Lecture #17
Respiration and Gas Exchange
Partial Pressure
• each gas in a mixture of gases exerts its own
pressure = partial pressure
– partial pressures denoted as “p”
– applies to gases in air and gases dissolved in liquids
• total pressure is sum of all partial pressures
– atmospheric pressure (760 mm Hg) = pO2 + pCO2 +
pN2 + pH2O
– to determine partial pressure of O2-- multiply 760 by
% of air that is O2 (21%) = 160 mm Hg
Respiratory Media
• respiratory media – either air or water
• conditions for gas exchange depend on this media
– air is less dense and easier to move over respiratory surfaces
– it is easy to breathe air
– but humans only extract 25% of the O2 out of the air they breathe
• O2 is plentiful in air – is always 21% of the earth’s
atmosphere by volume
• gas exchange from water is much more demanding
– amount of O2 dissolved in water varies with the conditions of the
water
• warmer and saltier – less O2
– but it is always less than what is found in air
• 40 times more O2 in air than in water!!
– water is also more dense and viscous – requires considerably more
energy to move over the respiratory surface
Respiratory Surfaces
• ventilation = movement of the respiratory medium
over the respiratory surface
• O2 and CO2 exchange is by diffusion and occurs
across a moist surface
• rate of diffusion determined by three things:
– 1. surface area
– 2. thickness of respiratory membrane (e.g. alveolar
wall + capillary wall)
– 3. diffusion coefficient – CO2 20X higher vs. O2
– i.e. diffusion is faster when the area for diffusion is
large and the distance is short
Respiratory Surfaces
• simple animals – every cell is close enough to the external
environment – gases diffuse quickly across the body
surface
– sponges, cnidarians and flatworms
• some animals have modified their skin to act as a
respiratory organ – dense network of capillaries below the
surface
– earthworms and some amphibians like frogs
• however this is not true for larger animals – development of
more complex structures like gills and lungs
• fish gas exchange
– to exchange enough O2 – fish must pass large
quantities of water across the gill surface
– water flows in the mouth and out the
operculum (slit-like opening in the body wall)
– flows over the gills
– most fishes have a pumping mechanism to
move water into the mouth and pharynx and
out through the opercula
– some elasmobranchs and open ocean bony
fishes (e.g. tuna) – keep their mouth open
during swimming – ram ventilation
– gills are supported by gill arches – contain
larger arteries and veins (branchial artery and
vein)
– 2 gill filaments extend from each arch and are
made up of plates called lamellae
– each lamella contains extensive capillary beds
Gills
Gill
arch
Blood
vessels
Gill arch
Water
flow Operculum
Gill filaments
O2-poor blood
O2-rich blood
Lamella
Water flow
Blood flow
– gas exchange across the lamellae – countercurrent or parallel
exchange depending on the fish
• parallel exchange – the blood flows in the same direction as the water
through the gills
– exchange will stop once the difference between water and blood O2 levels
disappears
• countercurrent exchange – the blood and water flow in opposite
directions
– there always exists a small gradient so that oxygen flows into the blood from
the water
Counter-current exchange
Parallel exchange
• amphibian gas exchange:
– requires a moist surface
– skin can function as a respiratory organ through
cutaneous respiration
• the majority of its total respiration
– gas exchange also occurs along the moist surfaces of the
mouth and pharynx – buccopharyngeal respiration
• amphibian gas exchange:
– contribution of cutaneous and buccopharyngeal
respiration to total gas exchange is relatively constant
• so their rate cannot be increased if metabolic rate goes up
• an alternate means of increasing respiration is required
– so amphibians also possess lungs
• pulmonary ventilation occurs through a buccal pump mechanism
• muscles of the mouth and pharynx create a positive pressure to
