ANATOMY OF RESPIRATORY ORGANS
Respiratory system can be divided into upper respiratory system and lower respiratory system.
Upper Respiratory System
It comprises of nose, pharynx and
associated structures.
Lower Respiratory System
It comprises of larynx, trachea,
bronchi and lungs.
Nose
Nose is a specialized organ that marks
the beginning of respiratory system.
It is divided into two parts.
External Nose
It is the visible portion of the nose
which is formed by bones and hyaline cartilage.
Bones
of the external nose include one frontal bone, a pair of nasal bones and a pair
of maxillae.
Cartilage of the external nose
consists of septal cartilage, lateral cartilages and alar cartilages.
The two openings of the external nose
are called nostrils (external nares).
Internal Nose
Internal nose consists of a large
cavity known as nasal cavity.
Anterior to the nasal cavity is nasal
vestibule. Nasal cavity is vertically divided into right and left sides by a septum.
Anterior part of the septum consists
of hyaline cartilage and its posterior part is formed by vomer and ethmoid
bone.
Each half of the nasal cavity
contains three turbinates called chonchae which in turn subdivide each side of
the nasal cavity into three groove-like passage ways called meatuses i.e.,
superior, middle and inferior meatuses.
Both the chonchae and meatuses
enhance the surface area in the internal nose and during expiration they trap
water droplets and thereby prevent dehydration of the nasal cavity.
Ducts from the paranasal sinuses
(air- filled cavities in the cranial and facial bones) and nasolacrimal ducts
open into the nasal cavity.
On both sides of the nasal cavity,
an olfactory epithelium is present in the region of superior nasal chonchae and
the adjacent nasal septum.
The olfactory epithelium functions
as olfactory receptors (detect the smell).
Region of the nasal cavity
inferior to the olfactory epithelium is lined with mucous membrane composed of
thin capillaries and pseudostratified ciliated columnar epithelium (with
goblets cells).
These goblet cells are responsible
for secreting mucus. which serves to moisten the air and trap dust particles.
The small, thin capillaries in the
nasal cavity helps to warm the inspired air and thereby bring it close to the
body temperature.
This warmed and moistened air is
then led into deeper air passages.
Functions of Nose
1. The basic function of nose is too
warm, humidify and filter the inspired air.
2. The olfactory epithelium helps
to detect the olfactory stimuli.
3. The paranasal sinuses function
as resonance chambers for speech.
Nasal cavity
It is divided into right and left
portions by means of nasal septum.
The nasal cavity is lined by mucous
membrane.
The entrance to nasal cavity is formed by
anterior nostrils.
The back of nasal cavities contains
posterior nostrils. They form the entrance to nasopharynx.
Pharynx (Throat)
It is a funnel shaped tube that
extends from the posterior part of the nasal cavity to the level of larynx.
It lies behind the nasal cavity,
oral cavity and above the larynx.
Its wall is made up of skeletal
muscles and is lined with a mucous membrane.
It is divided into three regions.
It lies behind the nasal cavity and extends
till the soft palate.
Nasal cavity opens into the
nasopharynx via two openings known as choanac or internal nares.
Nasopharynx in turn opens into the
oropharynx.
Additionally, there are two
openings of auditory tubes (Eustachian tubes) which open into each middle ear.
On its posterior wall, nasopharynx
contains pharyngeal tonsils (commonly known as adenoids) made up of lymphoid
tissue.
Oropharynx
It lies behind the oral cavity (mouth).It extends from the inferior part of the soft palate to the hyoid bone.
It contains one pair of palatine tonsils and one pair of lingual tonsils.
Laryngopharynx
It is the lower part of pharynx (hypopharynx).
It extends from the inferior part of the oropharynx and opens posteriorly into the esophagus and anteriorly into the larynx.
Functions of Pharynx
Pharynx performs both respiratory and
digestive functions as it is a common pathway for food, drink and air.
Tonsils present in the pharynx
consist of a mass of lymphatic tissue and hence are involved in providing
protection against microbes.
Auditory tubes carry air into each
middle ear and thus help in hearing.
Pharynx receives warm and humid air
from the nose and functions to warm and humidify it further.
