2. Trachea
Trachea is made up of 16 to 20 tracheal
cartilages.
Each of these is a C-shaped mass of hyaline
cartilage.
The gaps between the cartilage ends are filled
in by smooth muscle and fibrous tissue.
The connective tissue in the wall of the trachea
contains many elastic fibres.
3.
4. Trachea
The lumen of the trachea is lined by mucous membrane that
consists of a lining epithelium and an underlying layer of
connective tissue.
The lining epithelium is pseudostratified ciliated columnar.
It contains numerous goblet cells, and basal cells that lie next
to the basement membrane.
Numerous lymphocytes are seen in deeper parts of the
epithelium.
6. Lungs
Main function is oxygenation of blood.
These are the three features of lungs:
1. To bring air and circulating blood close
together
2. System of tubes required to bring air.
3. Rich network of capillaries for exchange.
8. Intrapulmonary passages
Each terminal bronchiole ends by dividing into
respiratory bronchioles.
These are so called because they are partly
respiratory in function as some air sacs arise from
them.
Each respiratory bronchiole ends by dividing into a
few alveolar ducts.
The alveoli are blind sacs having very thin walls
through which oxygen passes from air into blood, and
carbon dioxide passes from blood into air.
9. Respiratory Tract
The following changes in structure are observed when we move
from bronchi to alveoli:
The cartilages in the walls of the bronchi become irregular in
shape, and are progressively smaller.
Cartilage is absent in the walls of bronchioles: this is the
criterion that distinguishes a bronchiole from a bronchus.
The amount of muscle in the bronchial wall increases as the
bronchi become smaller.
10. Spasm of this muscle constricts the bronchi and can cause
difficulty in breathing. This is specially likely to occur in
allergic conditions and leads to a disease called asthma.
Lymphoid tissue increases in quantity as bronchi become smaller.
Glands become fewer, and are absent in the walls of
bronchioles.
The trachea and larger bronchi are lined by pseudostratified
ciliated columnar epithelium.
As the bronchi become smaller the epithelium first becomes
simple ciliated columnar, then non-ciliated columnar, and
finally cuboidal (in respiratory bronchioles).
11. Clara cells
Produce a secretion
forming a film that
reduces surface tension
Argyrophil
Contain argyrophil granules
and secrete hormones
12. Structure of Alveolar wall
Each alveolus has a very thin wall. The
wall is lined by an epithelium consisting
mainly of flattened squamous cells.
The epithelium rests on a basement
membrane.
Deep to the basement membrane there is a
layer of delicate connective tissue through
which pulmonary capillaries run.
13. Electron Microscopy of Alveolar
cells
Cells of epithelial lining are also known as pneumocytes.
The most numerous cells are the squamous cells.
They are called type I alveolar epithelial cells and these cells are
reduced to a very thin layer (0.05 to 0.2 µm).
They form the lining of 90% of the alveolar surface.
Type II alveolar epithelial cells which is scattered in the
epithelial lining there are rounded secretory cells bearing
microvilli on their free surfaces.
14.
15. Their cytoplasm contains secretory granules that appear to
be made up of several layers (and are, therefore, called
multilamellar bodies).
These cells are believed to produce a secretion that forms a
film over the alveolar epithelium.
This film or pulmonary surfactant reduces surface tension
and prevents collapse of the alveolus during expiration.
Type III alveolar cells, or brush cells, of doubtful function.
They may be playing role in clearance of dust etc., although
alveolar macrophages are doing the same.
