Respiratory System
-The respiratory system is responsible for the exchange of
oxygen and carbon dioxide by the body .
-The gas exchange organs are the lungs . In
addition to gas exchange , the lungs
have a variety of other functions, including contributing to the
bodys acid-base balance, speech,
metabolism of certain vasoactive compounds , and defense against infection.
-Metabolizing cells require a
continuous supply of oxygen and continuously produce carbon dioxide.
-To provide adequate oxygen to
the cells and remove sufficient amounts of carbon dioxide from them, the lungs
must be ventilated and perfused at all times.
-Gas exchange is limited if the
lungs ( or a region of the lungs) are ventilated but not receiving a dequate
blood flow or vice versa.
-Major functional evets of respiration include ventilation, is how air moves in and out
of the alveoli ; diffusion of oxygen ( O2) and CO2 between the blood and
alveoli ; transport of O2 and CO2 to and from the peripheral tissues ; and
regulation of respiration.
Anatomy of the lungs
-Air passes into the
trachiobronchial tree ( air ways) through the nose and mouth.
-It is warmmed to body temperature, humidified , and , if entering through the nose , filtered .
-The airways bring the inspired
air to the gas- exchange region of the lungs ; these are the respiratory
bronchioles, alveolar ducts and alveoli.
- From the trachea to the alveolar sacs , there are approximately 23generations of sequential branching.
-The first 16 of so generations constitute the conducting zone. The
blood flow to this portion of the airways is primarily to provide nutrients to
the smooth muscle of the air ways that constitute the conducting zone.
- Because no gas is
exchanged in these airways , the conducting zone constitutes the anatomic dead
space.
- Each generation of branching increases the collective cross sectional
area of the air ways, while reducing the radius of each individual airway &
the velocity of the air flow within that airway.
-Around the sixteenth or seventeenth branching generation ( between the terminal and respiratory bronchioles ) , there is a
transitional zone. -Generations 17 to 23 constitute the respiratory zone , comprising
respiratory bronchioles , alveolar ducts ( with smooth muscle sphincters only )
, and alveolar sacs.
- It is in the respiratory zone , principally the alveolar sacs , that
gas exchange occurs.
-There are approximately300 million alveoli in the lung, each with an
average diameter of 0.3 mm.
-These multiple generations of branching translate into an enormous
area for gas exchange ( 50 to 100
m2 ,average 70 m2).
-The large contact area between alveolar gas and pulmonary capillary
blood , coupled with a very thin alveolar- capillary barrier ( approximately
0.5 µ ) , highly soluble respiratory
gases ( O2 and CO2 ) , and driving pressures for both O2 and CO2 between the
alveoli and pulmonary capillary blood endow the respiratory system with the
ideal characteristics for exchanging gas by passive diffusion. -The
alveoli are lined by two types of epithelial cells , Type I cells are flat
cells with large cytoplasmic extensions and are the primary lining cells. -Type II cells (granular pneumocytes
) are thicker and contain numerous lamellar inclusion bodies. These cells
secrete surfactant. -There may be other special types of
epithelial cells, and the lung also
contains pulmonary alveolar macrophages
( PAMS ) , lymphocytes , plasma
cells and mast cells.
The process of
respiration can be divided into four major events:- (1) - Pulmonary ventilation; which means the inflow and outflow of air
between the atmosphere and the lung alveoli.
(2)- Diffusion of oxygen and carbon dioxide between the alveoli and the
blood. (3)- Transport of O2 and CO2 in the blood and body fluids to and from
the cells. (4)- Regulation of ventilation .
Pulmonary
ventilation -Pulmonary
ventilation includes inspiration and expiration.
-The lungs and the chest wall are elastic structures.
- Lung movement during inspiration results from the forces generated by
the respiratory muscles. Muscles of respiration Inspiration occurs through :-
-The
diaphragm is the most important muscle of inspiration. ( it is the only respiratory muscle used
during rest.) - Contraction
of the diaphragm elongates the thoracic cavity , causing the lungs to expand.
-During strenuous activity ,
muscles of inspiration also include
the external intercostals and accessory muscles in the neck , which pull the
rib cage up ward and forward in a” bucket handle” motion , increasing the
thickness of the chest cavity.
Expiration occurs :- -Passively during quiet breathing by elastic
recoil of the lungs and chest wall. The lungs and chest wall elastic and tend
to return to their resting positions following inspiration.
-Expiration becomes an active process during exercise and other
strenuous activities in which breathing increases greatly.
-Expiration is also active when
airway resistance is increased in diseases
such as asthma.
-The major muscles of expiration are the abdominal muscles.
-Contraction of these forces the
abdominal viscera upward against the
bottom of the diaphragm . -The
internal intercostals help with expiration by pulling the chest cage downward
and inward , which decreases the thickness of the chest cavity.
The breathing cycle
-Lung volume increases and decreases as the thoracic cavity expands and
contracts. -The lungs float
freely in the thoracic cavity ; whenever the length or thickness of the
thoracic cavity increases or decreases ,
simultaneous changes in lung volume must also occur.
-The space between the visceral pleura of the lungs and parietal pleura
of the thoracic cage is called the intrapleural. -Continuous absorption of
fluid by lymphatic channels keeps the space nearly empty except for a few
milliliters of pleural fluid that provide lubrication for the moving lungs.
-So normally, there is no more than a thin layer of fluid between the lungs and the chest
wall.
-The lungs slide easily on the chest wall but resist being pulled away from it.
-The pressure in the space
between the lungs and chest wall (
intrapleural pressure ) is subatmospheric. -The lungs are
stretched when they are expanded at birth, and at the end of quiet expiration
their tendency to recoil from the chest wall is just balanced by the tendency
of the chest wall to recoil in the opposite direction. -If the
chest wall is opened , the lungs collapse; and if the lungs lose their
elasticity , the chest expands and becomes barrel – shaped .
During rest before
inspiration begins -The
pressures in all parts of the pulmonary tree are equal to the atmospheric
pressure, which is considered to be zero
cm.water. -Thus,
there is no pressure gradient for air to flow.
-Intrapleural pressure is subatmospheric (about – 2.5 mmHg) because of
the opposing tendencies of lungs to collapse
and the chest to expand. During inspiration -Inspiration
is an active process.
-The contraction of the inspiratory muscles ( primarily the diaphragm )
increases intrathoracic volume. -The intrapleural
pressure at the base of the lungs , which is normally about (-2.5 mmHg or –5 cm
H2O )(relative to atmospheric ) . At the start of inspiration , decreases to
about ( -6 mmHg ). –The lungs are pulled into a more expanded position. -The pressure in the airway becomes slightly
negative, and air flows into the lungs. At the end of inspiration,
alveolar pressure is again zero
and air flow ceases.
During expiration
-When inspiration terminates, the diaphragm relax, and the elastic
recoil of the lungs compresses the alveolar gases, increasing alveolar pressure
to about +1 cm
water. -Since alveolar
pressure is now greater than atmospheric pressure, air flows out of the lungs.
Intrapleural pressure returns to its preinspiratory level. -Thus, during a normal respiratory cycle, intrapleural pressure
fluctuates solely in the negative range between about –5 and –8 cm water. -At the end of expiration, alveolar pressure is once again equal to atmospheric
pressure, causing air flow to stop.