Certain epithelial portions of the
pharynx bear olfactory nerve endings for sense of taste.
Pharynx also acts as a resonating
chamber for the sound coming from the larynx.
Larynx or Voice Box
Larynx is a small passageway which
connects the lower part of the pharynx i.e., laryngopharynx
with the
trachea.
It lies in front of the
laryngopharynx.
It is made up of the following
cartilages.
Thyroid Cartilage (1)
It is commonly known as Adam 's
apple.
It is formed by two broad plates of
hyaline cartilage which are fused incompletely to form a V-shaped notch above.
Thyroid cartilage is connected to the
hyoid bone by the thyrohyoid membrane.
Cricoid Cartilage (1)
It is a ring-shaped hyaline cartilage
that lies just below the thyroid cartilage.
It is connected to the thyroid
cartilage by cricothyroid ligament.
Arytenoid Cartilages (2)
These are a pair of triangular-shaped
hyaline cartilages located posteriorly on the superior borders of the cricoid
cartilage.
Corniculate Cartilages (2)
These are a pair of horn-shaped
elastic cartilages located above each arytenoid cartilage.
Cuneiform Cartilages (2)
These are also a pair of club-shaped
elastic cartilages that lie anterior to the corniculate cartilages.
They act as supporting structures to
the vocal cords and to the lateral portion of the epiglottis.
Epiglottis (1)
It is a leaf-shaped elastic
cartilage with a stem below.
The stem of the epiglottis is fixed
to the anterior wall of thyroid cartilage and hyoid bone whereas the leaf part
of the epiglottis is movable.
During swallowing, epiglottis moves
down and closes the glottis by forming a lid-like covering over it and thereby
prevents the accidental entry of food into the larynx and eventually into the
airways.
The superior part of the larynx is
lined with stratified squamous epithelium whereas the inferior part is lined
with pseudostratified ciliated columnar epithelium containing goblet cells.
Any dust particles which might not
have been trapped by the upper passages, get trapped in the larynx by the mucus
produced by the goblet cells.
Functions
of Larynx
Larynx is involved in the
production of sound which comprises of basically three properties
pitch,
volume and resonance.
It acts as a passageway for air.
Larynx also continues to filter,
warm and humidify the inspired air.
During swallowing, ascent (moving
up) of the larynx is followed by closure of its opening by the flap-like
epiglottis.
This helps to direct the food into
the esophagus and prevents its entry into the respiratory tract.
Trachea
Trachea is known as the windpipe as
it acts as a passage for air that moves into and out of the lungs.
It lies anterior to the esophagus
and extends till the larynx.
At the level of 5th
thoracic vertebra, trachea divides into right and left primary bronchi.
It is made up of about 16-20 incomplete rings of hyaline cartilage which
resemble 'C' shape.
These are arranged horizontally
throughout the length of the trachea with space between each ring.
The rings are incomplete towards the posterior wall of the trachea where
they are surrounded by a fibromuscular membrane comprising of smooth muscle
fibres termed as the trachealis muscle.
This muscle enables the trachea to
change its diameter during respiration.
The C-shaped cartilaginous rings support the trachea and prevent its
wall from collapsing inwards during breathing.
Functions of Trachea
1.
Trachea functions to pass air between the larynx and the primary
bronchi.
2. The C-shaped cartilaginous rings
prevent the blockade of airways when the head or neck is moved.
They also prevent collapse of trachea
especially after forced expiration as during this period the pressure inside
the trachea is less than the pressure in the thoracic cavity.
3. The cilia lining the mucous membrane
of the trachea beat synchronously.
This helps to transport mucus along
with any entrapped particles towards the larynx from where the mucus is either
swallowed or spat out.
4. Larynx, trachea and bronchi bear nerve
endings that are highly sensitive to irritation.
Upon
irritation, these nerve endings initiate nerve impulses, which are propagated
to the respiratory center in the brain stem via the vagus nerve.
This in turn initiates a cough reflex
as a result of which mucus or the irritants are expelled out.
Lungs
Lungs are a pair of cone-shaped organs that lie on either side of the mediastinum (an area which
divides the thoracic cavity into two chambers) in the thoracic cavity.
