LOWER RESPIRATORY TRACT - ENGLISH
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LOWER RESPIRATORY TRACT-
• RESPIRATORY
SYSTEM-The respiratory system includes the organs which provides the route by
which the oxygen present in the atmospheric air enters the body, and it
provides the route of excretion for carbon dioxide. The parts of
respiratory system are divided into two
groups-
• Upper
Respiratory Tract.
• Lower
Respiratory Tract.
• LOWER
RESPIRATORY TRACT- Lower Respiratory Tract includes-
• Trachea
• Bronchial
tree (Bronchous, Bronchioles, Alveolar duct, Alveoli)
• Lungs
and
• pleura
TRACHEA-
The trachea or windpipe, is a tubular passageway for air
that is about 12 cm (5 in.) long and 2.5 cm (1 in.) in diameter. It is located
anterior to the esophagus and extends from the larynx to the superior border of
the fifth thoracic vertebra (T5), where it divides into right and left primary
bronchi (singular- bronchus)
The trachea is composed of three
layers of tissue, and held open by between 16 and 20 incomplete (C-shaped)
rings of hyaline cartilage lying one above the other. The rings are incomplete
posteriorly. The cartilages are embedded in a sleeve of smooth muscle and connective tissue, which also forms the
posterior wall where the rings are incomplete. Trachea is lined ciliated
columnar epithelium, containing mucus-secreting goblet cells
FUNCTIONS OF TRACHEA-
• Support
and patency- Tracheal cartilages hold the trachea permanently open
(patent), but the soft tissue bands in between the cartilages allow flexibility
so that the head and neck can move freely without obstructing or kinking the
trachea.
• Mucociliary
escalator- This is the synchronous and regular beating of the cilia of the
mucous membrane lining that wafts mucus with adherent particles upwards towards
the larynx where it is either swallowed or coughed up
• Cough
reflex- Nerve endings in the larynx, trachea and bronchi are sensitive to
irritation. The reflex motor response is deep inspiration followed by closure
of the glottis. The abdominal and respiratory muscles then contract and
suddenly the air is released under pressure expelling mucus and/or foreign
material from the mouth.
• Warming,
humidifying and filtering-These continue as in the nose, although air is
normally saturated and at body temperature when it reaches the trachea.
BRONCHIAL TREE-
• The
two primary bronchi are formed when the trachea divides, at about the level of
the 5th thoracic vertebra
• The
right bronchus- This is wider, shorter and more vertical than the left bronchus.
It is approximately 2.5 cm long. After entering the right lung at the hilum it
divides into three branches.
• The
left bronchus- This is about 5 cm long and is narrower than the right.
After entering the lung at the hilum it divides into two branches, one to each
lobe.
• The
bronchial walls are composed of the same tissues as the trachea, and are lined
with ciliated columnar epithelium. The primary bronchi progressively subdivide
into secondary bronchi, tertiary brochi, bronchioles, terminal bronchioles,
respiratory bronchioles, alveolar ducts and finally, alveoli, where gas
exchange take place. The wider passages are called conducting airways because
their function is to bring air into the lungs, and their walls are too thick to
permit gas exchange.
• As
the bronchi divide and become progressively smaller, their structure changes to
match their function. The bronchi contain cartilage rings like the trachea, but
as the airways divide, these rings become much smaller plates, and at the
bronchiolar level there is no cartilage present in the airway walls at all. As
the cartilage disappears from airway walls, it is replaced by smooth muscle.
This allows the diameter of the airways to be increased or decreased through
the action of the autonomic nervous system, regulating airflow within each
lung. The ciliated epithelium is gradually replaced with non-ciliated
epithelium and than simple squamous epithelium.
LUNGS-
• The
lungs ( lightweights, because they are spongy) are paired cone-shaped
organs in the thoracic cavity. They are separated from each other by the heart
and other structures in the mediastinum, which divides the thoracic cavity into
two anatomically distinct chambers. As a result, if trauma causes one lung to
collapse, the other may remain expanded
• The
lungs extend from the diaphragm to just slightly superior to the clavicles and
lie against the ribs anteriorly and
posteriorly. The broad inferior portion of the lung, the base, is
concave and fits over the convex area of the diaphragm. The narrow superior
portion of the lung is the apex. The surface of the lung lying against
the ribs, the costal surface, matches the rounded curvature of the ribs.
The mediastinal (medial) surface of each lung contains a region, the hilum,
through which bronchi, pulmonary blood vessels, lymphatic vessels, and
nerves enter and exit.
• One
or two fissures divide each lung into lobes . The left lung have two lobes
superior and inferior while right lung have three lobes superior, middle and
inferior. Each lobe receives its own secondary (lobar) bronchus. So, the right
primary bronchus gives rise to three secondary (lobar) bronchi called the superior,
middle, and inferior secondary (lobar) bronchi, and the left primary
bronchus gives rise to superior and inferior secondary (lobar)
bronchi.
