Respiratory System and Gas Exchange
Gas exchange – intake of oxygen and elimination of
carbon dioxide
• Gas exchange ultimately relies on diffusion
• All animal respiratory systems share two features
that facilitate diffusion:
1. Respiratory system must remain moist (gases
must be dissolved in water to diffuse into or out
of cells)
2. Respiratory system must have large surface
area in contact with environment to allow
adequate gas exchange
• Most animals have evolved specialized respiratory
systems
In general, gas exchange in most respiratory systems occurs in
the following stages:
1. air or water, containing oxygen, is moved past a
respiratory system by bulk flow (fluids or gases move in
bulk through relatively large spaces, from areas of higher
pressure to areas of lower pressure) – commonly
facilitated by muscular breathing movements
2. oxygen and carbon dioxide are exchanged through the
respiratory surface by diffusion; oxygen is carried into
capillaries of circulatory system and carbon dioxide is
removed
3. Gases are transported between respiratory system and
tissues by bulk flow of blood as it is pumped throughout
body by heart
4. gases are exchanged between tissues and circulatory
system by diffusion (oxygen diffuses out into tissue and
carbon dioxide diffuses into capillaries based on
concentration gradients)
Four types of surfaces have evolved for gas
exchange – the animal’s own body surface,
gills, tracheal tubes, and lungs
1. Small animals or those with low energy
demands who live in a moist environment
rely solely on simple diffusion
• unicellular organisms, nematodes,
flatworms, earthworms, jellyfish, sponges
2. gills are the respiratory structures of many aquatic
animals
• elaborately branched or folded tissue for
maximum surface area
• have a dense profusion of capillaries just
beneath outer membrane (bring blood close to
surface for gas exchange)
• gas exchange occurs as a result of a
countercurrent exchange system
• blood flows in capillaries of gills in the opposite
direction as the water flowing over gills
• maximizes difference in concentration of gases
between blood and water
3. terrestrial animals have internal respiratory
structures
• air has more oxygen than water but presents
problems with being dry
• all respiratory surfaces must remain moist for
diffusion of gases to occur
• Insects use a system of elaborately branching
internal tubes called tracheae (tracheal tubes)
• air enters through tiny openings in body surface
called spiracles
• air is taken directly to cells through tubes
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Most terrestrial animals respire by means
of lungs – chambers containing moist,
delicate respiratory surfaces that are
protected in body
water loss is minimized (bathed in body
fluids) and body wall provides support
highly subdivided to increase surface
area for gas exchange
Human Respiratory System (and other
vertebrates) is divided into two parts: the
conducting portion (series of passageways that
carry air to gas-exchange portion) and the gas
exchange portion (gas is exchanged with the blood
in tiny sacs in lungs)
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nostrils – air enters body
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nasal cavities – air is warmed, filtered and
moistened
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lined with ciliated epithelium that trap
particles in mucus and move it to the throat
to be swallowed
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pharynx – back of nasal cavities that is
continuous with the throat
larynx – opening in pharynx leads to larynx or
“voice box”
contains the vocal cords
cartilage is embedded in walls to prevent
collapse
epiglottis – flap of tissue that automatically
covers opening to larynx during swallowing –
prevents food from entering lungs
trachea (“windpipe”) – tube that carries air down
to lungs – reinforced with cartilage to prevent
collapse
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bronchi – trachea branches into two bronchi
– each one leads to a lung
bronchioles – bronchi branch into tiny tubes
lungs – large, paired spongy organs
– right lung is divided into three lobes
– left lung divided into two lobes
– each is covered by a pleural membrane – forms a sac
and lines the thoracic cavity
– secretes a fluid that provides lubrication between lungs
and chest wall
alveoli – each bronchiole ends in a cluster of tiny air sacs
– walls of alveoli are extremely thin (one cell thick) and
surrounded by capillaries
– gases diffuse freely through the wall of the alveolus
and capillary (oxygen diffuses into blood and carbon
dioxide diffuses into alveoli)
Transportation of Oxygen
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Oxygen binds loosely to hemoglobin in blood – forms
oxyhemoglobin
each hemoglobin binds to 4 oxygen molecules
nearly all oxygen is transported in blood by hemoglobin
removal of oxygen from solution in the plasma by
hemoglobin maintains a concentration gradient that
favors the diffusion of oxygen from air into blood
blood can carry 70x more oxygen because of hemoglobin
than if oxygen were simply dissolved in the plasma
when hemoglobin binds to oxygen, it undergoes a slight
change in shape which alters its color
deoxygenated blood is dark red and appears bluish
through the skin
oxygenated blood is bright red
Composition of inspired (atmospheric air), alveolar,
and expired air (percentage composition by
volume)
Gas Inspired air
alveolar air
expired air
Oxygen 20.95
13.8
16.4
CO2
0.04
5.5
4.0
