The above Presentation is related to the Lungs Histology for 1st year MBBS student. it covers the trachea, lungs, bronchi upto the level of Alveoli. Also, it will help students to learn that what different type of epithelium are present at which region.

2. RESPIRATORY SYSTEM
INTRODUCTION
The LUNGS are a pair of primary organs of respiration, present in the
thoracic cavity beside the mediastinum. They are covered by a thin
double-layered serous membrane called the pleura.
The Respiratory System consist of the lungs and the air passage that
lead to and forms the lungs. The air passage branch as they enter the
lung to finally form alveoli.
The system has 3 function:
 Air conduction
 Air filtration
 Gas exchange

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5. The Conduction component is characterised by air passages composed
of ciliated pseudostratified columnar epithelium, (respiratory epithelium).
• Nasal cavities
• Nasopharynx and oropharynx
• Larynx
• Trachea
• Paired primary bronchi
Conducting portion consist of:

6. Respiratory portion is the part of the tract where gaseous exchange takes place.
Respiratory portion progresses from ciliated cuboidal epithelium to squamous
epithelium and includes:
• Respiratory Bronchioles
 Alveolar Ducts
 Alveolar Sacs
 Alveoli

7. LUNGS
The conduction portion of the lung begins at the trachea and extends to the
terminal bronchioles. Outside the lungs, the conduction system consists of the
Nasal Cavities, Nasopharynx, Larynx, And Trachea. Within the lungs, the
conducting portion spits into Paired Main Bronchi.
The bronchi begin as a branching pattern, splitting next into Lobar (Secondary)
Bronchial branches and then again into Segmental (Tertiary) Bronchi.
The Tertiary Bronchi continue to divide into small bronchioles where the first
change in histology takes place as cartilage is no longer present in the
bronchioles.
 Most of the respiratory epithelium is ciliated pseudostratified columnar
epithelium.

9. RESPIRATORY FUNCTIONS OF LUNGS
1. Respiration: The lungs are the primary organs of respiration, where the
exchange of gases takes place. In the alveoli, which are the functional
units of lungs, the oxygen is taken up, and carbon dioxide is removed
from the bloodstream through the alveolar-capillary bed.
2. Air conditioning: The function of conducting part is not only to lead the
air but also acts as an efficient air conditioner. This conditioning is done
by warming/cooling the inhaled air to bring it to the level of body
temperature, humidifying the air, and also the removal of all the foreign
particles present in it.
The removal of foreign particles like dust, bacteria, virus, etc., is done
by mucous secretion

10. TRACHEA
The Trachea connects the larynx to primary bronchi and contains
cartilage rings. The wall of the trachea contains ciliated, pseudo-
stratified epithelium, cartilage and glands. it consist of 4 components:
 Mucosa
 Sub mucosa
 Cartilage and intrinsic smooth muscle
 Adventitia

11.  The Mucosa is lined by pseudostratified columnar ciliated epithelium
containing many goblet cells. The lamina propria is thin and contains many
elastic fibres.
 The Sub Mucosa consists of loose connective tissue and contain numerous
simple branched tubuloacinar glands of the mixed(seromucous) type.
 The Lumen of the trachea is kept patent by 16 to 20 C shaped rings of
hyaline cartilage which are arranged one above the other. The gap
between the posterior free end of each c shaped cartilage is bridged by a
band of fibroelastic connective tissue and a bundle of smooth muscle fibres
known as TRACHEALIS MUSCLE.
 The adventitia forms the outer most covering of the trachea .it contains
blood vessels and autonomic nerves which supply the trachea.

13. BRONCHI
The Trachea branches into two primary bronchi, which branch into
secondary and then tertiary bronchi.
Bronchi are histologically similar to the trachea. They are lined with
pseudostratified ciliated columnar epithelium interspersed with goblet
cells.
The cartilage is no longer circular but divided into several plates that
surround the lumen of the bronchus. The lamina propria of bronchi
contains much more numerous submucosal glands.
The main function of the bronchi is to provide a passageway for air to
move into and out of the lungs. The repiratory epithelium and mucus that
line the multiple levels of the bronchial tree trap debris and pathogens.

14. BRONCHIOLES
Bronchi transform into even smaller Bronchioles. The walls of
bronchioles are composed only of epithelium and smooth muscle.
Bronchioles do not have cartilage or submucosal glands, which,
along with their smaller size, distinguish them from bronchi. Larger
bronchioles are lined by ciliated or non-ciliated simple columnar
epithelium that transitions to cuboidal epithelium in the smaller
bronchioles.
 Terminal bronchioles are present at the end of the conducting
zone.

15. TERMINAL BRONCHIOLES
The Terminal Bronchioles are lined by a single layer of cuboidal ciliated
cells. It is scattered by some non ciliated cells known as Clara Cells.
The Clara cells have dome shaped apices which protrudes into the lumen
of bronchioles. Apices of these cells contain secretory granules.
Function of Clara cells are:
 It secretes surfactant
 It secretes protein that protects the epithelium.
 It degrades the toxins contained in the inhaled air.
 It also serves as stem cells.

16. RESPIRATORY BRONCHIOLES
The respiratory bronchioles are short, branching tubes which
arises from the terminal bronchioles. Functionally, It constitute a
transitional zone between the conducting and respiratory
division of the respiratory tract.

18. ALVEOLAR DUCTS AND ALVEOLAR SACS
The Respiratory Bronchioles terminates in an elongated airways called
Alveolar Ducts. An Alveolar Ducts is characterized by the presence of
numerous pulmonary alveoli with almost no evidence of a bronchial wall.
The Alveolar duct terminates in Pulmonary atria.
Each alveolar sac consist of a collection of an alveoli opening into a
central slightly larger chamber.
No smooth muscle present in it. All are supplied by a rich network of
blood capillaries.

19. ALVEOLI
THE PULMONARY Alveoli are cup shaped structure having very
thin walls through which gaseous exchange takes place.
The pulmonary alveoli are lined by two types of epithelial cell:
 Type I Alveolar cells.
 Type II Alveolar cells.

20. TYPE I ALVEOLAR CELLS
Type I Alveolar cells is lined by thin squamous epithelial cell. It
lines around 95% of the surfactant of the alveoli. Adjacent
alveolar cells are joined to each other by occluding junction
which prevents tissue leakage into the alveolar lumen. Its
principal function id to provide a barrier of minimal thickness that
permits the gaseous exchange between the air and blood .
Type I Alveolar cells DO NOT have the capability to divide
MITOTICALLY.

21. TYPE II ALVEOLAR CELLS
Type II Alveolar cells are cuboidal cells. IT COVERS ONLY 5% of
the alveolar surface. The Type II Alveolar cells secretes secretory
granules that contains SURFACTANT Protein. It reduces the
surface tension of the Type I Alveolar cell. Reduction of surface
tension facilitates breathing by diminishing the force needed to
inflate alveoli.
Type II Alveolar cells serves as Stem Cells & have the capability
to divide Mitotically themselves as well as Type I Alveolar cells.

23. ALVEOLAR MACROPHAGE
Alveolar Macrophage are derived from the blood monocytes
and belong to the mononuclear phagocyte system. It helps
in maintaining the sterile environment by ingesting the
inhaled particulates like Dust, pollen and bacteria.

24. PLEURA
The pleura is a serous membrane that lines the walls of the
thoracic cavity and consist of a thin layer of connective tissue
stroma covered by al layer of simple squamous epithelium. It
produces a fluid film that lubricates the surface of the lungs and
provides surface tension for lung expansion.

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