The Nasal cavity is the first of the respiratory organs and consists of a
large irregular cavity divided into two equal passages by a septum. The
posterior bony part of the septum is formed by the perpendicular plate of
ethmoid bone and the vomer. The roof is formed by cribriform plate of
ethmoid, sphenoid, frontal and nasal bones. The floor is formed by the roof of
mouth consists of soft palate and hard palate.
The para nasal sinuses are air filled cavities in certain of the skull bones,
lined by mucous membrane and communicating with the nasal cavity. The
main sinuses are maxillary sinuses, frontal, sphenoidal and ethmoidal sinuses.
Pharynx is the passage extending from the base of skull to the level of
6th cervical vertebra where it is continous with the oesophagus. 13 cm length,
35 cm width.
Pharynx is divided into three parts. Naso Pharynx , Oro Pharynx,
Naso pharynx is the nasal part of the pharynx is situated behind the nasal
cavity and above the level of the soft palate. Oro pharynx extends from the
level of the soft palate to the level of the upper border of the epiglottis, Laryngo
pharynx extends from the upper border of the epiglottis to the lower border of
the cricoid cartilage.
The larynx is the voice box and serves as an air passages. Extends from
the root of the tongue at the inlet of the larynx to the commencement of the
trachea at the level of the 6th cervical vertebra. 4.3 cm length.
Trachea is the wind pipe. It starts at the lower border of the cricoid
cartilage and ends at the level of the upper border of the 5th thoracic vertebra by
dividing into two bronchi right and left. 11 – 12 cm length.
Bronchi and Smaller Air Passage
The two bronchi are formed when the trachea divides at the level of 5th
thorcic vertebra. The right bronchus is a wider, shorter tube than the left
bronchus and it lies in a more vertical position. It is approximately 2.5 cm long.
After entering the right lung at the hilum it divides into three branches. Each
branch divides into numerous smaller branches.
The left bronchus is about 5 cm long and is narrower than the right.
After entering the lungs at the hilum, it divides into two branches one for each
lobe. Each lobe branch then sub divides into progressively smaller tubes with
in the lung substance.
Bronchi are composed of the same tissue the trachea. They are lined
ciliated columnar epithelium. The bronchus progressively subdivides into
bronchioles, terminal bronchioles respiratory bronchioles, alveolar duct and
Respiratory Bronchioles and Alveoli
Lobules are blind ends of the respiratory tract, distal to the terminal
bronchus consist of respiratory bronchioles, alveolar duct and alveoli. The
walls gradually thinner until muscle and connective tissue fade out leaving a
single layer of simple squamous epithelial cells in the alveolar duct and alveoli.
These distal respiratory passages are supported by a loose network of
capillaries. The exchange of gases during respiration takes place across two
membranes – alveolar and capillary membrane.
Interspersed between the squamous cells are other cells that secrete
surfactant, a phospholipid, fluid which prevents the alveoli from drying out. In
addition, surfactant reduces the surface tension and prevents alveolar walls
collapsing during expiration.
Lungs are paired organs of respiration. They are situated one on each
side of the mediastinum with the thoracic cavity. Each lung resembles a half
cone. It has an apex, a base, medial surface and costal surface.
Right lung is broader than the left lung and weight 220zs,and is divided
in to three lobes, where as the left lung weight 200zs and is divided in to two
lobes.The apex is rounded and rises into the roof of the neck about 25mm
above the level of middle third of the clavicle. The base is concave and
semilunar in shape and is closely associated with the thoracic surface of the
The costal surface in convex and in closely associated with the costal
cartilages, the ribs and the inter costal muscles. The medial surface in concave
and has roughly triangular shaped area, called hilum at the level of 5th, 6th, 7th
thoracic vertebra. Structures that enter and leave at the hilum are 1 bronchus, 1
pulmonary artery, 2 pulmonary veins, 1 bronchial artery, 1 bronchial vein,
lymph vessels, parasympathetic and sympathetic nerves. The area between the
lungs is the mediastinum. It is occupied by heart, great vessels, trachea, right
and left bronchi, oesophagus, lymph nodes, lymph vessels and nerves.
Pleura and Pleural Cavity
Each lung is covered by the pleural cavity. The pleura consist of a
closed sac of serous membrane, which contain small amount of serous fluid.
The visceral pleura is adherent to the lungs covering each lobe and posses into
the fissures which separates them. The parietal pleura is adherent to the inside
of the chest wall and the thoracic surface of the diaphragm and is continuous
with the visceral pleura at the hilum.