Download Respiratory system

CHAPTER 49
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The Respiratory System
Chapter 49
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
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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
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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
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Gills
• Branchial chambers
– Provide a means of pumping water past
stationary 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
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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
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Gills
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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
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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
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Amphibians and Arthropods
• 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
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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
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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
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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 6000 m the atmospheric pressure is
380 mm Hg
– PO2 = 380 x 20.95% = 80 mm Hg
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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
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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
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Lungs
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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
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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
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Lungs
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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
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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
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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
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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
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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
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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
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Lung Structure and Function
• Neurons are sensitive to blood PCO2 changes
• A rise in PCO2 --------- > Send impulses to
respiratory control center to increase rate of
breathing
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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
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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
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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
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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
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Transportation of Carbon Dioxide
• Other dissolved gases are also
transported by hemoglobin
– Nitric oxide (NO) and carbon monoxide (CO)
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