force air into the lungs
Tracheal System of Insects
• the most common terrestrial respiratory system
• air tubes that branch throughout the body
– largest tubes are called tracheae – open to the outside
– branch into smaller tubes = tracheoles – deliver air directly to the cells of the tissues
• passive movement of air into the tracheae and diffusion brings in enough O2 to
support cellular respiration
• larger insects with higher energy requirements – must ventilate air and out of
the tracheae – through body movements produced by muscles
Tracheae
Air sacs
Body
cell
Air
sac
Tracheole
Trachea
External opening
Air
Terrestrial Animals & the Lung
• lungs are localized, regional respiratory organs
• subdivided into numerous lobes, lobules and broncho-pulmonary
segments
• these divisions are supplied by a series of branching tubes
• lungs are supplied by the circulatory system – blood comes from
the right side of the heart
• the amphibian lung is quite small – most respiration is done by the
skin
• most reptiles, all birds and all mammals – respiration done lungs
The Lung
•
•
•
•
•
Primary bronchi supply each lung
Secondary bronchi supply each lobe of the lungs (3 right + 2 left)
Tertiary bronchi splits into successive sets of intralobular bronchioles that supply each
bronchopulmonary segment ( right = 10, left = 8)
IL bronchioles split into Terminal bronchioles -> these split into Respiratory
Bronchioles
each RB splits into multiple alveolar ducts which end in an alveolar sac
The Alveolus
•
Respiratory bronchioles
branch into multiple
alveolar ducts
• alveolar ducts end in a
grape-like cluster =
alveolar sac
• each grape = alveolus
Branch of
pulmonary vein
(oxygen-rich
blood)
Terminal
bronchiole
Branch of
pulmonary artery
(oxygen-poor
blood)
Nasal
cavity
Pharynx
Left
lung
Larynx
(Esophagus)
Alveoli
50 m
Trachea
Right lung
Capillaries
Bronchus
Bronchiole
Diaphragm
(Heart)
Dense capillary bed
enveloping alveoli (SEM)
Alveolus
• one cell thick - site of gas exchange by simple diffusion
• surrounded by a capillary bed fed by a pulmonary arteriole and collected by
a pulmonary venule
• deoxygenated blood flows over the alveolus picks up O2 and the oxygenated
blood leaves the alveolus -> heart
• Type I alveolar cells: simple squamous cells where gas exchange occurs
• Type II alveolar cells (septal cells): secrete alveolar fluid containing
surfactant
• Alveolar dust cells: wandering macrophages remove debris
Ventilation & Breathing
• ventilation = movement of the respiratory medium over
the respiratory surface
• amphibians – use positive pressure breathing
– inflate their lungs by forcing air into them
• mammals – use negative pressure breathing
– change the volume of the lungs to either increase or decrease
air pressure within it – moves the air in and out
• birds – unique mechanism involving negative pressure
breathing
• respiratory system is designed to be
efficient and to provide the flight
muscles with enough oxygen
• external nares located in the bill –
draws air in – eventually enters into the
bronchii
• bronchi connect to air sacs that occupy
much of the body & to the lungs
• lung does not contain alveoli – but
contains parabronchii – tiny channels
for gas exchange
• inspiration and expiration results from
increasing and decreasing the volume of
the thorax and from the expansion and
compression of the air sacs
• bird actually uses two rounds of
inhalation/exhalation to move a
volume of air through its respiratory
system
Birds
Anterior
air sacs
Posterior
air sacs
Lungs
Airflow
Air tubes
(parabronchi)
in lung
1 mm
Posterior
air sacs
2
Lungs
3
Anterior
air sacs
4
1
1 First inhalation
3 Second inhalation
2 First exhalation 4 Second exhalation
• 1st inhalation – air moves into the
posterior/abdominal air sacs
• 1st exhalation – posterior air sac
contracts – forces air into the lungs for
additional gas exchange
• 2nd inhalation – air passes from the
lungs into the anterior air sacs; new air
moves into the posterior air sacs