Superior rounded portion of the lungs is
known as the apex. Inferior
concave portion of the lungs is known as the base which lies on the diaphragm.
Each lung on its mediastinal surface contains
an area known as hilum
through
which bronchi, pulmonary blood vessels, nerves, etc., enter and leave.
The
right lung is divided into three lobes
by oblique and horizontal fissures.
The left lung is divided
into two lobes by an oblique fissure.
Each lobe is divided into many small compartments called lobules with
connective tissue between them.
Lungs are covered and
protected by a bilayered serous membrane known as pleural membrane.
The outer layer that lines
the wall of the thoracic cavity is known as parietal
pleura.
The inner layer that Covers
the lungs is known as visceral pleura.
The space between
parietal pleura
and visceral pleura is known as pleural
cavity.
Pleural cavity is
filled with a lubricating fluid known as serous
fluid which prevents friction between the two pleural layers during breathing.
Components of Lungs
Bronchi
and Bronchioles
The two primary bronchi which are the divisions of trachea, enter into each lung at hilum.
The right primary bronchus which is shorter and wider
than the tell bronchus, enters the
right lung.
The left primary bronchus which is long and narrow
enters the left lung.
In the lungs, the right primary bronchus divides into three branches (secondary bronchi), one entering into each lobe of the
lung and the left primary
bronchus divides into two
branches, one entering into each lobe
of the lung.
In each lobe, secondary bronchi further divide into tertiary bronchi which in turn divide into bronchioles
and terminal bronchioles.
Terminal bronchioles further subdivide into microscopic branches known as respiratory bronchioles, alveolar ducts and finally alveoli.
Such extensive branching
of bronchi makes it appear
as an inverted tree; therefore, it is also called as the bronchial tree.
Like
trachea, bronchi also contain cartilaginous rings and are lined by
pseudostratified ciliated columnar epithelium.
However, towards the
smaller bronchioles, cartilage rings disappear and
pseudostratified ciliated columnar epithelium is replaced by non-ciliated
cuboidal epithelium.
As the amount
of cartilage in bronchi and bronchioles decreases, the amount of smooth muscles
increases.
The junction where the
trachea bifurcates into right and left primary bronchi appears as a small
internal ridge which is formed by the terminal tracheal cartilage.
This ridge is termed as and it represents one of the most sensitive and potential area from where the
cough reflex can he triggered.
Alveoli
Several alveoli arise from each alveolar duct.
Each alveolus is a
cup-shaped air sac.
There are two types of
epithelial cells in the alveolar walls.
Type I Alveolar cells: Simple
squamous epithelial cells which are vast in number.
Type II Alveolar Cells: Cuboidal
epithelial cells known as septal cells which are few in number. These
cells are found in between the type I cells.
The walls of the alveoli
also consist of macrophages (dust cells) and phagocytes, which help to get rid
of any inhaled dust particles.
Septal cells Secrete an
alveolar fluid which functions to moisten the air and intercellular spaces.
This alveolar fluid also
contains a surfactant made up of phospholipids and lipoproteins.
This surfactant reduces
the interfacial tension between the alveoli and thereby prevents them from
collapsing.
Respiratory Membrane
Alveoli are surrounded by a network of blood
capillaries.
During external respiration, the exchange
of gases takes place between alveoli of the lungs and blood by diffusion across
the respiratory membrane.
This respiratory membrane comprises
of the following four layers.
1.Alveolar Wall
This is made up of a layer
each of type I and type II alveolar cells along with the macrophages
2.Epithelial
Basement Membrane
It is found below the alveolar wall.
3.Capillary
Basement Membrane
It is usually found to be closely
attached to the epithelial basement membrane.
4.Capillary
Endothelium
The respiratory membrane is about 0.5 pm in
thickness.
Such thickness allows rapid diffusion
of gases.
In simpler terms, respiratory
membrane helps in the passage of oxygen from the alveoli to the blood
capillaries and the movement of carbon dioxide from the capillaries into the
alveoli.
The surface area provided by the
alveolar walls for gaseous exchange is estimated to be around 70 m2,
which is almost 40 times the surface area of the skin.
