Lecture 23

Chapter 23
The Respiratory
System

Copyright © John Wiley & Sons, Inc. All rights reserved.
Upper
respiratory
tract
Lower
respiratory
tract
Respiratory System Anatomy
Structurally, the respiratory system is divided into
upper and lower divisions or tracts.
The upper respiratory tract
consists of the nose, pharynx
and associated structures.
The lower respiratory tract
consists of the larynx,
trachea, bronchi and
lungs.

Functionally, the respiratory system is divided into the
conducting zone and the respiratory zone.
The conducting zone is involved with bringing air to
the site of external respiration.
The respiratory zone is the main site of gas exchange
and consists of the respiratory bronchioles, alveolar
ducts, alveolar sacs, and alveoli.

Air passing through the respiratory
tract traverses the:
Nasal cavity
Pharynx
Larynx
Trachea
Primary (1o
) bronchi
Secondary (2o
) bronchi
Tertiary (3o
) bronchi
Bronchioles
Alveoli (150 million/lung)

The external nose is visible on the face.
The internal nose is a large cavity beyond the nasal
vestibule.

Three nasal conchae (or turbinates) protrude from each
lateral wall into the breathing passages.

The pharynx is a hollow tube that starts posterior to
the internal nares and descends to the opening of the
larynx in the neck.
It is formed by a complex arrangement of skeletal
muscles that assist in deglutition.
It functions as:
o a passageway for air and food
o a resonating chamber
o a housing for the tonsils

The pharynx has 3 anatomical regions:
The nasopharynx; oropharynx; and laryngopharynx

The nasopharynx lies behind the internal nares.
It contains the pharyngeal tonsils (adenoids) and the
openings of the
Eustachian tubes
(auditory tubes)
which come off
of it and travels
to the middle
ear cavity.

The oropharynx lies behind the mouth and participates
in both respiratory and digestive functions.
The main palatine tonsils (those usually taken in a
tonsillectomy) and small lingual tonsil are housed
here.
The laryngopharynx lies inferiorly and opens into the
larynx (voice box) and the esophagus.
It participates in both respiratory and digestive
functions.

The larynx, composed of 9 pieces of cartilage, forms a
short passageway connecting the laryngopharynx with
the trachea (the “windpipe”).
Cartilages include thyroid, Cricoid
and epiglottis.
Anterior view of the larynx

The epiglottis is a flap of elastic cartilage covered with a
mucus membrane, attached to the root of the tongue.
The epiglottis guards the entrance of the glottis, the
opening between the vocal folds.
o For breathing, it is held
anteriorly, then pulled back-
ward to close off the glottic
opening during
swallowing.

The rima glottidis (glottic opening) is formed by a pair of
mucous membrane vocal folds (the true vocal cords).
The vocal folds are situated high in the larynx just
below where the larynx and the esophagus split off
from the pharynx.

Cilia in the upper respiratory tract move mucous and
trapped particles down toward the pharynx.
Cilia in the lower respiratory tract move them up toward
the larynx.

Upper
respiratory
tract
Lower
respiratory
tract
As air passes from the laryngopharynx into the larynx, it
leaves the upper respiratory tract and enters the lower
respiratory tract.
Air passing through the respiratory
tract
Nasal cavity
Pharynx
Larynx
Trachea
Primary bronchi
Secondary bronchi
Tertiary bronchi
Bronchioles
Alveoli (150 million/lung)

The trachea is a semi-rigid pipe made of semi-circular
cartilaginous rings, and located anterior to the esophagus.
It is about 12 cm long and extends from the inferior
portion of the larynx into the mediastinum where it
divides into right and left primary (1o
, “mainstem”)
bronchi.
It is composed of 4 layers: a mucous secreting epithelium
called the mucosa, and three layers of CT (submucosa,
hyaline cartilage, and adventitia).

The tracheal cartilage rings are incomplete posteriorly,
facing the esophagus.
Esophageal masses can press into this soft part of the
trachea and make it difficult
to breath, or even
totally obstruct
the airway.

