Respiratory System - Paper 1999
1999_2nd
semester_Q5_Part_B
The
brain stem plays in important role in maintaining the normal breathing pattern.
The medulla regions is responsible for initiating the inspiration and expiration
phases of breathing. It contains two separate groups of neurons namely: Dorsal
Respiratory Group and Ventral Respiratory Group.
The
dorsal respiratory group has predominantly inspiratory neurons which initiate
the inspiration phase. These neurons fire just before inspiration and relay
their signals via the phrenic motor neurons which cause contraction of the
diaphragm. The contract of the diaphragm cause volume and pressure differences
hence inspiration is complete. The ventral respiratory group contains
inspiratory and expiratory neurons. The expiratory neurons fire just before
expiration and provide excitatory signals to the expiratory muscles, mainly the
abdominal muscles. These muscles contract and cause forceful expiration. They
also send inhibitory signals to the inspiratory neurons to prevent inspiration
from occuring. This way inspiration and expiration occurs.
The
pons region of the brain stem contains two centers which have effects on the DRG
and VRG. The pneumotaxic center is located in the upper pons area and when this
increases in activity, its reciprical signals to the dorsal respiratory group
neurons inhibit excitation of these neurons so that shorter breaths result. This
then initiates the expiration process. This is called phase switching. The
pneumotaxic center has an effect on the breathing rate.
The
apneustic center on the other hand is responsible for prolonging inspiration.
These send excitatory signals to the inspiratory neurons and prolong the
inspiration phase. This is responsible for controlling the tidal volume of each
breath.
Collectively all of these are responsible for maintaining the breathing pattern.
1999_2nd semester_Q4_Part_B
There
are many factors that are important when considering the transfer of oxygen and
carbon dioxide between the alveolar air and pulmonary blood. These are as
follows. The main factor is the maintenance of a concentration gradient between
the alveolar air and the pulmonary blood with respect to concentrations of the
gases involved, the presence of this concentration gradient, the surface area of
the gas exchange surface namely: the respiratory membrane. Together all of these
factors aid transfer of the gases mentioned above.
The
bulk flow of pulmonary blood to the alveolus and the bulk flow of air into the
lung is responsible for the maintenance of the concentration gradients of oxygen
and carbon dioxide. The bulk flow of the pulmonary requires energy which is
provided by the contractile force of the right ventricle of the heart. The bulk
flow of the air into the lungs is provided by energy from the contraction of
respiratory muscles.
When
oxygen enters the lung it is immediately humidified and warmed due to the water
vapour present. This vapour exerts its own partial pressure and therefore the
partial pressure of oxygen is reduced to about 150mmHg from 160mmHg. As the
oxygen moves deeper into the tree, the carbon dioxide concentration increases
and exerts its own partial pressures (about 40mmHg) and the partial pressure of
oxygen is reduced even further. The mixed venous blood has partial pressure of
oxygen about 40mmHg. As it passes the alveolar walls, there is a huge gradient
present and therefore oxygen will diffuse through the respiratory membrane and
into the blood. The carbon dioxide pressure in mixed venous blood is about
46mmHg and therefore it will tend to diffuse into the alveolus. The large
surface area provided by the alveoli of the lungs makes for efficient gas
exchange, as gas exchange is directly proportional to the surface area over
which it occurs. Also transfer of oxygen and carbon is aided by the thinness of
the respiratory membrane. Together all of these facts aid the gas exchange
mechanism.
Major
Points: Concentration gradient important >
how is it maintained? > by bulk flow of blood and air into/around lungs >
surface area >
large area means good transfer of gases > thinness of the respiratory
membrane