Download Respiratory Pulmonary Ventilation

Respiratory Pulmonary Ventilation
Pulmonary Ventilation
Pulmonary ventilation is the act of breathing and the first step in
the respiratory process. Pulmonary ventilation brings in air with a new
supply of oxygen and a very small amount of carbon dioxide from the
atmosphere into the alveoli. This mixture then participates in external
respiration, the exchange of oxygen and carbon dioxide between the
alveoli and pulmonary capillary blood across the respiratory
membrane. Internal respiration is the exchange of gasses between
the tissues of the body and the blood, which provides oxygen for aerobic
cellular respiration and removes carbon dioxide. Aerobic Cellular
respiration refers to the intracellular use of oxygen and the generation
of carbon dioxide waste through metabolic pathways.
Pressure
Breathing or ventilation, involves changes in pressure as a result of
mechanical work.
The physical movement of air into the lungs is a result of the production
of differences in total pressure between the interior (alveolar pressure)
and exterior (atmospheric pressure) of the respiratory zone.
Atmospheric pressure at sea level is typically 1 atmosphere or 760
mmHg, and this value will be used as the reference atmospheric
pressure here. One atmosphere is the air pressure that would push a
column of the mercury up in a thin tube a distance of 760 mm. Mercury
is used because it is a liquid at room temperature, and its density
changes very little over pressure and temperature ranges. In the process
of ventilation, air moves down pressure gradients going from an area of
higher pressure to an area of lower pressure. Assuming a person is at sea
level, if his or her intrapulmonary pressure, the pressure in the alveoli, is
less than 1 atm, air enters the lungs and fills the alveoli. If his or her
intrapulmonary pressure is greater than 1 atm, then air moves out of the
lungs into the environment.
There is another pressure often discussed in the mechanics of
ventilation, intrapleural (sometimes just pleural) pressure. This is the
pressure within the pleural sac (space). Intrapleural pressure is
maintained slightly less than the pressure in the alveoli. This pressure
difference aids in keeping the lungs slightly inflated at all times and
ensures that they do not collapse when exhaling.
Inspiration and Expiration
Gases have a property of distributing to fill whatever size and shape
container they occupy. If a closed container is made larger (an increase
in volume), the total number of gas molecules will stay the same, but
they will redistribute to fill the larger space. In doing so, they decrease
their concentration (remember that concentration is a property of mass
and volume). This increase in volume but not number of molecules of
the gas leads to a corresponding decrease in the pressure exerted by that
gas on the walls of the container. If the same closed container is made
smaller, the concentration of gases increases( even though the actual
number of gas molecules has not changed) and the pressure increases.
This is an example of a physical law calledBoyle’s Law.
The law is expressed mathematically as:
P1V1 = P2V2
where P = pressure, V = volume, 1 is the initial pressure and volume,
and 2 is the resulting pressure and volume.
Boyle’s Law states that, at a constant temperature, the pressure of a gas
is inversely proportional with the volume of its container. Very simply,
as volume goes up, pressure goes down, and as volume goes down,
pressure goes up. These properties become important for ventilation
because the passage of air into and out of the lungs is controlled by the
size of the lungs’ container, the thorax. The difference between the
example above and the lungs is that the lungs are not a closed system
(unless you have a closed glottis), with the opening to the lungs
maintained through the conducting zone. But over very short periods of
time Boyle’s law still applies. In this case, when we inhale, the
movement of the diaphragm and the ribs increases the volume of the
thorax. This immediately decreases the air pressure within the thorax,
creating a pressure gradient between the atmosphere and the alveoli
(the latter is called intrapulmonary pressure), and drawing air into the
lungs. Air will enter until the atmospheric and intrapulmonary
pressures are equal. However, the volume of the thorax remains larger
than before the start of inhalation. To exhale, the thorax is allowed to
decrease in volume (the ribs and diaphragm return to their original
positions), increasing the intrapulmonary pressure and creating a
gradient in the opposite direction resulting in the flow of air out to the
atmosphere. Changing the size of the thoracic cavity would be
impossible if the ribs were solidly attached to the sternum. If they were
solidly attached, it would be like trying to move the sides of a birdcage.
The costal cartilage allows the ribs to move and expand or contract the
chest wall.