The right and left primary (1o
or “mainstem”) bronchi
emerge from the inferior trachea to go to the lungs,
situated in the right and left pleural cavities.
The carina is an internal
ridge located at the junction
of the two mainstem
bronchi – a very sensitive
area for triggering the
cough reflex.

The 1o
bronchi divide to form 2o
and 3o
bronchi which
respectively supply the lobes and segments of each lung.
3o
bronchi divide into
bronchioles which in
turn branch through
about 22 more divisions
(generations).
o The smallest are the
terminal bronchioles.

The bronchi and bronchioles go through structural
changes as they branch and become smaller.
The mucous membrane changes and then disappears.
The cartilaginous rings become more sparse, and
eventually disappear altogether.
As cartilage decreases, smooth muscle (under the
control of the Autonomic Nervous System) increases.
o Sympathetic stimulation causes airway dilation,
while parasympathetic stimulation causes airway
constriction.

All the branches from the trachea to the terminal
bronchioles are conducting
airways – they do not
participate in gas
exchange.

The cup-shaped outpouchings which participate in gas
exchange are called alveoli.
The first alveoli don’t appear until
the respiratory
bronchioles
where they are
rudimentary and
mostly
nonfunctioning.

Respiratory bronchioles give way to alveolar ducts, and
the epithelium (simple cuboidal) changes to simple
squamous, which comprises the alveolar ducts, alveolar
sacs, and alveoli.

Taken together, these structures form the functional unit
of the lung, which is the pulmonary lobule.
Wrapped in elastic
C.T., each pulmonary
lobule contains a
lymphatic vessel, an
arteriole, a venule
and a terminal
bronchiole.
The pulmonary lobule

As part of the pulmonary lobule, alveoli are delicate
structures composed chiefly of type I alveolar cells,
which allow for exchange of gases with
the pulmonary capillaries.
Alveoli make up a large
surface area (750 ft2).
Type II cells secrete a
substance called surfactant
that prevents collapse of the
alveoli during exhalation.

Alveoli macrophages (also called “dust cells”) scavenge
the alveolar surface to engulf and remove microscopic
debris that has made it past the “mucociliary blanket”
that traps most foreign particles higher in
the respiratory tract.
The alveoli (in close proximity
to the capillaries) form the
alveolar-capillary membrane
(“AC membrane”).

Blood Supply to the Lungs
The lungs receive blood via two sets of arteries
Pulmonary arteries carry deoxygenated blood from
the right heart to the lungs for oxygenation
Bronchial arteries branch from the aorta and
deliver oxygenated blood to the lungs primarily
perfusing the muscular walls of the bronchi and
bronchioles

Ventilation-Perfusion Coupling
Ventilation-perfusion coupling is the coupling of
perfusion (blood flow) to each area of he lungs to match
the extent of ventilation (airflow) to alveoli in that area
In the lungs, vasoconstriction in response to hypoxia
diverts pulmonary blood from poorly ventilated areas of
the lungs to well-ventilated regions
In all other body tissues, hypoxia causes dilation of blood
vessels to increase blood flow

Understanding Gases
The respiratory system depends on the medium of the
earth’s atmosphere to extract the oxygen necessary for
life.
The atmosphere is composed of these gases:
Nitrogen (N2) 78%
Oxygen (O2) 21%
Carbon Dioxide (CO2) 0.04%
Water Vapor variable, but on average
around 1%

Understanding Gases
The gases of the atmosphere have a mass and a weight
(5 x 1018
kg, most within 11 km of the surface).
Consequently, the atmosphere exerts a significant
force on every object on the planet (recall that
pressure is measured as force applied per unit area,
P = F/A.)
We are “accustomed” to the tremendous force
pressing down on every square inch of our body.

Understanding Gases
A barometer is an
instrument that measures
atmospheric pressure.
Baro = pressure or weight
Meter = measure
Air pressure varies greatly
depending on the altitude
and the temperature.

