The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs.[1] Gas exchange in the lungs occurs in millions of small air sacs called alveoli in mammals and reptiles, but atria in birds. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.[2] These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

1. RESPIRATORY SYSTEM
TRACHEA AND LUNGS

2. TRACHEA
 The trachea, or windpipe is a tubular passageway for air that is about 12 cm long
and 2.5 cm in diameter. It is located anterior to the esophagus and extends from the
larynx to the superior border of the fifth thoracic vertebra, where it divides into
right and left primary bronchi
 The layers of the tracheal wall from deep to superficial, are the
1) Mucosa
2) Submucosa
3) Hyaline cartilage
4) Adventitia(composed of areolar connective tissue)

3.  The mucosa of the trachea consist of an epithelial layer of pseudo
stratified ciliated columnar epithelium and an underlying layer of
lamina propria that contains elastic and reticular fibers.
 The submucosa consists of areolar connective tissue that contains
seromucous glands and their ducts.
 The 16-20 incomplete horizontal rings of hyaline cartilage
resemble the letter C, are stacked one above the other, and are
connected together by dense connective tissue. They may be felt
through the skin inferior to the larynx.

4.  The open part of each C shaped cartilage ring faces posteriorly
toward the esophagus and is spanned by a fibromuscular
membrane.
 Within this membrane are transverse smooth muscle fibers called
the trachealis muscle and elastic connective tissue that allow the
diameter of the trachea to change subtly during inhalation and
exhalation which is important in maintaining efficient air flow.
 The solid C shaped cartilage rings provide a semi rigid support to
maintain patency so that the tracheal wall does not collapse
inward(during inhalation) and obstruct the air passage way.

5.  The adventitia of the trachea consist of areolar connective tissue
that joints the trachea to surrounding tissues.
BRONCHI
 At the superior border of the 5th thoracic vertebra, the trachea
divides into a right primary bronchus which goes into the right
lung, and a left primary bronchus, which goes into the left lung.
 The right primary bronchus is more vertical, shorter, and wider
than the left. As a result, an aspirated object is more likely to enter
and lodge in the right primary bronchus than the left.

6.  Like the trachea, the primary bronchi contain incomplete rings of
cartilage and are lined by pseudo stratified ciliated columnar epithelium.
 At the point where the trachea divides into right and left primary
bronchi an internal ridge called the carina is formed by a posterior and
somewhat inferior projection of the last tracheal cartilage.
 The mucous membrane of the carina is one of the most sensitive
areas of the entire larynx and trachea for triggering a cough reflex.
 Widening and distortion of the carina is a serious sign because it
usually indicates a carcinoma of the lymph node around the region where
the trachea divides.

7.  On entering the lungs the primary bronchi divide to form smaller
bronchi – the secondary (lobar) bronchi, one for each lobe of the
lung.
 The right lung has 3 lobes and the left lung has 2 lobes.
 Secondary bronchi continue to branch, forming still smaller bronchi
called tertiary bronchi, that divide into bronchioles.
 These bronchioles contain clara cells, columnar , non-ciliated cells
interspersed among the epithelial cells. Clara cells may protect against
harmful effects of inhaled toxins and carcinogens and produce
surfactant and function as stem cells.

8.  Bronchioles continue to branch forming smaller tubes called terminal
bronchioles. The terminal bronchioles represent the end of the
conducting zone of the respiratory system.
 The extensive branching from the trachea through the terminal
bronchioles resembles an inverted tree and is commonly referred to as the
bronchial tree.
 The mucous membrane in the bronchial tree changes from pseudo
stratified ciliated columnar epithelium in the primary bronchi, secondary
bronchi, and tertiary bronchi to ciliated simple columnar epithelium with
some goblet cells in larger bronchioles, to mostly ciliated simple cuboidal
epithelium with no goblet cells in smaller bronchioles, to mostly non-
ciliated simple cuboidal epithelium in terminal bronchioles.

9.  Plates of cartilage gradually replace the incomplete rings of cartilage in
primary bronchi and finally disappear in the distal bronchioles.
 As the amount of cartilage decreases, amount of smooth muscles
increases. Smooth muscle encircles the lumen in spiral bands and helps
maintain patency. However, because there is no supporting cartilage,
muscle spasm can close off the air ways. This is what happens during an
asthma attack which can be a life threatening situation.
 During exercise, activity in the sympathetic division of the autonomic
nervous system increases and the adrenal medulla releases the hormones
epinephrine and norepinephrine; both of these events cause relaxation
of smooth muscle in the bronchioles, which dilates the airways.

