Respiratory System - Paper 2000
2000_2nd
Semester_Q4_Part_A
The
conducting airways from the nasopharynx to the bronchioles contains many
intermediate structures through which air has to pass through in order to be
able to participate in gas exchange.
The
structures involved are: oropharynx which is shared between the digestive and
respiratory system, laryngopharynx, the epiglottis (structure protecting the
entry into the respiratory passageways), the larynx which contains two sheets of
tissue projecting into the lumen which forms the vocal cords and the opening
called glottis. The passage of air through this vibrates the sheets and produces
sound. Other structures include the trachea, which splits into two primary
bronchi supplying each lung at the carina. The primary bronchi split into
secondary bronchi each supplying the lobes of the lung, and the tertiary bronchi
are formed from the division of the secondary bronchi and these supply each of
the bronchopulmonary segments of the lung. From here the divisions occur
constantly, forming a bronchial tree and when the diameter becomes less than
1mm, it is called bronchioles.
There
are significant changes that occur in the mucosal lining of the airway wall as
we move down the respiratory tree. Firstly the epithelium changes from a
pseudostratified columnar epithelium to a simple columnar and them to a low
columnar and eventually to a cuboidal epithelium at the bronchioles. The
epithelium of the trachea contains lots of cilia (finger like projections from
the cell into the lumen) and lots of goblet cells that secrete mucus. As we move
down the tree, the amount of goblet cells decreases with only cilia being
present, and eventually the cilia become absent leaving macrophages to deal
foreign dust particles. The cartilage rings present in the tracheal wall also
decrease in number, and are irregularly arranged as we move down the tree. But
eventually these rings are absent at the stage of the bronchioles. The amount of
smooth muscles increases as we move down, therefore at the level of the
bronchioles, there is no cartilage present but lots of smooth muscle enabling
constriction of the airwarys. Throughout the passageways the connective tissue
that supports their structures have elastic fibrs which account for the stretch
recoil properties of the lung.
Major Points: Oropharynx > laryngeopharyx > larynx > vocal cords > glottis > epiglottis > trachea > 1, 2 and 3 bronchi and associated lung areas > bronchioles > epithelium changes > cilia changes > goblet cells changes > cartilage ring changes > smooth muscle changes > elastic fibres always present > functional significance.
2000_2nd
semester_Q4_PartB
The
term “airway resistance” refers to the amount of resistance the air
encounters when it enters the respiratory passages. This is the frictional force
present when the air moves through the conducting airways. Airway resistance is
an important consideration regarding chronic obstructive pulmonary diseases.
The
relationship between airway resistance and pulmonary pressure is that when one
increases, so does the other. That is, as resistance is increased, then the
amount of space available for the movement of air is reduced and therefore a
change in volume produces a change in pressure, hence pressure will increase.
Similar, an increase in pressure will mean that the volume will change and
therefore offer more resistance to the flow of air.
Airway
resistance can be altered by mucous accumulations inside the airways and also by
the constriction of the respiratory passageways and therefore cause an increase
in airway. Dilation of these passage, increases the cross sectional area and
therefore reduce the airway resistance.
2000_2nd
semester_Q5_Part_A
The
partial pressure of oxygen in normal arterial blood is about: 100mmHg. This is
in the pulmonary capillaries therefore we can expect this value to lower when
oxygen offloading occurs at the tissues. The partial pressure of oxygen in mixed
venous blood is about 40mmHg. This is in the pulmonary capillaries therefore we
can expect this value to be higher at the level of the tissues.
2000_2nd
semester_Q5_Part_B
The
structure involved in the response of the respiratory system to an acute fall in
arterial oxygen partial pressure to half its normal value is the peripheral chemo receptors
namely: the carotid bodies and the aortic bodies. The carotid bodies are located
within the blood vessels of the neck and thorax and mainly at the bifurcation of
the common carotid artery. It receives a large blood supply from the internal
carotid artery which is innervated by branches of the glossopharyngeal nerve.
There are two types of cells involved at the carotid bodies. These are the type
1 glomus cells, and the other is the type 2 cells. The type 1 glomus cells seem
to have transmitter vesicles located within the cells.
The
aortic bodies are located on the aortic arch of the aorta, and these are
innervated by branches of the vagus
nerve.
A
sudden fall in arterial oxygen partial pressure below 60-70mmHg causes the
stimulation of the carotid bodies and the aortic bodies, although the latter do
not have much influence on the respiration. A decrease will send signals to the
medullary rhythmic center and therefore cause increase in ventilation to bring
in more oxygen and therefore increase the arterial oxygen partial pressure,
bringing the body back to homeostasis.
If the hypoxemia becomes chronic, peripheral chemoreceptors becomes very sensitive to further changes in oxygen partial pressures within the arterial system. They also increase in size to respond to events quickly.