Understanding Gases
There are many different units used to measure
atmospheric pressure. At sea level, the air pressure is:
14.7 lb/in2 = 1 atmosphere
760 mmHg = 1 atmosphere
76 cmHg = 1 atmosphere
29.9 inHg = 1 atmosphere
At high altitudes, the atmospheric pressure is less;
descending to sea level, atmospheric pressure is greater.

Understanding Gases
Gases obey laws of physics called the gas laws.
These laws apply equally to the gases of the
atmosphere, the gases in our lungs, the gases
dissolved in the blood, and the gases diffusing into
and out of the cells of our body.
To understand the mechanics of ventilation and
respiration, we need to have a basic understanding of
3 of the 5 common gas laws.

Understanding Gases
Boyle’s law applies to containers with flexible walls – like
our thoracic cage.
It says that volume and pressure are inversely related.
o If there is a decrease in volume – there will be an
increase in pressure.
o V ∝ 1/P

Understanding Gases
Dalton’s law applies to a mixture of gases.
It says that the pressure of each gas is directly
proportional to the percentage of that gas in the
total mixture: PTotal = P1 + P2 + P3 …
Since O2 = 21% of atmosphere, the partial pressure
exerted by the contribution of just O2 (written pO2
or PAO2) = 0.21 x 760 mmHg = 159.6 mmHg at sea
level.

Understanding Gases
Henry’s law deals with gases and solutions.
It says that increasing the partial pressure of a gas
“over” (in contact with) a solution will result in more
of the gas dissolving into the solution.
The patient in this picture is getting
more O2 in contact with his
blood - consequently,
more oxygen goes
into his blood.
Medicimage/Phototake

Ventilation and Respiration
Pulmonary ventilation is the movement of air between
the atmosphere and the alveoli, and consists of inhalation
and exhalation.
Ventilation, or
breathing, is made
possible by changes
in the intrathoracic
volume.

In contrast to ventilation, respiration
is the exchange of gases.
External respiration (pulmonary)
is gas exchange between the
alveoli and the blood.
Internal respiration (tissue)
is gas exchange between
the systemic capillaries and
the tissues of the body.

Certain thoracic
muscles participate in
inhalation; others aid
exhalation.
The diaphragm is
the primary muscle
of respiration – all
the others are
accessory.

Measuring Ventilation
Ventilation can be measured using spirometry.
Tidal Volume (VT) is the volume of air inspired (or
expired) during normal quiet breathing (500 ml).
Inspiratory Reserve Volume (IRV) is the volume
inspired during a very deep inhalation (3100 ml –
height and gender dependent).
Expiratory Reserve Volume (ERV) is the volume
expired during a forced exhalation (1200 ml).

Spirometry continued
Vital Capacity (VC) is all the air that can be exhaled
after maximum inspiration.
o It is the sum of the inspiratory reserve + tidal
volume + expiratory reserve (4800 ml).
Residual Volume (RV) is the air still present in the
lungs after a force exhalation (1200 ml).
o The RV is a reserve for mixing of gases but is not
available to move in or out of the lungs.

Old and new spirometers used to measure ventilation.

A graph of spirometer volumes and capacities

Only about 70% of the tidal volume reaches the
respiratory zone – the other 30% remains in the
conducting zone (called the anatomic dead space).
If a single VT breath = 500 ml, only 350 ml will
exchange gases at the alveoli.
o In this example, with a respiratory rate of 12, the
minute ventilation = 12 x 500 = 6000 ml.
o The alveolar ventilation (volume of air/min that
actually reaches the alveoli) = 12 x 350 = 4200ml.

Exchange of O2 and CO2
Using the gas laws and understanding the principals of
ventilation and respiration,
we can calculate the
amount of oxygen and
carbon dioxide
exchanged between
the lungs and
the blood.

Dalton’s Law states that each gas in a mixture of gases
exerts its own pressure as if no other gases were present.
The pressure of a specific gas is the partial pressure Pp.
Total pressure is the sum of all the partial pressures.
Atmospheric pressure (760 mmHg) = PN2 + PO2 + PH2O
+ PCO2 + Pother gases
o Since O2 is 21% of the atmosphere, the PO2 is
760 x 0.21 = 159.6 mmHg.