10.  Because the air reaches the alveoli more quickly, lung ventilation
improves. The parasympathetic division of the ANS and mediators of
allergic reactions such as histamine have the opposite effect, causing
contraction of bronchiolar smooth muscle, which results in
constriction of distal bronchioles.

11. LUNGS
 The lungs are paired cone shaped organs in the thoracic cavity. They are
separated from each other by the heart and other structures of mediastinum,
which divides the thoracic cavity into 2 anatomically distinct chambers. As a
result if trauma causes one lung to collapse, the other may remain expanded.
 Each lung is enclosed and protected by a double layered serous membrane
called the pleural membrane. This superficial layer, called the parietal
pleura, lines the wall of the thoracic cavity; the deep layer, the visceral
pleura, covers the lungs themselves.

12.  Between the visceral and parietal pleurae is a small space, the
pleural cavity which contains a small amount of lubricating fluid
which reduces friction between the membranes allowing them to slide
easily over one another during breathing. Separate pleural cavity
surround left and right lungs.
 Inflammation of the pleural membrane, called pleurisy or
pleuritis may in its early stages cause pain due to friction between the
parietal and visceral layers of the pleura. If the inflammation persist,
excess fluid accumulates in the pleural space, a condition known as
pleural effusion.

13.  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. The narrow superior
portion of the lung is the apex.
 The surface of the lung line against the ribs, the coastal surface,
matches the rounded curvature of the ribs. The mediastinal surface of
each lung contains a region, the hilum, through which bronchi, pulmonary
blood vessels, lymphatic vessels, and nerves enter and exit. These structures
are held together by the pleura and connective tissue and constitute the root
of the lung. Medially, the luft lung also contains a concavity, the cardiac
notch, in which the apex of the heart lies. Due to the space occupied by the
heart, the left lung is about 10% smaller than the right lung.

14.  Apex of the lungs lies superior to the medial 3rd of the clavicles. And
this the only area that can be palpated. The anterior, lateral, and posterior
surfaces of the lungs lie against the ribs. The base of the lungs extends
from the 6th costal cartilage anteriorly to spinous process of the 10th
thoracic vertebra posteriorly.
 The pleura extends about 5 cm below the base from the 6th costal
cartilage anteriorly to the 12th rib posteriorly.
 Removal of excessive fluid in the pleural cavity can be accomplished
without injuring lung tissue by inserting a needle anteriorly through the
seventh intercostal space, a procedure called thoracentesis. The needle is
passed along the superior border of the lower rib to avoid damage to the
intercostal nerves and blood vessels.

15.  One or two fissures divide each lung into lobes. Both lungs have an
oblique fissure, which extends inferiorly and anteriorly; the right lung has a
horizontal fissure. The oblique fissure in the left lung separates the superior
lobe from the inferior lobe. In the right lung, the superior part of the
oblique fissure separates the superior lobe from the inferior lobe; the inferior
part of the oblique fissure separates the inferior lobe from the middle lobe,
which is bordered superiorly by the horizontal fissure.
 Each lobe receives its own secondary bronchus. Thus the right primary
bronchus gives rise to three secondary bronchi called the superior, middle,
and inferior secondary bronchi, and the left primary bronchus gives rise to
the tertiary bronchi. There are 10 tertiary bronchi in each lung.
 The segment of lung tissue that each tertiary bronchus supplies is called a
bronchopulmomary segment.

16.  Each bronchopulmomary segment of the lungs has many small
compartments called lobules; each lobule is wrapped in elastic connective
tissue and contains a lymphatic vessel, an arteriole, a, venule and a branch
from a terminal bronchiole.
 Terminal bronchioles divide into respiratory bronchioles. They also have
alveoli budding from their walls. Alveoli participate in gas exchange, and thus
respiratory bronchioles begin the respiratory zone of the respiratory system.
As the respiratory bronchioles penetrate more deeply into the lungs, the
epithelial lining changes from simple cuboidal to simple squamous.

17.  The respiratory bronchioles divide into several alveolar ducts which
consist of simple squamous epithelium. The respiratory passages from
the trachea to the alveolar ducts contain about 25 orders of branching.
 Around the alveolar ducts are numerous alveoli and alveolar sacs.an
alveolus is lined by simple squamous epithelium and supported by a thin
elastic basement membrane; an alveolar sac consists of two or more alveoli
that share a common opening. The wall of alveoli consists of two types of
epithelial cells.
1) The more numerous type 1 alveolar cells are simple squamous epithelial
cells that form a nearly continuous lining of the alveolar wall. They are the
main sites of gas exchange.

