The Respiratory System
1
Gas Exchange
• One of the major physiological challenges facing
all multicellular animals is obtaining sufficient
oxygen and disposing of excess carbon dioxide
• In vertebrates, the gases diffuse into the
aqueous layer covering the epithelial cells that
line the respiratory organs
• Diffusion is passive, driven only by the difference
in O2 and CO2 concentrations on the two sides
of the membranes and their relative solubilities
in the plasma membrane
2
Respiratory champion
• Elephant seals
– Can hold their breath for over 2 hours
– Can descend and ascend rapidly repeatedly
– Can dive great depths
3
Gas Exchange
• Gases diffuse directly into unicellular organisms
• However, most multicellular animals require
system adaptations to enhance gas exchange
• Amphibians respire across their skin
• Echinoderms have protruding papulae
• Insects have an extensive tracheal system
• Fish use gills
• Mammals have a large network of alveoli
4
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Amphibians
Single Cell Organisms
O2
CO2
CO2
O2
Epidermis
Blood vessel
a.
b.
5
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Insects
Echinoderms
Spiracle
Epidermis
Papula
Trachea
O2
CO2
O2
CO2
CO2
c.
O2
d.
6
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Fish
Mammals
CO2
O2
Alveoli
Blood
vessel
O2
CO2
Gill
lamellae
e.
f.
7
Lungs
• Air exerts a pressure downward, due to
gravity
• A pressure of 760 mm Hg is defined as
one atmosphere (1.0 atm) of pressure
• Partial pressure is the pressure
contributed by a gas to the total
atmospheric pressure
8
Lungs
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Blood flow
Bronchiole
Smooth muscle
Nasal cavity
Nostril
Pharynx
Glottis
Larynx
Trachea
Right lung
Left lung
Pulmonary venule
Pulmonary arteriole
Left
bronchus
Alveolar
sac
Diaphragm
Capillary
network on
surface
of alveoli
Alveoli
9
Gas Exchange
• Gas exchange is driven by differences in partial
pressures
• Blood returning from the systemic circulation,
depleted in oxygen, has a partial oxygen
pressure (PO2) of about 40 mm Hg
• By contrast, the PO2 in the alveoli is about
105 mm Hg
• The blood leaving the lungs, as a result of this
gas exchange, normally contains a PO2 of about
100 mm
10
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Peripheral tissues
Alveolar gas
PO2 = 105 mm Hg
CO2
O2
PCO2 = 40 mm Hg
PO2 = 40 mm Hg
Alveolar gas
PO2 = 105 mm Hg
PCO2 = 46 mm Hg
PCO2 = 40 mm Hg
Lung
CO2
CO2
Pulmonary
artery
O2
O2
PO2 = 100 mm Hg
Pulmonary
vein
PCO2 = 40 mm Hg
Systemic veins
PO2 = 40 mm Hg
Systemic arteries
Peripheral tissues
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
PCO2 = 46 mm Hg
CO2
O2
11
Lung Structure and Function
• Outside of each lung is covered by the
visceral pleural membrane
• Inner wall of the thoracic cavity is lined by
the parietal pleural membrane
• Space between the two membranes is
called the pleural cavity
– Normally very small and filled with fluid
– Causes 2 membranes to adhere
– Lungs move with thoracic cavity
12
Lung Structure and Function
• During inhalation, thoracic volume
increases through contraction of two
muscle sets
– Contraction of the external intercostal
muscles expands the rib cage
– Contraction of the diaphragm expands the
volume of thorax and lungs
• Produces negative pressure which draws
air into the lungs
13
Lung Structure and Function
• Thorax and lungs have a degree of
elasticity
• Expansion during inhalation puts these
structures under elastic tension
• Tension is released by the relaxation of
the external intercostal muscles and
diaphragm
• This produces unforced exhalation,
allowing thorax and lungs to recoil
14
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Inhalation
Muscles
contract
Sternocleidomastoid
muscles contract
(for forced inhalation)
Air
Lungs
Diaphragm
contracts
a.
Exhalation
Muscles
relax
Air
Diaphragm
relaxes
b.
Abdominal muscles
contract (for forced
exhalation)
15
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16
Lung Structure and Function
• Tidal volume
– Volume of air moving in and out of lungs in a person
at rest
• Vital capacity
– Maximum amount of air that can be expired after a
forceful inspiration
• Hypoventilation
– Insufficient breathing
– Blood has abnormally high PCO2
• Hyperventilation
– Excessive breathing
– Blood has abnormally low PCO2
17
Lung Structure and Function
• Each breath is initiated by neurons in a
respiratory control center in the medulla
oblongata
• Stimulate external intercostal muscles and
diaphragm to contract, causing inhalation
• When neurons stop producing impulses,
respiratory muscles relax, and exhalation occurs
• Muscles of breathing usually controlled
automatically
– Can be voluntarily overridden – hold your breath
18
Lung Structure and Function
• Neurons are sensitive to blood PCO2 changes
• A rise in PCO2 causes increased production of
carbonic acid (H2CO3), lowering the blood pH
• Stimulates chemosensitive neurons in the aortic
and carotid bodies
• Send impulses to respiratory control center to
increase rate of breathing
• Brain also contains central chemoreceptors that
are sensitive to changes in the pH of
cerebrospinal fluid (CSF)
19
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Medulla
oblongata
Stimulus
Signal to Chemosensitive
respiratory neuron
system
Stimulus
Increased tissue
Metabolism
(i.e., muscle contraction)
Negative
feedback
Stimulus
Increased blood CO2
concentration (PCO2)
Cerebrospinal
fluid (CSF)
Sensor
Sensor
Decreased blood pH
H+ + HCO3–
H2O + CO2
H+ +
Decreased
CSF pH
HCO3–
H2CO3
Capillary
blood
Comparator
Comparator
H2O + CO2
Choroid
plexus of
brain
H2CO3
(–)
Inadequate
breathing
Central chemoreceptors
stimulated (in the brain)
Peripheral chemoreceptors stimulated
(aortic and carotid bodies)
(+)
CO2
Effector
Impulses sent to
respiratory control center
in medulla oblongata
a.