Each gas diffuses across a permeable membrane (like the
AC membrane) from the side where its partial pressure is
greater to the side where its partial pressure is less.
The greater the difference, the faster the rate of
diffusion.
Since there is a higher PO2 on the lung side of the AC
membrane, O2 moves from the alveoli into the blood.
Since there is a higher PCO2 on the blood side of the
AC membrane, CO2 moves into the lungs.

PN2 = 0.786 x 760 mmHg = 597.4 mmHg
PO2 = 0.209 x 760 mmHg = 158.8 mmHg
PH2O = 0.004 x 760 mmHg = 3.0 mmHg
PCO2 = 0.0004 x 760 mmHg = 0.3 mmHg
Pother gases = 0.0006 x 760 mmHg = 0.5 mmHg
Total = 760.0 mmHg
Partial pressures of gases in inhaled air for sea level

Transport of O2 and CO2
In the blood, some O2 is dissolved in the plasma as a gas
(about 1.5%, not enough to stay alive – not by a long
shot!). Most O2 (about 98.5%) is carried attached to Hb.
Oxygenated Hb is called oxyhemoglobin.

CO2 is transported in the blood in three different forms:
1. 7% is dissolved in the plasma, as a gas.
2. 70% is converted into carbonic acid through the
action of an enzyme called carbonic anhydrase.
o CO2 + H2O H2CO3 H+
+ HCO3
-
3. 23% is attached to Hb (but not at the same binding
sites as oxygen).

The amount of Hb saturated with O2 is called the SaO2.
Each Hb molecule can carry 1, 2, 3, or 4 molecules of
O2. Blood leaving the lungs has Hb that is fully
saturated (carrying 4 molecules of
O2 – oxyhemoglobin).
o The SaO2 is close to 95-98% .
When it returns, it still has 3 of
the 4 O2 binding sites occupied.
o SaO2 = 75%

Measuring SaO2 has
become as commonplace
in clinical practice as
taking a blood pressure.
Pulse oximeters which
used to cost $5,000
can now be purchased
at your local
pharmacy.
3660 Group,
Inc/NewsCom

Although PO2 is the most important determinant of SaO2,
several other factors influence the affinity with which
Hb binds O2 .
Acidity (pH), PCO2 and blood temperature shift the
entire O2 –Hb saturation
curve either to the left
(higher affinity for O2), or
to the right (lower affinity
for O2).

Fetal and Maternal Hemoglobin
Fetal hemoglobin (Hb-F) has a higher affinity for oxygen
(it is shifted to the left) than adult hemoglobin A, so it
binds O2 more strongly.
The fetus is thus able
to attract oxygen
across the placenta
and support life,
without lungs.

Initial Response
Mucous layer thickens.
Goblet cells over-secrete
mucous.
Basal cells proliferate.
Advanced Response to Irritation
Mucous layer and goblet cells disappear.
Basal cells become malignant & invade deeper tissue.
Normal columnar epithelium
in the respiratory tract
Response to Pollutants

Diseases and Disorders
Asthma is a disease of hyper-reactive airways (the major
abnormality is constriction of smooth muscle in the
bronchioles, and inflammation.) It presents as attacks of
wheezing, coughing, and excess mucus production.
It typically occurs in response to allergens; less often
to emotion.
Bronchodilators and anti-
inflammatory corticosteroids
are mainstays of treatment.
Pulse Picture Library/CMP mages /Phototake

Chronic bronchitis and emphysema are caused by
chronic irritation and inflammation leading to lung
destruction. Patients may cough up
green-yellow sputum due to
infection and increased mucous
secretion (productive cough).
They are almost exclusively
diseases of cigarette smoking.

Pneumonia is an acute infection of the lowest parts of
the respiratory tract.
The small bronchioles and alveoli become filled with
an inflammatory fluid exudate.
o It is typically caused by infectious agents such as
bacteria, viruses, or fungi.

Normal Lungs Pneumonia Patient
Du Cane Medical Imaging, Ltd./Photo Researchers, Inc

Lecture 23

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Lecture 23