The Respiratory System
Gas Exchange at a Major and Minor Scale
Christin DeMoss
Audience & Scope
Introductory physiology students will utilize this description to gain a more precise
understanding of the respiration process. These physiology students will have prior
knowledge of the subject through lectures and online readings. This description will not
serve as a replacement to the lectures and readings, but will instead serve to enhance,
clarify, and better organize existing knowledge. This description will extend the
students' respiration knowledge and will comprehensively outline the process to help
the students understand the material.
Introduction
The respiratory system is a physiological process involving inspiration, expiration, and
gas exchange. The average adult human breathes about 8 to 16 times a minute1. With
an average respiratory rate of 12 breaths a minute and 1,440 minutes in a day, the
average adult takes approximately 17,280 breaths a day. It is imperative introductory
physiology students understand this frequent, life sustaining physiological process.
The process of respiration occurs in a stepwise sequence that involves inhaling
oxygenated air and exhaling deoxygenated air. The components that convert air from
oxygenated to deoxygenated are elaborated throughout this technical description. The
major components of the respiration process are displayed below in diagram 1.
In addition to the steps of respiration, it is equally important to fully understand how
respiration is controlled. This technical description also elaborates on the basics of
homeostatic regulation.
Diagram 1: Simplified process of respiration
Inspiration
Gas
Exchange
Expiration
Overview
The respiratory system is comprised of two divisions, the conduction division (major
scale) and the respiratory division (minor scale)2. Table 1 shows the major components
of each division.
Table 1: Conduction and Respiration Division Overview
1. The Conduction Division
2. The Respiration Division
 Nose and nasal cavity
 The Alveoli (site of gas exchange)
 Oral cavity
 Pharynx
The alveoli and the capillaries that surround
 Larynx
them can be observed in figure 2.
 Trachea
 Bronchial Tree
These components can be observed
In figure 1.
Figure 1:
Figure 2:
Figure 3: The pathway of inspired air through both the conducting and respiration
divisions.
1. Inspiration
8.
Expiratio
n
7.
nose/mo
uth,
pharynx,
larynx,
and
trachea
6.
brachial
tree
2.
nose/mo
uth,
pharynx.
larynx,
and
trachea
3.
brachial
tree
4. alveoli
5. gas
exchange
Figure 3 demonstrates the pathway that inspired air takes during the process of
respiration. As shown in the figure, oxygenated air is inspired and travels through the
components of the conduction division to the alveoli located at the terminal ends of the
bronchioles.
Once oxygenated air reaches the alveoli, O2 diffuses across the membrane into the
blood, while CO2 from the blood diffuses into the alveoli. Once in the blood, the O2 is
transported to the heart and pumped through the body.
The CO2 in the alveoli is carried with the deoxygenated air back through the conduction
system and expired through the mouth and nose.
The details of the respiratory system are elaborated on in the following sections.
Division I: Conduction
1. Inspiration: For air to be inspired, the diaphragm must first contract and pull the
lungs downward. This contraction increases the volume of the lungs while decreasing
the intrapulmonary pressure in the lungs. Once the atmospheric pressure becomes
greater than the decreased pressure in the lungs, air rushes into the nose and mouth.2
Figure 4 demonstrates this process.
Figure 4: Inhalation vs. Exhalation
Intrapulmonary pressure >
atmospheric pressure
Atmospheric pressure >
intrapulmonary pressure
2. Mouth/nasal cavity -> pharynx -> larynx -> trachea: Inspired air travels into the body
through the mouth and nasal cavity where it is warmed and cleaned along the way. As
the air moves through the pharynx then to the trachea, it is further cleaned and
humidified2. The larynx located at the top of the trachea functions to ensure only air
enters the lungs.
Figure 5: Respiratory anatomy
Inspired Air
3. Bronchial Tree: The trachea splits into two bronchi, the right primary bronchus and
left primary bronchus. Each primary bronchus continues to branch to the smallest
bronchi called respiratory bronchioles2. Figure 6 shows the succession of bronchi
branching and figure 7 shows a visual of the brachial tree.
Figure 6: Brachial Tree
Figure 7: Visual of Brachial Tree
Primary Bronchi
Secondary Bronchi
Tertiary Bronchi
Bronchioles
Terminal Bronchioles
*Please note that the terminal bronchioles are too small to be shown on this
diagram.
Division 2: Respiration
4. Alveoli: The alveoli located at the terminal end of the terminal bronchioles and are
surrounded by a dense covering of capillaries. Figure 8 displays the alveoli structure.
Figure 8: Alveoli
5. Gas exchange: When air reaches the alveoli, the O2 from the air diffuses across the
alveolar membrane and into the blood. At the same time, CO2 from the deoxygenated
blood diffuses into the alveoli. The oxygenated blood returns to the heart to be
pumped to the rest of the body. The deoxygenated air travels back through the
conducting division of the respiratory system to be expired through the mouth and
nose. Figure 9 demonstrates this process.
Figure 9: Gas Exchange
The O2 is moving from the alveoli to
the capillary and the CO2 is moving
from the capillary to the alveoli.
