Download Chapter 48 - The Respiratory System

Chapter 48
Lecture and
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The Respiratory System
Chapter 48
2
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
3
Gas Exchange
• Rate of diffusion between two regions is
governed by Fick’s Law of Diffusion
• R = Rate of diffusion
• D = Diffusion constant
• A = Area over which diffusion takes place
• Dp = Pressure difference between two sides
• d = Distance over which diffusion occurs
DA Dp
R=
d
4
Respiratory champion
• Elephant seals
– Can hold their breath for over 2 hours
– Can descend and ascend rapidly repeatedly
– Can dive great depths
5
Gas Exchange
• Evolutionary changes have occurred to
optimize the rate of diffusion R
– Increase surface area A
– Decrease distance d
– Increase concentration difference Dp
6
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
7
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Single Cell Organisms
Amphibians
O2
CO2
CO2
O2
Epidermis
Blood vessel
a.
b.
8
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Insects
Echinoderms
Spiracle
Epidermis
Papula
Trachea
O2
CO2
O2
CO2
CO2
c.
O2
d.
9
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fish
Mammals
CO2
O2
Alveoli
Blood
vessel
O2
CO2
Gill
lamellae
e.
f.
10
Gills
• Specialized extensions of tissue that
project into water
• Increase surface area for diffusion
• External gills are not enclosed within body
structures
– Found in immature fish and amphibians
– Two main disadvantages
• Must be constantly moved to ensure contact with
oxygen-rich fresh water
• Are easily damaged
11
Gills
• Branchial chambers
– Provide a means of pumping water past
stationary gills
– Internal mantle cavity of mollusks opens to
the outside and contains the gills
• Draw water in and pass it over gills
– In crustaceans, the branchial chamber lies
between the bulk of the body and the hard
exoskeleton of the animal
• Limb movements draw water over gills
12
Gills
• Gills of bony fishes are located between
the oral (buccal or mouth) cavity and the
opercular cavities
• These two sets of cavities function as
pumps that alternately expand
• Move water into the mouth, through the
gills, and out of the fish through the open
operculum or gill cover
13
Gills
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Buccal cavity Operculum
Oral valve
Water
Mouth opened, Gills Opercular
jaw lowered
cavity
Mouth closed,
operculum opened
14
Gills
• Some bony fish have immobile opercula
– Swim constantly to force water over gills
– Ram ventilation
• Most bony fish have flexible gill covers
• Remora switch between ram ventilation
and pumping action
15
Gills
• 3–7 gill arches on each side of a fish’s
head
• Each is composed of two rows of gill
filaments
• Each gill filament consist of lamellae
– Thin membranous plates that project into
water flow
– Water flows past lamellae in 1 direction only
16
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Operculum Gills
Water
flow
OxygenOxygendeficient
rich
blood
blood
Water
flow
Gill arch
Water flow
Gill raker
Oxygenrich
blood
Oxygendeficient blood
Gill
filaments
Gill
filament
Lamellae with
capillary networks
Blood flow
17
Gills
• Within each lamella, blood flows opposite
to direction of water movement
– Countercurrent flow
– Maximizes oxygenation of blood
– Increases Dp
• Fish gills are the most efficient of all
respiratory organs
18
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Countercurrent Exchange
Water (100%
Blood (85%
O2 saturation)
O2 saturation)
Water (50%
Blood (50%
O2 saturation)
O2 saturation)
85%
100%
80%
90%
70%
80%
60%
70%
50%
60%
50%
50%
40%
50%
40%
60%
30%
40%
30%
70%
20%
30%
20%
80%
10%
15%
10%
90%
Blood (0%
O2 saturation) Water (15%
O2 saturation)
a.
Concurrent Exchange
No further
net diffusion
Blood (0%
O2 saturation) Water (100%
O2 saturation)
b.
19
Gills
• Many amphibians use cutaneous
respiration for gas exchange
• In terrestrial arthropods, the respiratory
system consists of air ducts called
trachea, which branch into very small
tracheoles
– Tracheoles are in direct contact with individual
cells
– Spiracles (openings in the exoskeleton) can
20
be opened or closed by valves
Lungs
• Gills were replaced in terrestrial animals
because
– Air is less supportive than water
– Water evaporates
• The lung minimizes evaporation by moving
air through a branched tubular passage
• A two-way flow system
– Except birds
21
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
22
Lungs
• Partial pressures are based on the % of
the gas in dry air
• At sea level or 1.0 atm
– PN2 = 760 x 79.02% = 600.6 mm Hg
– PO2 = 760 x 20.95% = 159.2 mm Hg
– PCO2 = 760 x 0.03% = 0.2 mm Hg
• At 6,000 m the atmospheric pressure is
380 mm Hg
– PO2 = 380 x 20.95% = 80 mm Hg
23
Lungs
• Lungs of amphibians are formed as
saclike outpouchings of the gut
• Frogs have positive pressure breathing
– Force air into their lungs by creating a positive
pressure in the buccal cavity
• Reptiles have negative pressure breathing
– Expand rib cages by muscular contractions,
creating lower pressure inside the lungs
24
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Nostrils open
External nostril
Air
Nostrils closed
Buccal cavity
Esophagus
Air
Lungs
a.
b.
25
Lungs
• Lungs of mammals are packed with
millions of alveoli (sites of gas exchange)
• Inhaled air passes through the larynx,
glottis, and trachea
• Bifurcates into the right and left bronchi,
which enter each lung and further
subdivide into bronchioles
• Alveoli are surrounded by an extensive
capillary network
26
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
27
Lungs
• Lungs of birds channel air through very
tiny air vessels called parabronchi
• Unidirectional flow
• Achieved through the action of anterior
and posterior sacs (unique to birds)
• When expanded during inhalation, they
take in air
• When compressed during exhalation, they
push air in and through lungs
28
Lungs
• Respiration in birds occurs in two cycles
– Cycle 1 = Inhaled air is drawn from the
trachea into posterior air sacs, and exhaled
into the lungs
– Cycle 2 = Air is drawn from the lungs into
anterior air sacs, and exhaled through the
trachea
• Blood flow runs 90o to the air flow
– Crosscurrent flow
– Not as efficient as countercurrent flow
29
Lungs
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Cycle 1
a.
Inhalation
Exhalation
Parabronchi of lung
Anterior
air sacs
Posterior
Trachea
Anterior
air sacs
Lung
air sacs
Posterior
air sacs
Trachea
Cycle 2
Inhalation
Exhalation
a.
b.
30
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
31
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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
32
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
33
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
34
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
35
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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)
36
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37
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
38
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
39
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)
40
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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
41
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
42
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
43
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
44
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Healthy Lungs
Cancerous Lungs
a: © Clark Overton/Phototake; b: © Martin Rotker/Phototake
45
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
46
The structure of the adult hemoglobin protein
47
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
48
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
49
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
50
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
51
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”
52
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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
53
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)
54