• 2nd exhalation – anterior air sacs
contract and air moves out of body;
posterior air sacs contract and a new
volume of air moves in to lung
• due to this arrangement – birds have a
near continuous movement of O2 rich
air over the respiratory surfaces of the
lungs
Birds
Anterior
air sacs
Posterior
air sacs
Lungs
Airflow
Air tubes
(parabronchi)
in lung
1 mm
Posterior
air sacs
2
Lungs
3
Anterior
air sacs
4
1
1 First inhalation
3 Second inhalation
2 First exhalation 4 Second exhalation
Mammalian Breathing
•
•
•
to understand mammalian ventilation - must understand the physical relationship
between the lungs and the thoracic cavity
Pleural cavity is potential space between ribs & lungs
– the lungs do not fill the entire pleural cavity
– pressure of air inside the lungs is greater than the pressure in the pleural cavity
lungs and thoracic cavity are lined with membranes
– Visceral pleura covers lungs
– Parietal pleura lines ribcage & covers upper surface of diaphragm
Respiratory pressures
• two different pressures need to be considered
– 1. atmospheric (barometric) pressure
• caused by the weight of air on objects on the Earth’s surface
– 2. intrapulmonary (intra-alveolar) pressure
• pressure within the lungs (within each alveolus)
• when not ventilating – pressure of air inside the lungs = pressure of air outside the
body
• ventilation happens because of a pressure gradient between AP and IP
Mammalian Ventilation: Boyle’s law
• Inhalation - the diaphragm drops and
the rib cage swings up and out – the
thoracic cavity increases in volume
• fluid adhesion holds the visceral and
parietal pleural membranes together
• so when the parietal the movement
of the thoracic cavity “pulls” the
lungs with it
• this expands the lungs in volume –
air pressure in the lung (i.e. IP) drops
below atmosphere (i.e. AP)
Rib cage
expands.
Air
inhaled.
Lung
Diaphragm
Boyle’s law:
As the size of closed container
decreases, pressure inside is
increased
As the size of a closed container
increases, pressure decreases
Mammalian Ventilation: Boyle’s law
• Exhalation – the diaphragm
comes back up and the rib
cage swings back down – the
thoracic cavity decreases in
volume
• PLUS – elastic recoil of the lung
tissue decreases volume
• lung volume decreases and the
air pressure within the lungs
increases vs. atmospheric
• air moves out to equilibrate
Rib cage gets
smaller.
Air
exhaled.
Mammalian Ventilation: Boyle’s law
• additional muscles can be
used to increase and
decrease the volume of
the thoracic cavity more
than normal
• other animals use the
rhythmic movement of
organs in their abdomen
to increase breathing
volumes
Rib cage gets
smaller.
Air
exhaled.
Respiratory Volumes and Capacities
•inspiratory capacity (IC) = max. amnt of air taken in after a normal exhalation, 3500 ml
•vital capacity = max. amnt of air capable of inhaling,
IRV + TV + ERV = 4600 ml
• total lung capacity = VC + RV = 6000ml
•(TV) = amnt of air that enters or exits the lungs
500 ml per inhalation
•inspiratory reserve volume
(IRV) = IC + TV, 3000 ml
•expiratory reserve
volume (ERV) = amnt
of air forcefully
exhaled, 1100 ml
•residual volume (RV) = amnt
of air left in lungs after forced
expiration = 1200 ml
•functional
residual capacity
=
ERV + RV, 2300 ml
Control of Breathing
• controlled by three clusters of
neurons that make up the
Respiratory Center
• 1. medullary rhythmicity area – in
the medulla oblongata
– controls the rate and depth of
breathing
• 2. pneumotaxic area – in the pons
– shortens the breath
• 3. apneustic area – in the pons
– prolongs the breath
• detects changes in the pH of the
CSF surrounding the brain
CO2 is the major determinant
for breathing rate
• the major determinant of CSF pH is the
blood’s pH
• the major determinant of blood pH is the
dissolution of CO2 into the plasma
• CO2 combines with the water of the
plasma to create carbonic acid
• carbonic acid dissociates into H+ ions
(pH) and bicarbonate ions (HCO3-)
Figure 42.29
Homeostasis:
Blood pH of about 7.4
CO2 level
decreases.
