like the
trachea and main bronchi.
Note: Smaller generations of secondary bronchi are
lined by ciliated simple columnar epithelium and their
adventitia contains isolated smaller cartilages.
Bronchioles
These are smaller in diameter. Its structure is as follows:– Mucosa is folded and contains: ciliated simple columnar cells with goblet
cells.
– Muscularis mucosa: relatively thick smooth muscle layer.
– Submucosa: no glands or lymphoid tissue.
– Adventitia: loose connective tissue, no cartilage.
– The bronchioles give rise to terminal bronchioles, which give respiratory
bronchioles, that receives the alveoli.
Terminal bronchioles
These are the smallest branches of the conducting system.
Their mucosa is characterized by:
– The epithelium is cuboidal ciliated.
– Absent goblet cells.
– Presences of bronchiolar secretary cells called Clara cells. They are
non-ciliated with projecting apex-carrying microvilli. They secrete
surfactant, a phospholipid that alters the surface tension of the fluid layer
covering the surface.
The Respiratory bronchioles are lined with simple cuboidal
ciliated epithelium, some of them are ciliated, and others are
Clara cells.
Alveolar Ducts
– The wall is deficient except in small areas lined by cuboidal cells in
between the alveolar sacs. It looks like a long corridor along which open
many lobbies (the alveolar sacs or antrum). The lobbies lead to the
rooms (the alveoli).
The Alveoli
– The alveoli are lined by 2 types of cells:
Flattened simple squamous cells (type I pneumocytes).
Cuboid cells (type II pneumocytes) called secretory cells or septal cells that
bulge into the alveolar lumen and contain multi-lamellar bodies. Their
secretion is rich in phospholipids “surfactant” because it reduces the surface
tension and prevents collapse of alveoli during expiration. Absence of the
surfactant cells leads to respiratory distress syndrome (RDS).
The spaces between the alveoli are called inter-alveolar septa,
which consists of C.T. containing:
– Fibres: reticular and elastic.
– Septal cells (type II pneumocytes).
– Other Cells: Fibroblasts, mast cells, leucocytes, macrophages. Other
septal cells contain bundles of actin and myosin filaments which contract
in response to hypoxia (their role is unknown).
– Blood capillaries (continuous non-fenestrated);
N.B.: the capillary endothelial cells and type I flat cells share
fused basement membranes.
SUMMARY OF STRUCTURES OF THE RESPIRATORY TREE
The structure of respiratory changes according to the functional
need:
– The air is inhaled through mouth or nose where it is humidified, warmed
and the suspended particles trapped by the hair of nasal cavity and the
mucous present on the mucosa. These particles are ultimately expelled
out by the cilia.
– As we go down the respiratory tree the cartilage decreases till it is
completely absent in the bronchioles. In trachea it is present as a single
semilunar plate while in secondary and tertiary bronchi, it is present in
few discontinuous patches.
– The height of the epithelium decreases down the tree till it becomes
simple cuboidal in the terminal bronchioles.
– The cilia decrease in number as we go down till they are absent in the
terminal bronchioles.
– The goblet cells disappear with the absence of cilia.
– The smooth muscle increase in quantity till it is maximum in the
bronchioles where it serves to act as a sphincter or control valve for
regulation of air.
LOCAL DEFENSE MEHCHANISM
– Alveolar Macrophages
Present in the interalveolar septum and on alveoli, help in
phagocytosis and disposal of antigens reaching the alveoli.
Bacteria and viruses are easily phagocytosed and degraded 
Indigested inert particles as carbon remain in macrophages for a long
time and are deposited in islands of collagen (scar tissue).  Heavy
exposure to asbestos, coal and other industrial particles, toxic gases
also lead to their accumulation in scar tissues.
Alveolar Fluid
– This neutralizes bacteria and viruses by carrying the
secretory IgA produced by B-lymphocytes (in local lymphoid
tissue). There are also lymph nodes present in hila of the
lungs, so both humoral and cell-mediated immune
responses play a role in lung defense against infection.
– Cigarette smoke interferes with the normal macrophage
and immune function of the lung and increase susceptibility
to infection.
CLINICAL NOTES
– Absence of cartilage from the wall of bronchioles is
a potential hazard, since these airways can
constrict to a point of closing if the tone of their
muscles is increased. This is the problem of asthma
which is an allergic condition to non-specific lung
irritant. Wheezing noises and difficulty in breathing
occurs during expiration rather than inspiration.
– Pulmonary Surfactant: Type II pneumocytes
secrete phospholipid surfactant that decreases the
alveolar surface tension forces to a minimal level
thus preventing the alveoli from collapse. The
presence of this secretion is important for the
newborn to obtain their first breath of air. In
premature delivery type II cells are immature. This
leads to fatal respiratory difficulty in new born
(respiratory distress syndrome).