Reduced HCO3− levels (and
corresponding drop in CSF pH) result
in increased respiration, which
subsequently results in lower arterial
PCO2.
Response
b.
Diaphragm stimulated
to increase breathing
20
Respiratory Diseases
• Chronic obstructive pulmonary disease
(COPD)
– Refers to any disorder that obstructs airflow
on a long-term basis
– Asthma
• Allergen triggers the release of histamine, causing
intense constriction of the bronchi and sometimes
suffocation
21
Respiratory Diseases
• Chronic obstructive pulmonary disease
(COPD) (cont.)
– Emphysema
• Alveolar walls break down and the lung exhibits
larger but fewer alveoli
• Lungs become less elastic
• People with emphysema become exhausted
because they expend three to four times the
normal amount of energy just to breathe
• Eighty to 90% of emphysema deaths are caused
by cigarette smoking
22
Respiratory Diseases
• Lung cancer accounts for more deaths than any
other form of cancer
• Caused mainly by cigarette smoking
• Follows or accompanies COPD
• Lung cancer metastasizes (spreads) so rapidly
that it has usually invaded other organs by the
time it is diagnosed
• Chance of recovery from metastasized lung
cancer is poor, with only 3% of patients surviving
for 5 years after diagnosis
23
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Healthy Lungs
Cancerous Lungs
a: © Clark Overton/Phototake; b: © Martin Rotker/Phototake
24
Hemoglobin
• Consists of four polypeptide chains: two a and
two b
• Each chain is associated with a heme group
• Each heme group has a central iron atom that
can bind a molecule of O2
• Hemoglobin loads up with oxygen in the lungs,
forming oxyhemoglobin
• Some molecules lose O2 as blood passes
through capillaries, forming deoxyhemoglobin
25
The structure of the adult hemoglobin protein
26
Hemoglobin
• At a blood PO2 of 100 mm Hg, hemoglobin is
97% saturated
• In a person at rest, the blood that returns to the
lungs has a PO2 about 40 mm Hg less
• Leaves four-fifths of the oxygen in the blood as a
reserve
• This reserve enables the blood to supply body’s
oxygen needs during exertion
• Oxyhemoglobin dissociation curve is a graphic
representation of these changes
27
Hemoglobin
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Percent saturation
100
80
Amount of O2 unloaded
to tissues at rest
60
Amount of O2 unloaded
to tissues during exercise
40
Veins
(exercised)
20
Veins
(at rest)
Arteries
0
0
20
40
60
PO2 (mm Hg)
80
100
Oxyhemoglobin dissociation curve
28
Hemoglobin
• Hemoglobin’s affinity for O2 is affected by
pH and temperature
• The pH effect is known as the Bohr shift
– Increased CO2 in blood increases H+
– Lower pH reduces hemoglobin’s affinity for O2
– Results in a shift of oxyhemoglobin
dissociation curve to the right
– Facilitates oxygen unloading
• Increasing temperature has a similar effect
29
Hemoglobin
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100
pH 7.60
90
Percent oxyhemoglobin saturation
Percent oxyhemoglobin saturation
100
pH 7.20
80
pH 7.40
70
60
50
20% more O2 delivered to the
tissues at the same pressure
40
30
20
10
0
0
20
40
60
80
100
120
140
20°C
90
80
70
60
50
20% more O2 delivered to the
tissues at the same pressure
40
30
20
10
0
0
20
PO2 (mm Hg)
a. pH shift
43°C
37°C
40
60
80
100
120
140
PO2 (mm Hg)
b. Temperature shift
The effect of pH and temperature on the oxyhemoglobin dissociation curve
30
Transportation of Carbon Dioxide
• About 8% of the CO2 in blood is dissolved in
plasma
• 20% of the CO2 in blood is bound to hemoglobin
• Remaining 72% diffuses into red blood cells
– Enzyme carbonic anhydrase combines CO2 with H2O
to form H2CO3
– H2CO3 dissociates into H+ and HCO3–
– H+ binds to deoxyhemoglobin
– HCO3– moves out of the blood and into plasma
– One Cl– exchanged for one HCO3– – “chloride shift”
31
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Capillary
endothelium
Capillary
Erythrocyte
Nucleus of capillary
endothelial cell
CO2 dissolved
in plasma (8%)
CO2 + H2O
H2CO3
CO2 combines with
hemoglobin (20%)
H2CO3
H+ + HCO3–
H+ combines
with hemoglobin
Cl–
HCO3–
(72%)
CO2
Tissue cells
a.
Alveolar
epithelium
Nucleus of
alveolar cell
Nucleus of capillary
endothelial cell
Erythrocyte
Capillary
endothelium
Capillary
CO2 dissolved
in plasma
Hemoglobin
+ CO2
HCO3–
Alveoli
CO2 + H2O
H2CO3
HCO3– + H+
H2CO3
Cl–
CO2
32
b.
Transportation of Carbon Dioxide
• When the blood passes through
pulmonary capillaries, these reactions are
reversed
• The result is the production of CO2 gas,
which is exhaled
• Other dissolved gases are also
transported by hemoglobin
– Nitric oxide (NO) and carbon monoxide (CO)
33