Pulmonary ventilation, also known as breathing, is the process of air flowing into and out of the lungs. During inspiration, air enters the respiratory system through the mouth and nose, then travels through the pharynx, larynx, trachea, bronchi and bronchioles before reaching the alveoli in the lungs where gas exchange occurs. Expiration is the opposite process where air is exhaled from the lungs out through the mouth and nose. The diaphragm and intercostal muscles control inspiration and expiration by fluctuating the pressure and volume of the chest cavity.

1. Pulmonary Ventilation
Dr. K. Rama Rao
Department of Zoology
Phone: 9010705687

2. 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 the gases
inside the lungs.

3. Air enters the respiratory
system through the
mouth and the nasal
cavity, passing through
the pharynx then larynx
(where sounds are
produced for speech) and
finally the trachea which
enters the chest cavity.

4. These passages (the nasal cavity in
particular) are lined with cilia (tiny hair like
structures) that act as a filter, blocking
germs from entering the respiratory system
as well as warming and moistening the air.

5. At the top of the
oesophagus is a plate
of flexible elastic
cartilage called the
epiglottis which
closes over the larynx
as we swallow food to
ensure the food
travels to the
stomach and not into
the lungs

6. In the chest cavity the trachea splits into two
smaller tubes called the left and right
bronchi.
These further divide into secondary bronchi
which lead directly into the lungs, where
they divide into many smaller tubes called
bronchioles

7. The bronchioles
finally connect to
tiny structures
called alveoli where
gas exchange
occurs, as can be
seen in the adjacent
diagram.

8. Respiratory muscles and their role in
respiration
When we breathe in, air flows into the lungs
down what is known as a ‘pressure gradient’
from an area of high pressure to an area of
low pressure.
The pressure gradient within the lungs is
controlled by a muscle called the diaphragm.

9. When contracted the diaphragm flattens
and pushes downwards resulting in an
enlargement of the chest cavity (area within
which the lungs sit) and a decrease of
pressure in the lungs.
This reduction in pressure causes air to be
sucked into the lungs and we breathe in, as
seen in the following image.

11. As we breathe out the opposite happens
as the diaphragm relaxes and pushes
upwards, as seen in the right image
above.
This reduces the size of the chest cavity
making the pressure within the lungs
higher and results in air being exhaled.
In turn the pressure gradient within the
body returns to its original state and the
process starts again.

12. During exercise when inspiration increases,
the external intercostal muscles are
recruited to help with the increase in
ventilation rate.
They work to lift the ribs up and outwards,
further increasing the chest cavity and
enabling more air to be inspired, as seen on
the image below.

14. Expiration is generally a passive process
that does not require muscle
contraction.
As the muscles of inspiration relax and
return to their resting positions air is
forced out of the lungs as the pressure
within them increases.

15. However, when needed, active and or
forced expiration can be completed by
voluntary contraction of the abdominals
and internal intercostal muscles, as seen
in the above image.
This further reduces the volume within the
chest cavity and more air is exhaled.

16. Pulmonary ventilation summarised
The following images and table provides a
usefull summary of pulmonary ventilation
or 'breathing

17. Inspiration Expiration
1. Diaphragm contracts
pushing downward
2. Lungs expand as
pressure Inside chest
Decreases
3. Air drawn in through
nose and mouth
1. Diaphragm relaxes
pushing upward
2. Lungs made smaller
and more pressurized
by relaxing diaphragm
3. Air passes out
through nose and
mouth