Pulmonary ventilation is a major function event of respiration. It is the movement of air in and out of alveoli.
The passage of air contains following parts;
Nasal Cavity:Air is drawn into the nasal cavity from nose through a pair of nostrils where two nasal sacs are present.
Pharynx:The nasal cavity opens into a passage through glottis called pharynx.
Trachea:It is a long wind pipe about 12 cm long. The walls of trachea are supported by incomplete rings of
cartilage which keep the trachea open and resist it from collapsing.
Bronchi And Bronchioles:The trachea branches into two bronchi which have almost same structure
as the trachea but smaller diameter. Each bronchi splits into numerous branches called bronchioles.
In all areas of the trachea and bronchi not occupied by cartilage plates, the walls are composed
mainly of smooth muscles. Also , the walls of the bronchioles are almost entirely smooth muscles,
with the exception of nose terminal bronchioles called the respiratory bronchioles which is mainly
pulmonary epithelium and unlying fibrous tissue plus a few smooth muscles fibers.
Alveoli:At the end of bronchioles tiny air sacs are present called alveoli. Each lung contains about
300 Millions alveoli. The air we breathe in eventually reaches the alveoli. Each alveolus is
surrounded by a network of thin blood vessels. O2 passes through the wall of the alveoli into
the blood vessels. The blood then carries te O2 to the body cells. Meanwhile, the CO2
present in the blood passes out of the blood vessels and into the alveoli.
Lungs:Lungs are most advance type of respiratory organs. They are situated in thorax.
They are seperated from each other by a pleural membrane present inside the pleural cavity.
Pulmonary ventilation or breathing is the exchange of air between the atmosphere and the lungs.
As air moves into and out of the lungs,it travels from regions of high air pressure to the regions of
low air pressure.
• To identify the muscles used during ventilation.
• To understand how volume changes in the thoracic cavity cause pressure changes that lead to
• To identify factors which influence airway resistance and lung compliance.
Pulmonary ventilation is the process by which gasses flow between the atmosphere and lung alveoli. Air
moves into the lungs when air pressure inside the lungs is less than the air pressure in the atmosphere and out
of the lungs when the pressure inside the lungs is greater than the atmosphere pressure.
Breathing in is called inhalation or inspiration. Just before each inhalation air pressure inside the alveoli
is equal to the atmospheric pressures, 760 rnmHg, and the intrapleural pressure is 756 mmHg. With
inspiration, the cavity of the thorax is enlarged as external intercostals muscles and the diaphragm contract
causing a decrease in the intrapleural pressure to about 754 mmHg. The parietal pleura lining the cavity is
pulled outward in all direction and the visceral pleura and lungs are pulled along with it. As the lung volume
increases in this way the pressure inside the lungs i.e. 760 mmHg drops to 758 mmHg. Thus a pressure
difference is established between the atmosphere and alveoli and air flows into the lungs from the
atmosphere.Inhalation or inspiration is an active process.
Breathing out or exhalation or expiration starts when the inspiratory muscles and diaphragm relaxes,
the dome of the diaphragm moves up, the ribs are depressed leading to decrease in the lung volume and
increase in the lung pressure (763 rnm Hg). Air flows out form the area of higher pressure to the area of lower
pressure in the atmosphere. The elastic recoil of the chest wall and the lungs is due to the recoil of elastic
fibers and the inward pull of surface tension due to the alveolar fluid. A thin layer of alveolar fluid coats the
surface of the alveoli and exerts a force known as surface tension. Exhalation or expiration is a passive
Mechanics of Pulmonary Ventilation
Muscles That Cause Lung Expansion and Contraction
The lungs can be expanded and contracted in two ways
(1)By downward and upward movement of the diaphragm to lengthen or shorten the chest cavity:- During
inspiration, contraction of the diaphragm pulls the lower surfaces of the lungs downward. Then, during
expiration,the diaphragm simply relaxes, and the elastic recoil of the lungs, chest wall, and abdominal structures
compresses the lungs and expels the air. During heavy breathing, however, the elastic forces are not powerful
enough to cause the necessary rapid expiration, so that extra force is achieved mainly by contraction of the
abdominal muscles, which pushes the abdominal contents upward against the bottom of the diaphragm, thereby
compressing the lungs.