PLEURA-
The pleura consists of a closed
sac of serous membrane (one for each lung) it has two layers:
The visceral pleura-This
is adherent to the lung, covering each lobe and passing into the fissures that
separate them.
The parietal pleura-This
is adherent to the inside of the chest wall and the thoracic surface of the
diaphragm.
The space between these two
layers is called pleural cavity. This is only a potential space and contains no
air. In health, the two layers of pleura are separated by a thin film of serous
fluid (pleural fluid), which allows them to glide over each other, preventing
friction between them during breathing. The serous fluid is secreted by the
epithelial cells of the membrane.
PHYSIOLOGY OF RESPIRATION
The term respiration means the
exchange of gases between body cells and the environment. This involves two
main processes:
• Breathing
(pulmonary ventilation)- This is movement of air into and out of the
lungs.
• Exchange
of gases- This takes place:
• in
the lungs: external respiration
• in
the tissues: internal respiration
BREATHING-
Each breath consists of three
phases:
• Inspiration
• Expiration
• Pause.
INSPIRATION-
• Inspiration
is the process of taking air inside the
lungs. The process of inspiration is active, as it needs energy for
muscle contraction. Simultaneous contraction of the external intercostal
muscles and the diaphragm help in this
process. As lung expands due to action of these muscles and diaphragm the air
is pulled in.
EXPIRATION-
• Expiration
is the process of forcing air outside in
the environment. The process of expiration is passive, as it does not
needs energy for muscle contraction. Relaxation of the external intercostal
muscles and the diaphragm results in downward and inward movement of the
ribcage and elastic recoil of the lungs.
As this occurs, pressure inside the lungs rises and expels air from the respiratory
tract.
PAUSE-
• after
expiration there is a pause before the next cycle begins.
LUNG VOLUME AND CAPACITIES-
• Anatomical
dead space- In normal quiet
breathing there are about 15 complete respiratory cycles per minute. The lungs
and the air passages are never empty and, as the exchange of gases takes place
only across the walls of the alveolar ducts and alveoli, the remaining capacity
of the respiratory passages is called the anatomical dead space- (about 150 ml).
• Tidal
volume (TV)-This is the amount of air passing into and out of the lungs
during each cycle of breathing (about 500 ml at rest).
• Inspiratory
reserve volume (IRV)-This is the extra volume of air that can be inhaled
into the lungs during maximal inspiration.
• Expiratory
reserve volume (ERV)
• This
is the largest volume of air which can be expelled from the lungs during
maximal expiration.
• Inspiratory
capacity (IC)-This is the amount of air that can be inspired with maximum
effort. It consists of the tidal volume (500 ml) plus the inspiratory reserve
volume.
• Functional
residual capacity (FRC)- This is the amount of air remaining in the air
passages and alveoli at the end of quiet expiration. The functional residual
volume also prevents collapse of the alveoli on expiration.
• Residual
volume (RV)- This cannot be directly measured but is the volume of air
remaining in the lungs after forced expiration.
• Vital
capacity (VC)- This is the maximum volume of air which can be moved into
and out of the lungs.
• Total
lung capacity (TLC)- This is the maximum amount of air the lungs can hold.
In an adult of average build, it is normally around 6 litres . Total lung
capacity represents the sum of the vital capacity and the residual volume
• Alveolar
ventilation- This is the volume of air that moves into and out of the
alveoli per minute. It is equal to the tidal volume minus the anatomical dead
space, multiplied by the respiratory rate.
•
Exchange of gases- This takes place:
•
in the lungs: external respiration-
This is exchange of gases by diffusion between the alveoli and the blood in the
alveolar capillaries, across the respiratory membrane. Each alveolar wall is
one cell thick and is surrounded by a
network of tiny capillaries (the walls of which are also only one cell
thick). Venous blood arriving at the
lungs contains high levels of carbon-dioxide and low levels of oxygen. Carbon dioxide diffuses from venous blood
down its concentration gradient into the alveoli until equilibrium with
alveolar air is reached. By the same process, oxygen diffuses from the alveoli
into the blood. The relatively slow flow of blood through the capillaries
increases the time available for gas exchange to occur. When blood leaves the
alveolar capillaries, the oxygen and carbon dioxide concentrations are in
equilibrium with those of alveolar air
•
in the tissues: internal respiration-
This is exchange of gases by diffusion between blood in the capillaries and the
body cells. Blood arriving at the tissues has oxygenated and saturated with
oxygen. This creates concentration gradients between capillary blood and the
tissues, and gas exchange therefore occurs . Oxygen diffuses from the bloodstream through the
capillary wall into the tissues. Carbon-dioxide diffuses from the cells into the bloodstream towards the venous end of the
capillary.
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