Alveoli
**Expelled air returns to the conduction division of the Respiratory System after Gas Exchange**
6. Brachial Tree: The deoxygenated air travels back through the brachial tree.
Figure 10: Brachial Tree
Terminal Bronchioles
Bronchioles
Tertiary bronchi
Secondary bronchi
Primary bronchi
7. Trachea -> Larynx -> Pharynx -> Mouth/nasal cavity: As the deoxygenated air moves
back through the conduction division, the trachea, larynx, pharynx, and mouth/nasal
cavity all act to dehumidify the air. Reducing the amount of water in exhaled air
reduced the amount of water lost via breathing.
8. Expiration: Once gas exchange has occurred, the diaphragm relaxes and compresses
the lungs. This compression decreases the lung volume and increases the
intrapulmonary pressure of the lungs. Deoxygenated air can be expired when the
pressure inside the lungs is greater than the atmospheric pressure.2 Please refer to
figure 4 on page 4.
Homeostatic Regulation
Homeostatic regulation also occurs in a stepwise process like respiration2. Homeostatic
regulation is comprised of:
 The respiration control centero Located at the base of the brain
o Includes the medulla oblongata and pons
o Controls breathing rate
o Neurons run from control center to diaphragm
 Chemoreceptors
o Located in arteries
o Monitor CO2, H+, and O2 blood concentrations

Neurons
o Afferent (from muscle to control center)
o Efferent (from control center to muscle)
It is important to note that carbon dioxide in the blood combines with water and yields
hydrogen protons and bicarbonate. Equation 1 demonstrates this process below:
Equation 1:
Since CO2 breaks down into H+ and HCO3- in the blood, high levels of H+ signify high levels
of CO2 and low levels of H+ signify low levels of CO2. How the body reacts to the level of
H+ in the blood is summarized below.
Table 2: Homeostatic Regulation Overview
High H+ in Blood
1. Chemoreceptors detect a change in
equilibrium
2. Chemoreceptors send a message to
the respiratory control center via
afferent neurons
3. Efferent neurons from the control
center tell the diaphragm to
contract more quickly
4. Breathing increases and CO2 is
expelled
Low H+ in Blood
1. Chemoreceptors detect a change in
equilibrium
2. Chemoreceptors send a message to
the respiratory control center via
afferent neurons
3. Efferent neurons from the control
center tell the diaphragm to
contract less frequent
4. Breathing decreases, increasing
CO2 concentration
Conclusion
There is no doubt that the respiratory process functions as one of the most important
physiological pathways. A full understanding of this process is necessary to comprehend
other physiological processes. This technical description, in conjunction with lectures
and online readings, serves to clarify and organize respiration for introductory
physiology students.
References
Content
1. Dugdale, David C. "Rapid Shallow Breathing: MedlinePlus Medical Encyclopedia."
U.S National Library of Medicine. U.S. National Library of Medicine, 25 May 2011.
Web. 18 Oct. 2012.
<http://www.nlm.nih.gov/medlineplus/ency/article/007198.htm>.
2. "Respiratory System Tutorial." Biology 142: Physiology Laboratory: Spring 2012.
N.p., n.d. Web. 18 Oct. 2012. <http://cms.psu.edu/>.
Figures
-Figure 1
"THE RESPIRATORY SYSTEM." Respiratory System. N.p., 18 May 2010. Web. 23 Oct.
2012.
<http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookrespsys.html>.
-Figure 2
"Respiratory System Tutorial." Biology 142: Physiology Laboratory: Spring
2012. N.p., n.d. Web. 18 Oct. 2012. <http://cms.psu.edu/>.
-Figure 4
"The Mechanics of Breathing." Respiration_page. N.p., n.d. Web. 23 Oct. 2012.
<http://magnetscience.inspiringteachers.com/ap/group2/Respiratory/Respiratio
n_page.html>.
-Figure 5
"RESPIRATORY MANAGEMENT IN SPINAL CORD INJURY: NORMAL BREATHING
AND THE RESPIRATORY TRACT." Normal Breathing and the Respiratory Tract.
N.p., n.d. Web. 23 Oct. 2012.
<http://calder.med.miami.edu/pointis/normbr.html>.
-Figure 7
"Bronchi, Bronchial Tree, & Lungs." SEER Training: Bronchi, Bronchial Tree, & Lungs.
N.p., n.d. Web. 23 Oct. 2012.
<http://training.seer.cancer.gov/anatomy/respiratory/passages/bronchi.html>.
-Figure 8
"File:Alveoli Diagram.png." Wikipedia. N.p., n.d. Web. 23 Oct. 2012.
<http://en.wikipedia.org/wiki/File:Alveoli_diagram.png>.
-Figure 9
"Human Respiration." Human Respiration, Excretion, and Locomotion. N.p., n.d. Web.
23 Oct. 2012.
<http://www.goldiesroom.org/Note%20Packets/13%20Human%20Other/00%20
Human%20Other%20Systems--WHOLE.htm>
*Note- Figures 3, 6, and 10 along with tables 1 and 2 were hand made and no
reference was used.
Equations
Equation 1
"Respiratory System Tutorial." Biology 142: Physiology Laboratory: Spring
2012. N.p., n.d. Web. 18 Oct. 2012. <http://cms.psu.edu/>.