Nitrogen 79.01
80.7
79.6
• Blood arriving in lungs has a relatively high
concentration of carbon dioxide and
relatively low concentration of oxygen
– both gases diffuse down their
concentration gradients to equalize
between blood and air
Partial gas pressures
• partial pressure is usually used to compare
the proportion of gases in a mixture
• the partial pressure of a gas in a mixture of
gases is the pressure exerted by that gas
(measured in kilopascals, kPa)
• ex.at sea level, total atmospheric pressure
is 101.3 kPa
• atmosphere contains 21% oxygen, therefore
oxygen has a partial pressure of
.21 x 101.3 kPa or 21.3 kPa
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Hemoglobin and the transport of oxygen
oxygen enters blood from alveoli and diffuses
into red blood cells
oxygen then combines with hemoglobin to
form oxyhemoglobin (HbO2)
as hemoglobin picks up the first molecule of
oxygen, it increases its affinity for oxygen and
picks up the next molecule even faster, the
third and fourth are picked up even faster
the degree of oxygenation of hemoglobin is
determined by the partial pressure of oxygen
(p(O2)) in the immediate surroundings
• If p(O2) is low (as in the capillaries at the
tissues needing oxygen) hemoglobin
releases oxygen and carries relatively
small amounts of oxygen
• If p(O2) is high (such as at the alveoli)
hemoglobin becomes almost saturated
with oxygen
• an oxygen dissociation curve shows the degree
of hemoglobin saturation with oxygen plotted
against different values of p(O2) – the curve is Sshaped
• at p(O2) close to zero there is no oxygen bound to
the hemoglobin
• at low p(O2), the polypeptide chains are tightly
bound together, making it difficult for an oxygen
molecule to attach to iron in heme group
• as one molecule of oxygen attaches, the
polypeptide chain opens up exposing the other
heme groups to oxygen and allowing oxygen to
attach – the curves rises sharply
• at very high p(O2), the hemoglobin becomes
saturated and the curve levels off
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oxygen at the muscles is taken over and
stored by myoglobin
– myoglobin has a much higher affinity for
oxygen than hemoglobin
– it binds with oxygen at a high rate and does
not dissociate its oxygen unless the p(O2)
drops to very low levels
– myoglobin stores oxygen in muscles until the
demand becomes very great – during heavy
exercise, muscles will get oxygen from
hemoglobin first, then when supply oxygen
from hemoglobin is exhausted, myoglobin will
begin to release its oxygen
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Fetal hemoglobin
– mother and child have separate circulatory
systems
– fetus must be able to take oxygen from
mother’s hemoglobin in placenta
– fetal hemoglobin is structurally different from
maternal hemoglobin (slightly different) – has a
slightly higher affinity for oxygen than adult
hemoglobin – oxygen released by maternal
hemoglobin is bonded to fetal hemoglobin
Transportation of Carbon Dioxide
Carbon dioxide is transported three ways in the
blood:
• about 70% of carbon dioxide is transported in
plasma in the form of bicarbonate ions (HCO3)
• carbon dioxide combines with water to form
carbonic acid (catalyzed by enzyme – carbonic
anhydrase)
• carbonic acid dissociates forming hydrogen ions
and bicarbonate ions
• H+ tend to lower pH of blood
• H+ bind to hemoglobin (acts as a buffer) – forms
acid hemoglobin
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about 20% of CO2 binds to hemoglobin
(has already released O2 at tissues) to be
carried back to lungs
10% remains dissolved in the plasma as
CO2
both the production of bicarbonate ion and
the binding of CO2 to hemoglobin reduces
the concentration of CO2 in blood to
increase the gradient for CO2 to flow out of
body cells into blood
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Breathing (ventilation) is the mechanical
process of moving air from the environment into
the lungs and expelling air from the lungs
Inspiration (inhalation) – volume of thoracic
cavity is increased
diaphragm (dome-shaped muscle forming floor
of thoracic cavity) contracts and moves
downward
External intercostal rib muscles contract lifting
ribs up and out (internal intercostal muscles
relax)
this increases volume of thoracic cavity which
lowers air pressure
air from outside rushes into lungs to equalize
air pressure
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Expiration (exhalation) – volume of thoracic
cavity is decreased
diaphragm relaxes and returns to domeshape (moves up)
Internal intercostal rib muscles contract
(external intercostal muscles relax) causing
ribs to drop back down
this decreases the volume of thoracic
cavity which increases air pressure
air from inside lungs rushes out to equalize
air pressure
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Breathing rate is controlled by the
respiratory center in brain – located in
medulla just above spinal cord
muscles are stimulated to contract by
impulses from respiratory center
nerve cells in respiratory center
generate cyclic bursts of impulses that
cause the alternating contraction and
relaxation of respiratory muscles
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respiratory center receives input from
several sources and adjusts breathing
rate and volume to meet body’s changing
needs
CO2 concentration in blood is the most
important chemical stimulus for regulating
rate of respiration
chemoreceptors in medulla, and in walls
of the aorta and carotid arteries are
sensitive to changes in arterial CO2
concentration