18.  Type 2 alveolar cells also called septal cells are found between type 1
alveolar cells. They are rounded or cuboidal epithelial cells with free
surfaces containing microvilli, secrete alveolar fluid, which keeps the
surface between the cells and the air moist. Included in the alveolar
fluid is surfactant, a complex mixture of phospholipids an lipoproteins.
It lowers the surface tension of alveolar fluid, which reduces the
tendency of alveoli to collapse and thus maintains their patency.
 Associated with the alveolar walls are alveolar macrophages, phagocytes
that remove fine dust particles and other debris from the alveolar
spaces. Fibroblasts present produce reticular and elastic fibers.

19.  The exchange of oxygen and carbon dioxide between the air spaces in the
lungs and the blood takes place by diffusion across the alveolar and capillary
walls, which together form the respiratory membrane.
 Respiratory membrane consists of four layers:
1) A layer of type 1 and type 2 alveolar cells and associated macrophages
2) An epithelial basement membrane
3) A capillary basement membrane
4) The capillary endothelium
 The lungs contain about 300 million alveoli.

20. BLOOD SUPPLY TO THE LUNGS
 The lungs receive blood via pulmonary arteries and bronchial arteries.
deoxygenated blood passes through the pulmonary trunk, which divides
into a left pulmonary artery that enters the left lung and a right
pulmonary artery that enters the right lung. These are the only arteries in
the body that carry deoxygenated blood.
 Return of the oxygenated blood to the heart occurs by ways of
pulmonary veins, which drain into left atrium.
 In the lungs, vasoconstriction in response to hypoxia diverts
pulmonary blood from poorly ventilated areas of the lungs to well

21. Ventilated regions for more efficient gas exchange. This phenomenon is
known as ventilation-perfusion.
 Bronchial arteries, which branch from aorta, deliver oxygenated blood to
the lungs. Most blood returns to the heart via pulmonary veins. Some
blood drains into bronchial veins, branches of the azygos system, and
returns to the heart via superior vena cava.
 The bony and cartilaginous frameworks of the nose, skeletal muscles of
the pharynx, cartilages of the larynx, C shaped rings of cartilage ,
surfactant in the alveoli helps maintain the patency of the system.

22. PULMONARY VENTILATION
 Pulmonary ventilation, or breathing is the inhalation and exhalation of
air and allows the exchange of air between the atmosphere and the alveoli
of the lungs.
 External respiration is the exchange of gases between the alveoli of the
lungs and the blood in pulmonary capillaries across the respiratory
membrane.
 Internal respiration is the exchange of gases between the blood in
systemic capillaries and tissue cells.

23. GAS EXCHANGE AND TRANSPORT IN LUNGS AND
TISSUES
Deoxygenated blood returning to the pulmonary capillaries in the
lungs contains carbon dioxide dissolved in blood plasma, carbon
dioxide combined with globin as carbaminohemoglobin and carbon
dioxide incorporated into bicarbonate within RBCs. The RBCs have
also picked up protons , some of which binds to and therefore is
buffered by hemoglobin. As blood passes through the pulmonary
capillaries, molecules of carbon dioxide dissolved in blood plasma
and carbon dioxide that dissociates from the globin portion of

24. hemoglobin diffuse into alveolar air and are exhaled. At the same time,
inhaled oxygen is diffusing from alveolar air into RBCs and is binding to
hemoglobin to form oxyhemoglobin. Carbon dioxide also is released from
bicarbonate when proton combines with bicarbonate inside RBCs. The
hydrogen bicarbonate formed from this reaction ten splits into carbon
dioxide, which is exhaled, and water. As the concentration of bicarbonate
declines inside RBCs in pulmonary capillaries, bicarbonate diffuses in from
the blood plasma, in exchange for chloride. In sum, oxygenated blood
leaving the lungs has increased oxygen content and decreased amounts of
carbon dioxide and protons.

25. LUNG VOLUMES AND
CAPACITIES
 The volume of one breath is called the tidal volume.
 The minute ventilation is the total volume of air inhaled and exhaled each
minute.
 The alveolar ventilation rate is the volume of air per minute that actually
reaches the respiratory zone.
 The inspiratory reserve volume is the air that is inhaled additionally.
 If the thoracic cavity is opened, the intrapleural pressure rises to equal the
atmospheric pressure and forces out some of the residual volume. The air

26. remaining is called the minimal volume.
 Inspiratory capacity is the sum of tidal volume and inspiratory reserve
volume.
 Functional residual capacity is the sum of residual volume and
expiratory reserve volume.
 Vital capacity is the sum of inspiratory reserve volume, tidal volume,
and expiratory reserve volume.
 Total lung capacity is the sum of vital capacity and residual volume.