Breathing and Lung Mechanics
Ventilation is the exchange of air between the external
environment and the alveoli. Air moves by bulk flow from an area of
high pressure to low pressure. All pressures in the respiratory system
are relative to atmospheric pressure (760mmHg at sea level). Air will
move in or out of the lungs depending on the pressure in the alveoli.
The body changes the pressure in the alveoli by changing the volume of
the lungs. As volume increases pressure decreases and as volume
decreases pressure increases. There are two phases of ventilation;
inspiration and expiration. During each phase the body changes the lung
dimensions to produce a flow of air either in or out of the lungs.
The body is able to change the dimensions of the lungs because of
the relationship of the lungs to the thoracic wall. Each lung is
completely enclosed in a sac called the pleural sac. 2 structures
contribute to the formation of this sac. The parietal pleura is
attached to the thoracic wall where as the visceral pleura is attached
to the lung itself. In-between these two membranes is a thin layer of
intrapleural fluid. The intrapleural fluid completely surrounds the
lungs and lubricates the two surfaces so that they can slide across
each other. Changing the pressure of this fluid also allows the lungs
and the thoracic wall to move together during normal breathing. Much
the way two glass slides with water in-between them are difficult to
pull apart, such is the relationship of the lungs to the thoracic wall.
The rhythm of ventilation is also controlled by the "Respiratory
Center" which is located largely in the medulla oblongata of the brain
stem. This is part of the autonomic system and as such is not
controlled voluntarily (one can increase or decrease breathing rate
voluntarily, but that involves a different part of the brain). While
resting, the respiratory center sends out action potentials that travel
along the phrenic nerves into the diaphragm and the external
intercostal muscles of the rib cage, causing inhalation. Relaxed
exhalation occurs between impulses when the muscles relax. Normal
adults have a breathing rate of 12-20 respirations per minute.
When one breathes air in at sea level, the inhalation is composed of
different gases. These gases and their quantities are Oxygen which
makes up 21%, Nitrogen which is 78%, Carbon Dioxide with 0.04% and
others with significantly smaller portions.
Diagram of the Pharynx.
In the process of breathing, air enters into the nasal cavity through the nostrils and is filtered by coarse hairs (vibrissae)
and mucous that are found there. The vibrissae filter macroparticles,
which are particles of large size. Dust, pollen, smoke, and fine
particles are trapped in the mucous that lines the nasal cavities
(hollow spaces within the bones of the skull that warm, moisten, and
filter the air). There are three bony projections inside the nasal
cavity. The superior, middle, and inferior nasal conchae. Air passes between these chonchae via the nasal meatuses.
Air then travels past the nasopharynx, oropharynx, and
laryngopharynx, which are the three portions that make up the pharynx.
The pharynx is a funnel-shaped tube that connects our nasal and oral cavities to the larynx. The tonsils
which are part of the lymphatic system, form a ring at the connection
of the oral cavity and the pharynx. Here, they protect against foreign
invasion of antigens. Therefore the respiratory tract aids the immune
system through this protection. Then the air travels through the larynx.
The larynx closes at the epiglottis to prevent the passage of food or
drink as a protection to our trachea and lungs. The larynx is also our
voicebox; it contains vocal cords, in which it produces sound. Sound is
produced from the vibration of the vocal cords when air passes through
them.
The trachea, which is also known as our windpipe, has
ciliated cells and mucous secreting cells lining it, and is held open
by C-shaped cartilage rings. One of its functions is similar to the
larynx and nasal cavity, by way of protection from dust and other
particles. The dust will adhere to the sticky mucous and the cilia
helps propel it back up the trachea, to where it is either swallowed or
coughed up. The mucociliary escalator extends from the top of the trachea all the way down to the bronchioles, which we will discuss later. Through the trachea, the air is now able to pass into the bronchi.
Inspiration
Inspiration is initiated by contraction of the diaphragm and in some cases the intercostals muscles when they receive nervous impulses. During normal quiet breathing, the phrenic nerves stimulate the diaphragm to contract and move downward into the abdomen.
This downward movement of the diaphragm enlarges the thorax. When
necessary, the intercostal muscles also increase the thorax by
contacting and drawing the ribs upward and outward.
The active increase of the thorax changes the stability set up in a
resting lung. As the thoracic wall moves away from lung which increases
the space between the thoracic wall and lung and decreases the pressure
in the intrapleural cavity. This decrease in pressure causes the
pressure in the alveoli to become greater than the elastic recoil that
is inherent in lung tissue. Thus, when contraction of the diaphragm and
the intercostal muscles actively increase the size of the thorax, the
lungs are passively forced to expand. This expansion increases the size
of the alveoli which decreases pressure in the alveoli. Pressure within
the alveoli is now lower than atmospheric pressure which allows air to
move into the lungs through the structures discussed above.
During quiet breathing, expiration is normally a passive process and
does not require muscles to work. When the lungs are stretched and
expanded, stretchy receptors within the alveoli send inhibitory nerve
impulses to the medulla oblongata, causing it to stop sending signals
to the rib cage and diaphragm to relax and rise. This elastic recoil
causes the lungs and chest cavity to shrink and increase the air
pressure within the lungs. This increased positive air pressure pushes
the air out of the lungs. Expiration happens as the diaphragm relaxes.
Although the respiratory system is primarily under involuntary control,
and regulated by the medulla oblongata, we have some voluntary control
over it also. This is due to the higher brain function of the cerebral
cortex.
When under physical or emotional stress, more frequent and deep
breathing is needed, and both inspiration and expiration will work as
active processes. Additional muscles in the rib cage forcefully
contract and push more air out of the lungs. (This cannot occur during
rest.) In addition to deeper breathing, when coughing or sneezing we
exhale forcibly. Our abdominal muscles will contract suddenly (when
there is an urge to cough or sneeze), raising the abdominal pressure.
The rapid increase in pressure pushes the relaxed diaphragm up against
the pleural cavity. This causes air to be forced out of the lungs.
Another function of the respiratory system is to sing and to speak.
Our exert of conscious control over our breathing is what allows us to
speak and sing.
Lung Compliance is the magnitude of the change in lung volume
produced by a change in pulmonary pressure. Compliance can be
considered the opposite of stiffness. A low lung compliance would mean
that the lungs would need a greater than average change in intrapleural
pressure to change the volume of the lungs. A high lung compliance
would indicate that little pressure difference in intrapleural pressure
is needed to change the volume of the lungs. More energy is required to
breathe normally in a person with low lung compliance. Persons with low
lung compliance due to disease therefore tend to take shallow breaths
and breathe more frequently.
Determination of Lung Compliance Two major things determine
lung compliance. The first is the elasticity of the lung tissue. Any
thickening of lung tissues due to disease will decrease lung
compliance. The second is surface tensions at air water interfaces in
the alveoli. The surface of the alveoli cells is moist. The attractive
force, between the water cells on the alveoli, is called surface
tension. Thus, energy is required not only to expand the tissues of the
lung but also to overcome the surface tension of the water that lines
the alveoli.
To overcome the forces of surface tension, certain alveoli cells secret a protein and lipid complex called ""Surfactant””.
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