(2) By elevation and depression of the ribs to increase and decrease the anteroposterior diameter of the chest
cavity:- The second method for expanding the lungs is to raise the rib cage. This expands the lungs because, in
the natural resting position, the ribs slant downward, thus allowing the sternum to fall backward toward the
vertebral column. But when the rib cage is elevated, the ribs project almost directly forward, so that the sternum
also moves forward, away from the spine, making the anteroposterior thickness
of the chest about 20 per cent greater during maximum inspiration than during expiration.Therefore,all the muscles
that elevate the chest cage are classified as muscles of inspiration, and those muscles that depress the chest
cage are classified as muscles of expiration.
External intercostals:The most important muscles that raise the rib cage.
Sternocleidomastoid muscles:They lift upward on the sternum.
Anterior serrate:Which lift many of the ribs.
Scalen:Which lift the first two ribs.
Abdominal recti:Which have the powerful effect of pulling downward on the lower ribs at the same time
that they and other abdominal muscles also compress the abdominal contents upward
against the diaphragm.
Internal intercostals:The muscles that pull the rib cage downward during expiration.
Pressures That Cause the Movement of Air In and Out of the Lungs
Pleural pressure is the pressure of the fluid in the thin space between the lung
pleura and the chest wall pleura.This is normally a slight suction, which means
a slightly negative pressure. The normal pleural pressure at the beginning of
inspiration is about –5 centimeters of water, which is the amount of suction
required to hold the lungs open to their resting level. Then, during normal
inspiration, expansion of the chest cage pulls outward on the lungs with
greater force and creates more negative pressure, to anaverage of about
–7.5 centimeters of water.
Alveolar pressure is the pressure of the air inside the lung alveoli. When the
glottis is open and no air is flowing into or out of the lungs, the pressures in all
parts of the respiratory tree, all the way to the alveoli, are equal to atmospheric
pressure, which is considered to be zero reference pressure in the airways,
that is, 0 centimeters water pressure. To cause inward flow of air into the alveoli
during inspiration, the pressure in the alveoli must fall to a value slightly below
atmospheric pressure (below 0). The second curve labeled ―alveolar pressure‖
in Figure demonstrates that during normal inspiration, alveolar pressure decreases
to about –1 centimeter of water. This slight negative pressure is enough to pull 0.5
liter of air into the lungs in the 2 seconds required for normal quiet inspiration.
During expiration, opposite pressures occur: The alveolar pressure rises to
about +1 centimeter of water, and this forces the 0.5 liter of inspired air out of the
lungs during the 2 to 3 seconds of expiration.
The difference between the alveolar pressure and the pleural pressure. This is called
the transpulmonary pressure. It is the pressure difference between that in the alveoli
and that on the outer surfaces of the lungs, and it is a measure of the elastic forces
in the lungs that tend to collapse the lungs at each instant of respiration, called the
Compliance of the Lungs
The extent to which the lungs will expand for each unit increase in transpulmonary
pressure (if enough time is allowed to reach equilibrium) is called the lung compliance.
The total compliance of both lungs together in the normal adult human being averages
about 200 milliliters of air per centimeter of water transpulmonary pressure. That is,
every time the transpulmonary pressure increases 1 centimeter of water, the lung
volume, after 10 to 20 seconds, will expand 200 milliliters.
Compliance of the Thorax and the Lungs Together
The compliance of the entire pulmonary system (the lungs and thoracic cage together)
is measured while expanding the lungs of a totally relaxed or paralyzed person.To do this,
air is forced into the lungs a little at a time while recording lung pressures and volumes.To
inflate this total pulmonary system, almost twice as much pressure is needed as to inflate
the same lungs after removal from the chest cage.Therefore, the compliance of the
combined lung-thorax system is almost exactly one half that of the lungs alone—110
milliliters of volume per centimeter of water pressure for the combined system, compared
with 200 ml/cm for the lungs alone. Furthermore, when the lungs are expanded to high volumes
or compressed to low volumes, the limitations of the chest become extreme.
Effect of the Thoracic Cage on Lung Expansibility
Thus far, we have discussed the expansibility of the lungs alone, without considering
the thoracic cage.The thoracic cage has its own elastic and viscous characteristics,
similar to those of the lungs; even if the lungs were not present in the thorax, muscular
effort would still be required to expand the thoracic cage.
Resistance Within Airways
• As air flows into the lungs, the gas molecules encounter resistance when they strike the walls of the
airway. Therefore the diameter of the airway affects resistance.
