☆What is Human respiratory system ?
In human respiratory system may be divided into two major components:
1- Respiratory tract
2- Respiratory organs
1- Respiratory tract
Respiratory tract serves as a passageway for the fresh air to flow from outside to the lungs and for the foul air to return from the lungs to the exterior. Gas exchange does not occur here.
” A passage which allows movement of exhale and inhale air in and out of the lungs is called as Respiratory tract.
a. Upper Respiratory tract
External Nares(Nostrils) -:
The two nostrils are openings of the nasal cavity and lie above the mouth. They are separated by the septum.
a- Vestibular Region -:
It lies just within the external naris.It is very short and lined with skin. It bears hair and contains some sweat and oil glands.The hairs act as filters and prevent the entry of dust particles.
b- Respiratory Region-:
It is middle region of nasal chamber.It lined with “Respiratory Epithelium(It is highly vascular and appears pink and reddish and act as an air conditioning and filtering unit in breathing).
- This is ciliated, pseudostratified columnar epithelium rich in gland cells.
- The latter include mucous and serous cells.
- The mucous cells secert a viscid fluid called mucus.
- The serous cells produce a watery fluid.
It is the upper region of nasal chamber.It is lined with “Olfactory epithelium”(This epithelium is confined to the upper part of nasal chamber and the superior nasal concha.It looks yellowish- brown.)
The olfactory region acts as an organ of smell.It detect the odour of the inspired air.If the odour is offensive or pungent,the air is not allowed to pass in.
Arising from the wall of each nasal chamber are three shallow bony ridges called nasal conchae.It covered with mucous membrane. They greatly increase the surface of nasal chambers .
These are cavities in the frontal , maxillary,ethmoid and sphenoid bones. Globet cells in their lining epithelium produce mucus that drains into the nasal chambers.
Internal nares (Choanae )
The nasal chambers open behind into the nasopharynx by internal nares.
The pharynx (Plural:- Pharynges )is the part of throat that is behind the mouth and nasal cavity and above the oesophagus and the larynx.
- The pharynx is a short, vertical, about 12 cm long tube.
- The food and air passages cross here.
- It is divided into three parts-
The upper part is called nasopharynx. The internal nares open into it.
The middle part is called oropharynx. The latter receives the buccal cavity.
The lower part is called laryngopharynx. It leads into two tubes: the one at the front is the windpipe or trachea and the one at the back is the foodpipe or oesophagus.
|Diagram of parts of Pharynx|
b. Lower Respiratory tract
- “The larynx is often called the “Adam apple”.
- The larynx is the upper part of the trachea. It is a short, tubular chamber supported by a cartilaginous framework. It opens into the laryngopharynx by a slit like aperture,the glottis (always remains open except during swallowing ).
- The glottis bears a leaf -life cartilaginous flap,the epiglottis,at its anterior margin.
- The epiglottis projects into the pharynx opposite the uvul
- During swallowing, the larynx moves upward to meet the epiglottis. This close the glottis to check the entry of food into it.
- Larynx produces sounds by vibrations of vocal cords when air is passed between them under pressure.
☆Difference between pharynx and Larynx
- The trachea is a thin- walled tube, about 11cm long and 2.5 cm wide. It is also called windpipe
- Its located on downward through the neck.
Bronchi and Bronchioles
- In the middle of the thorax,trachea divides into two tubes, the major ,or primary bronchi. One major bronchus enters the right lungs and the other , the left lung.
- The right primary bronchus further divides into three lobar or secondary bronchii and the left primary bronchus divides into two lobar or secondary bronchii.
- The lobar or secondary bronchi subdivide into smaller tertiary bronchi which divide into still smaller Bronchioles.
- The smaller terminal Bronchioles give off respiratory bronchioles which divide into alveolar ducts[ It have thin, nonciliated cuboidal epithelium].The latter enter the alveolar sacs.
- The respiratory passageway within the lungs are referred to as the bronchial tree.
- Wall of trachea,bronchi and bronchioles ( It have smooth muscle controlled by autonomic nervous system ) is composed of fibromuscular tissue and is lined by pseudostratified ciliated columnar epithelium rich in mucus- secreting cells.
- Cartilaginous rings, incomplete behind, support the walls of the trachea and the bronchi to prevent their collapsing. The rings gradually become thinner, and finally disappear over the bronchioles.