Inspiration
Inspiration is the act of inhaling. As stated above, inspiration occurs by
increasing the volume of the thorax. This active process involves the use
of chest and neck muscles. Mostly movement of the diaphragm achieves
resting inspiration. The relaxed shape of the diaphragm resembles a
shallow dome with the apex pointing toward the lungs, similar to the
shape of an open umbrella. When the diaphragm contracts, it tends to
flatten out, expanding the volume of the thorax in an inferior direction.
Consequently, the intrapleural and intrapulmonary pressures decrease
below atmospheric pressure resulting in air being pulled through the
conducting zone and into the lungs. The external intercostal muscles
work in conjunction with the diaphragm. The normal orientation of the
ribs is around the side of the thorax and angled inferiorly to the
sternum. When the external intercostal muscles contract, the ribs are
pulled up, also expanding the thoracic cavity in a horizontal direction. In
adults, ventilation occurs about 12 times a minute and moves roughly
500 ml of air during each breath.
A deep breath involves a greater shortening of the diaphragm and the
external intercostal muscles plus additional contractions of other neck
and chest muscles. The scalene muscles elevate the first two ribs. The
sternocleidomastoid muscles elevate the sternum, and the pectoralis
minor muscles help to elevate the third, fourth, and fifth ribs. Two
physical conditions that interfere with inspiration are obesity and
advanced pregnancy. In both cases, the abdominal organs are pushed
against the diaphragm, restricting its downward movement and
hindering the expansion of the thoracic cavity.
Expiration
Expiration is the act of exhaling. Resting expiration is a passive process.
The muscles used during inspiration relax, and allow the chest wall and
the diaphragm to move back to their original position, thus decreasing
the volume of the thorax and forcing air from the lungs. The
compression of the chest wall also aids in moving blood and lymph
through the vessels that drain the lungs. Expiration becomes an active
process when a more forceful exhale is required. The internal intercostal
muscles pull the ribs down, helping to compress the chest. The external
and internal oblique and transverse abdominal muscles press on the
abdominal organs, which move them up against the diaphragm and
force the diaphragm higher than it would normally go on relaxation,
further decreasing the volume of the thoracic cavity.
Airway resistance, alveolar surface tension, and lung compliance
One thing that works against the pressure gradient created by the
expansion of the thorax is the resistance found within the conducting
zone. Under normal conditions this resistance is quite low, and the
airways have little trouble passing air between the atmosphere and the
alveoli. The small amount of resistance found in the system stems
mainly from the bronchioles, which are analogous to the arterioles of the
vascular system. Both the bronchioles and the arterioles contribute a
large portion of the resistance of their entire system, and both have
smooth muscles in their walls that allow them to constrict or dilate.
Under conditions of bronchiolar constriction (bronchoconstriction),
resistance to airflow can increase dramatically. When this happens
rapidly, it is classified as an acute asthma attack. The afflicted individual
will need to generate much larger changes in intrapulmonary pressure
to maintain a normal flow rate of air during breathing. Recall that if
resistance increases an increased pressure gradient is necessary to
maintain the same flow. To achieve this, the person uses the accessory
muscles to breath, and will appear to be “straining” as he or she does so.
Treatment usually involves an inhaler containing a bronchodilator.
Resistance will also be affected if the airways become narrowed by
mucus (chronic asthma) or aspirated substances.
Along with the resistance of the airways, the compliance of the lung
tissue also determines how much effort breathing requires. Compliance
is a property that explains the relationship between a volume change
and pressure change. Typically it takes a 2-3 cm of water change in
pleural pressure to change volume in the lungs by 500 ml. This is a
compliance of about 200 ml/cm H2O. The compliance of the lungs is
dependent on the elasticity of the connective tissues of the lung as well
as the alveolar surface tension. Remember that a thin liquid layer
containing water as well as other molecules lines the interior lung
surface. Because of its polar nature, water exhibits cohesion, such that it
takes some effort to separate water molecules (you may have
experienced the force of this cohesion if you ever belly flopped into the
water). This surface tension is actually enough to collapse the alveoli
each time a person exhales. In order to prevent this from happening, the
type II alveolar cells make a combination of lipids and proteins that
serves as a surfactant in the alveoli. A surfactant is a chemical that acts
like a detergent breaking the surface tension of the water lining the
alveoli. This action allows the alveoli to remain open when exhaling.
Compliance of the lung can be decreased either by fibrosis of the lung
tissue creating a stiffening of the tissue (this can occur with conditions
such as tuberculosis) or lack of surfactant (common in premature
infants). Either way breathing becomes much more difficult, requiring
more effort and sometimes mechanical ventilation.