Response:
Rib muscles
and diaphragm
increase rate
and depth of
ventilation.
Stimulus:
Rising level of
CO2 in tissues
lowers blood pH.
medulla detects drop in
CSF pH
Sensor/control center:
Cerebrospinal fluid
Medulla
oblongata
Carotid
arteries
Aorta
neurons in
carotid and
aortic arch
sense
drop in blood
pH
Respiratory pigments
• CO2 dissolves in the water of the plasma
• but O2 dissolves poorly in plasma
– reduces the amount of O2 that the blood can carry
• so there is the need for a respiratory pigment to bind oxygen
• hemocyanin – respiratory pigment of molluscs, arthopods,
annelids
– has copper as it’s oxygen binding element
• hemoglobin used by most other animals
– uses iron to bind oxygen
– acts as an “oxygen sponge”
– allows for the transport of significant amounts of O2 in the blood
Hemoglobin
• comprised of 4 proteins called globin
• each globin has a heme group
• each heme group has an iron-containing pigment at its
core
• each iron atom binds one O2 molecule
– as one heme binds one O2 – the other three increase their
affinity for their O2 “partners”
– as one heme releases its O2 – the other three lose their
affinity for their O2
• so each Hb can carry four O2 molecules
100
O2 unloaded
to tissues
at rest
80
O2 unloaded
to tissues
during exercise
60
40
20
O2 saturation of hemoglobin (%)
O2 saturation of hemoglobin (%)
Hemoglobin & O2
100
pH 7.4
80
pH 7.2
60
Hemoglobin
retains less
O2 at lower pH
(higher CO2
concentration)
40
20
0
0
0
20
Tissues during
exercise
40
60
Tissues
at rest
PO 2 (mm Hg)
80
100
Lungs
(a) PO 2and hemoglobin dissociation at pH 7.4
0
20
40
60
80
100
PO2 (mm Hg)
(b) pH and hemoglobin dissociation
Bohr shift: low pH decreases the affinity of
Hb for O2
CO2 transport
Body tissue
CO2 produced
CO2 transport
from tissues
Interstitial
CO2
fluid
•
•
•
•
•
•
•
•
•
•
CO2 produced by tissue cells & diffuses into the
plasma
over 90% of CO2 then diffuses into the RBC
some CO2 combines with Hb
most CO2 reacts with the cytosol inside the RBC to
form carbonic acid – catalyzed by the enzyme
carbonic anhydrase
dissociation of carbonic acid into H+ and HCO3Hb binds the H+ ions and prevents the Bohr shift
most of the HCO3- diffuses out of the RBC into the
plasma
in the lungs – Hb releases the H+ ion – it combines
with the HCO3- to reform carbonic acid
carbonic acid breaks up into H2O and CO2; CO2 is
released by Hb
CO2 diffuses into the alveolar air
Plasma
within capillary CO2
H2O
Red
blood
cell
Capillary
wall
CO2
H2CO3
Hb
Carbonic
acid
HCO3 
Bicarbonate
HCO3
H+
To lungs
CO2 transport
to lungs
HCO3
HCO3 
H2CO3
Hemoglobin (Hb)
picks up
CO2 and H+.
H+
Hb
Hemoglobin
releases
CO2 and H+.
H2O
CO2
CO2
CO2
CO2
Alveolar space in lung
Diving Mammals
• humans can hold their breath for no more than 3 minutes
• seals – can dive to 200-500m and can hold their breath for
close to 20 minutes
• some whales can reach depths of 1500m and stay submerged
for close to 2 hours
• evolutionary adaptations:
– 1. ability to store large amounts of O2 in their muscle mass
– 2. adaptations to conserve O2 – little effort to swim and their
buoyancy allows them to change depths easily
– 3. regulatory mechanisms routes blood to the brain, spinal cord, eyes,
adrenal glands – shut off in other areas during a dive