☆ Difference between Tracheoles and Bronchioles
|Tracheoles and Branchioles|
The respiratory organs in human respiratory system provide the surface for the exchange of gases. In man, the respiratory organs are a pair of lungs.
A very narrow space exist between the two pleura.It is called the pleural cavity ,and contains a film of watery fluid,the pleura fluid, secreted by pleura.
The pleural cavity is air tight and pressure in it remains 3-4 mm Hg lower than that in the lungs.
The adhesive force of thin layer of pleural fluid.
The pleura of two sides meet to form a sort of septum named mediastinum.
- The lungs are soft , spongy, elastic organs with smooth shining surface.
- Theyar pinkish at birth but gradually turn grey and finally black with age due to environmental pollutants, particularly cigarette smoke.
- Theyare conical shape.
- Thelungs are divided externally into lobes by grooves called fissures.
- The left lung has two lobes :left superior and left inferior demarcated by an oblique fissures and right lung has three lobes: right superior, middle and inferior demarcated by transvarse and oblique fissures is show in the diagram –
|External features of Lungs|
*Each lung weighs about 450 grams.
=In the lung,each alveolar duct opens into a blind chamber,the alveolar sac or infundibulum.
= The latter consists of a central passage giving off several small pouches,the alveoli or air sacs on the all sides.(The alveoli have very thin [0.0001mm thick] wall composed of simple moist, nonciliated, squamous epithelium).
☆Difference between Right lung and left lung
☆Mechanism of human respiratory system
- By downward and upward movement of the diaphragm to expand or contract the chest cavity.
- By raising or lowering of the ribs to increase or decrease the diameter of the chest cavity.
- In large and heavy animals, movement of diaphragm plays a greater role than the movement plasma of ribs e.g. elephant.
- External Intercostal Muscles
These muscles slant forward and downward between the ribs . Their contraction pulls the ribs and sternum upward and outward. This enlarges the thoracic cavity from front – to – back as well as from side-to-side.
- Abdominal muscles
These muscles relax and allow compression of abdominal organs by the diaphragm .
- Movement of Fresh Air into the Lungs
As the lungs are held tightly against thoracic wall, enlargement of the thoracic cavity results in expansion of the lungs. This reduces the pressure of air in the lungs below atmospheric pressure by a few (-2 to -6) mm Hg.
External Nares———-> Nasal Chambers ———-> Internal Nares———-> Pharynx ———–>Glottis————> Larynx————> Trachea————>Bronchi ————-> Bronchioles————> alveolar ducts ————->Alveoli.
Conditioning and Filtration of Air
The peripheral muscle of the diaphragm relaxes. With this, the abdominal viscera, compressed during inspiration, push the diaphragm upward, making it convex.
- External Intercostal muscles
These muscles also relax. This brings the ribs and the sternum to their original position. This is aided by the elastic recoil (contraction) of the lungs and thoracic wall which are stretched during inspiration. With the above two events, the thoracic cavity becomes smaller.
- Abdominal and Internal Intercostal Muscles
In forced breathing,as during exercise, abdominal muscles and internal intercostal muscles come into action, making expiration also an active, energy – consuming process. These muscles contract and decrease the thoracic cavity further. The contraction of abdominal muscles presses the abdominal viscera against the diaphragm, bulging it further upward. This shortens the thoracic cavity vertically. The abdominal muscles are practically inactive during normal breathing, but become active during coughing and sneezing. Contraction of internal intercostal muscles moves the ribs downward and inward. This reduces the thoracic cavity from front – to – back and also from side – to – side. The abdominal and internal intercostal muscles are called expiratory muscles.
- Movement of Foul Air out the Lungs
All round decrease in the thoracic cavity, reduces the lungs and raises the pressure of air in the lungs above atmospheric pressure by +3 to +4 mm Hg. This positive pressure in the lungs pushes out the foul air from the lungs until the air pressure in the lungs falls to that of the atmosphere. On its return journey, the air again passes through the respiratory passage. Much air stays in the alveoli after expiration. This air keeps the walls of the alveoli from sticking together and collapsing.
The foul air follows the following routes –
Alveoli ———> Alveolar ducts ————-> Bronchioles ———–> Bronchi ———–> Trachea ————-> Larynx ————-> Glottis ———-> Pharynx ———-> Internal nares ———> Nasal chamber ————> External Nares ————> Atmospheres .