• When the bronchiole constricts, the diameter decreases, and the resistance increases. This is
because more gas molecules encounter the airway wall. Airflow is inversely related to resistance.
• Airflow equals the pressure difference between atmosphere and intrapulmonary pressure,
divided by the resistance.
• As the resistance increases, the airflow decreases.
• As the resistance decreases, the airflow increases.
• In healthy lungs, the airways typically offer little resistance, so air flows easily into and out of the
Factors Affecting Airway Resistance
• Several factors change airway resistance by affecting the diameter of the airways. They do this by
contracting or relaxing the smooth muscle in the airway walls, especially the bronchioles.
• Parasympathetic neurons release the neurotransmitter acetylcholine, which constricts bronchioles.
As you can see in the equation, increased airway resistance decreases airflow.
• Histamine, released during allergic reactions, constricts bronchioles. This increases airway
resistance and decreases airflow, making it harder to breathe.
• Epinephrine, released by the adrenal medulla during exercise or stress, dilates bronchioles, thereby
decreasing airway resistance. This greatly increases airflow, ensuring adequate gas exchange.
SURFACTANTS AND SURFACE TENSION
Surface tension:- is the tendency of molecules in a fluid to be pulled toward the center of the fluid. It is measured
as an energy per unit area (J/m2) or as the force across a line (N/m)
Surfactants:- They are substances that reduce surface tension
They prevent water droplets from blocking airways.
High surface tension would tend to decrease the surface area of the lungs, thus making it harder to absorb air.
The surface tension of pure water is about 70 mN/m. With lung surfactant, it can drop lower than 2 mN/m.
(Possmayer et al., 2001, Measurement online)
Components of lung surfactant:·
35-40% dipalmitoyl phosphatidylcholine (DPPC), a phospholipid
30-45% other phospholipids
5-10% protein (SP-A, B, C, and D)
cholesterols (neutral lipids) and trace amounts of other substances
Minute Respiratory Volume Equals Respiratory Rate Times Tidal Volume
The minute respiratory volume is the total amount of new air moved into the respiratory passages each minute;
this is equal to the tidal volume times the respiratory rate per minute. The normal tidal volume is about 500
milliliters, and the normal respiratory rate is about 12 breaths per minute. Therefore, the minute respiratory volume
averages about 6 L/min. A person can live for a short period with a minute respiratory volume as low as 1.5 L/min
and a respiratory rate of only 2 to 4 breaths per minute. The respiratory rate occasionally rises to 40 to 50 per
minute, and the tidal volume can become as great as the vital capacity, about 4600 milliliters in a young adult man.
This can give a minute respiratory volume greater than 200 L/min, or more than 30 times normal. Most people
cannot sustain more than one half to two thirds these values for longer than 1 minute.
The ultimate importance of pulmonary ventilation is to continually renew the air in the gas
exchange areas of the lungs, where air is in proximity to the pulmonary blood. These areas include the alveoli,
alveolar sacs, alveolar ducts, and respiratory bronchioles.The rate at which new air reaches these areas is called
Rate of Alveolar Ventilation
Alveolar ventilation per minute is the total volume of new air entering the alveoli and adjacent gas
exchange areas each minute. It is equal to the respiratory rate times the amount of new air that enters these areas
with each breath.
VA = Freq • (VT – VD)
where VA is the volume of alveolar ventilation per minute, Freq is the frequency of respiration per minute, VT is
the tidal volume, and VD is the physiologic dead space volume. Thus, with a normal tidal volume of 500 milliliters,
a normal dead space of 150 milliliters, and a respiratory rate of 12 breaths per minute, alveolar ventilation equals
12 ¥ (500 – 150), or 4200 ml/min.
Alveolar ventilation is one of the major factors determining the concentrations of oxygen and carbon dioxide in
the alveoli.Therefore, almost all discussions of gaseous exchange in the following chapters on the respiratory
system emphasize alveolar ventilation.
Muscle activity causes changes in the volume of the thoracic cavity during breathing.
Changing the thoracic cavity volume causes intrapulmonary and intrapleural pressure changes,
which allow air to move from high pressure to low pressure regions.
Airway resistance is normally low, but nervous stimulation and chemical factors can change the
diameter of bronchioles, thereby altering resistance and airflow.
Lung compliance is normally high due to the lung's abundant elastic tissue and surfactant's ability
to lower the surface tension of the alveolar fluid.
Guyton and Hall textbook of medical physiology
Medical physiology LANGE William F. Ganong
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