1. Respiration and Gas Exchange
Lecture #17
Respiration and Gas Exchange

2. 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

3. 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

4. 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

5. 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

6. Gills 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
Operculum
Water
flow
Blood
vessels
Gill arch
Gill filaments
O2-poor blood
Water flow
Blood flow
Lamella
O2-rich blood

7. 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

8. 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

9. 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

10. 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
External opening
Trachea
Air
sac
Tracheole
Body
cell

11. 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

12. 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

13. The Alveolus
Respiratory bronchioles branch into multiple alveolar ducts
alveolar ducts end in a grape-like cluster = alveolar sac
each grape = alveolus
Pharynx
Larynx
(Esophagus)
Trachea
Right lung
Bronchus
Bronchiole
Diaphragm
(Heart)
Capillaries
Left
lung
Dense capillary bed
enveloping alveoli (SEM)
50 m
Alveoli
Branch of
pulmonary artery
(oxygen-poor
blood)
pulmonary vein
(oxygen-rich
Terminal
bronchiole
Nasal
cavity

14. 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

15. 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

16. Birds
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
Anterior
air sacs
Posterior
Lungs
1 mm
Airflow
Air tubes
(parabronchi)
in lung
Second inhalation
First inhalation 3 2 4 1
Second exhalation
First exhalation

17. Birds 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
Anterior
air sacs
Posterior
Lungs
1 mm
Airflow
Air tubes
(parabronchi)
in lung
Second inhalation
First inhalation 3 2 4 1
Second exhalation
First exhalation

18. 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

19. 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

20. 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

21. 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
Air
exhaled.
Rib cage gets
smaller.

22. 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
Air
exhaled.
Rib cage gets
smaller.

23. 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
functional residual capacity =
ERV + RV, 2300 ml
inspiratory reserve volume
(IRV) = IC + TV, 3000 ml
residual volume (RV) = amnt of air left in lungs after forced expiration = 1200 ml
expiratory reserve volume (ERV) = amnt of air forcefully
exhaled, 1100 ml

24. 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

25. 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-)

26. Carotid arteries Aorta Homeostasis: Blood pH of about 7.4 CO2 level
Figure 42.29
Homeostasis:
Blood pH of about 7.4
CO2 level
decreases.
Stimulus:
Rising level of
CO2 in tissues
lowers blood pH.
Response:
Rib muscles
and diaphragm
increase rate
and depth of
ventilation.
Carotid
arteries
Aorta
Sensor/control center:
Cerebrospinal fluid
Medulla
oblongata
neurons in
carotid and
aortic arch sense
drop in blood pH
medulla detects drop in CSF pH
Figure Homeostatic control of breathing.

27. 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

28. 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

29. Hemoglobin & O2 (b) pH and hemoglobin dissociation PO (mm Hg) 20 40 60
(a) PO and hemoglobin dissociation at pH 7.4
Tissues during
exercise
Tissues
at rest
Lungs
PO (mm Hg) 20 40 60 80
100
O2 unloaded
to tissues
during exercise
O2 saturation of hemoglobin (%)
(b) pH and hemoglobin dissociation
PO (mm Hg) 2 20 40 60 80
100
Hemoglobin
retains less
O2 at lower pH
(higher CO2
concentration)
pH 7.2
pH 7.4
O2 saturation of hemoglobin (%)
Bohr shift: low pH decreases the affinity of Hb for O2

30. CO2 transport CO2 produced by tissue cells & diffuses into the plasma
Body tissue
Capillary
wall
Interstitial
fluid
Plasma
within capillary
CO2 transport
from tissues
CO2 produced
CO2
H2O
H2CO3 Hb
Red
blood
cell
Carbonic
acid
Hemoglobin (Hb)
picks up
CO2 and H+. H+
HCO3
Bicarbonate 
To lungs
to lungs
Hemoglobin
releases
Alveolar space in lung
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 HCO3-
Hb 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

31. Diving Mammals humans can hold their breath for no more than 3 minutes
seals – can dive to m 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