Download Handout 1 Ventilatory system review

Principal structures of the ventilatory system
Smooth muscle tissue is found on the walls of some of our internal hollow organs. It produces smooth,
rhythmical actions. We cannot consciously control the action of smooth muscle. It is subsequently
termed involuntary. (movement of blood and air in the lungs)
The nostrils are fringed with coarse hair, which strains large particles out of the airstream and may also
serve to protect the nasal cavity from invasion by insects. The interior of the nasal cavity contains
projections of considerable surface area. These projections, nasal conchae, make the airstream
turbulent and subsequently warm and hydrate it. Mucus in the nasal cavity also filters as well as
moisten the air. Thanks to the structure of the nose, air entering
the trachea is virtually 100% humidified.
Air passes through the 3 portions of the pharynx, which provides a
low resistance path for airflow, to the trachea via the larynx. In
addition to its function as the voice box the larynx protects the
trachea from invasion by foods and fluids.
The cartilaginous trachea, branches into the two main bronchi. The
trachea is a thin walled tube about the diameter of an average
garden hose. It is composed of very thin, tough connective tissue
and is strengthened at intervals by incomplete rings of cartilage.
The trachea muscle runs down the posterior wall of the trachea. This is an
example of smooth muscle. The lining of the tracheobronchial system is
designed to protect the lungs from dehydration and invasion by foreign
particles, including micro-organisms.
The lungs themselves develop at the end of the bronchi. They are elastic
spongy organs. Gas exchange is carried out by a complex of structures at
the end of each terminal bronchioles. They are simple thin walled
structures which also have numerous thin-walled out pocketings called
alveoli, which are specialized for the function of gaseous exchange.
Questions:
1. Describe the function of smooth muscle.
2. How is air cleaned and moistened during inhalation
3. List the structures air passes through during inhalation.
The mechanics of ventilation in the human lungs
To understand how a person breathes, you need to know that a substance called pleural fluid lies
between the lungs and the chest wall. Have you ever put two pieces of wet glass together (e.g.
microscope slides) and found that you could not easily pull them apart. This phenomenon results from a
combination of forces – surface tension, molecular cohesion and atmospheric pressure.
Think of the walls of the chest and the lungs as the two wet slides and the pleural fluid as the film of
water. When the chest expands during breathing, the film of pleural fluid causes the membranous walls
of the lungs to be pulled outward along with the chest walls. This means the space within the lungs
increases. The air molecules in the lungs now move momentarily farther apart, so that the pressure in of
the air in the lungs falls below the pressure of the atmosphere outside the body. Consequently, air from
outside rushes down the trachea and into the lungs until the two pressures are equal again. This is the
process of inspiration.
Observation of the skeleton reveals that each rib pivots about a vertebral
joint. If it is lifted upward it also swings outward, with the thoracic cavity
being enlarged anteriorly and superiorly. This is the task in quiet
breathing of the external intercostal muscles. At the same time the ribs
are lifted, the diaphragm (the muscular floor of the thoracic cavity)
contracts downward enlarging the thoracic cavity inferiorly. This process
enlarges the cavity twofold.
Expiration is almost entirely a passive process that depends on the
elasticity of the lungs and chest structures, as well as fluid film surface
tensions within the lungs. When inspiratory muscles are relaxed, air simply leaves the lung, much as it
would leave an untied balloon.
This above description is for quiet breathing. When one speaks or runs, the abdominal muscles press
upon the abdominal contents, squeezing them upwards against the
diaphragm. The internal intercostal muscles oppose the external
intercostals and pull the ribcage downward, helping to decrease the
thoracic cavity volume and forcibly empty the lungs. The diaphragm may
also function in forcible expiration.
In labored inspiration (e.g. accompanying exercise) many of the muscles of
the upper trunk are also recruited. They are only indirectly attached to the
ribs and are inefficient as respiratory muscles. (Pectoralis major and minor,
Trapezius, Rhomboids)
The pressure of oxygen arriving at the alveoli is high, and the pressure of it
in the capillaries is low. Therefore oxygen diffuses from the alveoli into the
blood. The opposite is true for carbon dioxide.
Questions:
1. Discuss the process of inhalation. (include all terms necessary)
2. Discuss the process of expiration. (include all terms necessary)
Respiratory Terms
Pulmonary ventilation is commonly referred to as breathing. It is the process of air flowing into the lungs
during inspiration (inhalation) and out of the lungs during expiration (exhalation). Air flows because of
pressure differences between the atmosphere and gases inside the lungs. Air, like other gases, flows
from a region with higher pressure to a region with lower pressure. Muscular breathing movements and
recoil of elastic tissues create the changes in pressure that result in ventilation. Pulmonary ventilation
involves three different pressures, atmospheric pressure, intraalveolar (intrapulmonary) pressure and
intrapleural pressure. Atmospheric pressure is the pressure of the air outside the body. Intraalveolar
pressure is the pressure inside the alveoli of the lungs. Intrapleural pressure is the pressure within the
pleural cavity. These three pressures are responsible for pulmonary ventilation.It is important to
understand the various volumes and capacities of the lungs in order to appreciate the effects of exercise
on the respiratory system.
Total lung capacity is the volume of air in the lungs after a maximal inhalation (can be calculated by
adding vital capacity to residual volume of the lungs). Vital capacity is the maximal volume of air that
can be exhaled after a maximal inhalation. During normal, quiet respiration, about 500mL of air is
inspired. The same amount of air moves out with expiration. This volume of air is called the tidal volume.
We can also forcibly exhale. This is termed the expiratory reserve volume. Even after the expiratory
reserve volume is expelled, some air is still trapped in the lungs because of pressure. This is called the
residual volume. When we forcibly take a deep breath, we can take in up to 3100mL above the tidal
volume. This additional air is the inspiratory reserve volume.