Pulmonary Circulation
Blood circulation from the heart to
the lungs and from the lungs back to the heart is known as pulmonary circulation.
Pulmonary circulation
carries deoxygenated blood from the right ventricle of
the heart to the alveoli of the
lungs and carries oxygenated blood from the alveoli of the lungs to the left
atrium of the heart.
Pulmonary artery that
arises from the right ventricle of the heart divides into two (right and left) branches.
The right pulmonary artery enters the right lung and the left pulmonary artery enters the left lung.
In each lung, these pulmonary arteries further divide into two branches.
These arteries subdivide
into arterioles and capillaries within the lungs.
The capillaries form a
network around the alveoli.
In the alveoli, carbon dioxide from the blood capillaries
diffuses into the alveoli of the lungs and oxygen from the alveoli diffuses into the blood capillaries.
This exchange of gases takes place across the respiratory membrane which is formed by the
membranes of alveoli and capillaries.
Capillaries with
oxygenated blood unite to form two pulmonary veins in each lung.
These pulmonary veins carry oxygenated blood to the left atrium of the heart.
These pulmonary blood vessels
differ from other blood vessels in the following two aspects.
1. Pulmonary arteries carry deoxygenated blood from the heart to the
lungs in contrast to other arteries which carry oxygenated blood.
2. In response to low levels
of oxygen (a condition termed hypoxia), other blood vessels dilate.
Moreover, oxygenated blood from
the heart to the lungs especially to the muscular walls of bronchi and
bronchioles is supplied by the bronchial arteries which arise from the aorta.
Similarly, pulmonary
veins, which are four in number
carry oxygenated blood and drain it
into the left atrium.
However, under
hypoxic conditions pulmonary blood vessels undergo constriction due to which
blood from poorly ventilated areas is made to flow into those
areas which are highly
ventilated.
Such a phenomenon
has been described as ventilation-perfusion coupling because the amount of blood
directed towards a particular area of the lung is according to the air flow to
that area.
Oxygenated blood from the lungs is predominantly returned to heart via pulmonary veins and
minorly via the superior vena cava.
Functions of Lungs
1.The most important function of lungs is to facilitate external respiration, a process by which the O2 present in the alveolar walls diffuses into the capillaries while CO2 in the capillaries diffuses in the reverse direction. Therefore, lungs supply essential nutrients (oxygen) to the tissues and helps to remove carbon dioxide, a waste product of metabolism from the body.
2. Since lungs are involved in regulating the levels of CO2 in blood, they are responsible for maintaining the blood pH within the desired range. When the CO2 level increases, respiratory rate also increases to eliminate excess CO2
3.Lungs help to filter out small clots in the veins.
4.There are also involved in providing protection. Since lungs are devoid of any ciliated epithelium or goblet cells and mucus, defense against microbes is ensured by phagocytic cells like macrophages and polymorphonuclear lymphocytes and antibody producing cells.
5.Lungs are responsible for Converting angiotensin I to angiotensin 11 in the presence of angiotensin-converting enzyme (ACE). The product so formed is involved in the regulation of blood pressure.
6. Lungs act as shock absorbers tot heart as they surround it.
7. Immunoglobulin A (IgA) is secreted in bronchial secretions. It functions to provide protection against respiratory infection.
Respiratory Muscles or Muscles of Breathing
Diaphragm and intercostal
muscles are the major muscles involved in breathing.
Diaphragm
It is
a dome-shaped skeletal muscle that separates the thoracic Cavity and the
abdominal cavity. it is supplied with phrenic nerves.
When the diaphragm undergoes
Contraction. It moves downwards the thereby increasing the volume of the
thoracic Cavity.
This decreases the
intra-thoracic pressure and increases the intra-abdominal pressure.
Intercostal Muscles
They are located in the spaces between the ribs hence are known as intercostal muscles.
They are divided into
external intercostal muscles and internal intercostal muscles based on
their arrangement as external or internal layer respectively.
They are supplied with intercostal nerves. When the intercostal muscles contract,
the ribs are pulled upwards thereby
decreasing the intra-thoracic pressure.
These muscles are essential
for maintaining respiration during anesthesia.
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