Biology 6A
02 Gas Exchange & Circulation v2
Brian McCauley
Why is gas exchange important?
Gas Exchange &
Circulation
Read Ch. 42
start with 42.5:
Gas Exchange in
Animals
Taking up oxygen
Respiration:
C6H12O6 + O2 è Energy + CO2 + H2O
Photosynthesis:
Energy + CO2 + H2O è C6H12O6 + O2
Diffusion
Diffusion is the
only way.
Molecules spread out by
random motion.
concentration gradient
Random walks in 3 dimensions
Taking up oxygen
v  For very small organisms,
diffusion is fast enough to supply
O2.
v  O2 use is correlated with body
mass (or volume), but diffusion
rate depends on surface area...
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surface area/volume ratio
Brian McCauley
Taking up oxygen
v  For very small organisms,
diffusion is fast enough to supply
O2.
v  O2 use is correlated with body
mass (or volume), but diffusion
rate depends on surface area...
v  and larger organisms have lower
surface area/volume ratios.
Taking up oxygen
v  Diffusion alone is enough for animals
up to about 1 mm thick...
Taking up oxygen
v  Diffusion alone is enough for animals
up to about 1 mm thick...
v  depending on rate of O2 use.
Taking up oxygen
Fick’s Law of Diffusion
Rate of diffusion is
proportional to:
C1
Larger, faster animals need tricks
to speed up diffusion.
C2
Surface area
Concentration gradient
u  Diffusion distance
u  Diffusion constant
u
u
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O2 requirements and Diffusion
Brian McCauley
Speeding up Diffusion
v  Gases diffuse from
higher to lower
concentration.
v  Will organism get enough O2?
Depends on rate of O2 use and
diffusion.
v  O2 concentration
must be lower in
organism than in
environment.
v  Large, active animals need tricks to
speed up diffusion.
Speeding up diffusion: gills
Fish gills compared to invertebrates
v  Higher metabolic rate
v  More O2
v  Large surface area
v  Short diffusion
distance (thin
epithelium)
Gill area is proportional to body size
log gill area, mm2
Bigger fish have bigger gills...
v  Bigger relative gill
area
Gill size depends
on body size and
metabolic rate.
Allometry
Scaling: the relationship
between the size of an
organism and the size of
any of its “parts”
log body mass, g
but not relative to their body size.
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Gill area vs. body size
Brian McCauley
Faster fish have bigger gills.
index of relative gill surface area
Mackerel: 2551
Toadfish: 137
gill area per unit body mass compared to
body mass
Diffusion isn’t enough: ventilation
How do gas molecules move?
v  Bony fish: opercular pumping
v  At very small scales,
diffusion is the only
way.
v  Diffusion is extremely
slow at larger scales.
v  Supplements diffusion with
convection.
Speeding up diffusion: ventilation
v  Convection (bulk
flow) is needed.
Angel sharks: buccal pumping
v  Many sharks
have
spiracles:
Opercular pumping:
one-way water flow
v  modified gill
slits for water
intake
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Speeding up diffusion: ventilation
v  At higher speed, ram
ventilation is used.
Brian McCauley
How do gas molecules move?
v  At very small scales,
diffusion is the only
way.
v  Diffusion is extremely
slow at larger scales.
v  Convection (bulk
flow) is needed.
Fish gills: countercurrent exchange
Countercurrent exchange of O2 in
fish gills (2)
Countercurrent
exchange of O2 in fish
gills (1)
Countercurrent vs. Concurrent Exchange
v  Water & blood flow
opposite directions.
v  This maximizes the
concentration gradient &
speeds up diffusion.
v  Equilibrium is never
reached.
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02 Gas Exchange & Circulation v2
Brian McCauley
Fick’s Law of Diffusion
Rate of diffusion is
proportional to:
C1
Gas Exchange &
Circulation
C2
air vs. water
Surface area
u  Concentration gradient
u  Diffusion distance
u  Diffusion constant
u
Fick’s Law of Diffusion
C1
Diffusion between gas & liquid
C2
v  Closed cylinder
contains 1 liter
water and 1 liter
air (at 1 atm).
How is concentration measured?
u
u
v  Oxygen diffuses
between air &
water.
moles/liter
gas pressure
Air: composition &
partial pressures
Partial pressure & concentration
v  N2: 78%; PN = 0.78 atm
210 ml O2
v  O2: 21%; PO = 0.21 atm
2
PO2=0.2 atm
2
v  CO2: 0.03%; PCO2= 0.0003 atm
Other gases bring total up
to 1 atmosphere.
5.8 ml O2
PO =0.2 atm
2
v  The amount of O2
dissolved in water
depends on
solubility and
pressure.
v  The air & water
are at equilibrium
if they have the
same PO .
2
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Brian McCauley
The diving bell spider
Air vs. Water
Problems with breathing water
Problems with breathing water
v Water is dense & viscous.
v Osmoregulation (gain or loss of
water or salt).
v Water doesn’t hold much O2.
1 liter water: 5 ml O2
1 liter air: 200 ml O2
Water vs. Air
1 liter water: 1 kg
1 liter air: 1.2 g
v Water has high thermal
conductivity & heat capacity.
Water conducts heat
25x as fast as air.
Water vs. Air
Air’s hard tradeoff
v Air has plenty of O2...
v But breathing air causes
evaporation.
Anything that speeds up
gas exchange in air will
also speed up evaporation.
Gills work well
in water, but
not in air.
Water vs. Air
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Most terrestrial animals
have lung-like structures.
Banana slug
(phylum
mollusca)
Lungs are like inside-out gills.
Brian McCauley
Air-breathing snails
v Gas exchange through
skin and lung-like
mantle cavity.
v They dry out easily.
v Don’t pump air through
“lung.”
v Don’t use much O2.
Insect Gas Exchange
v Animals in dry environments must
limit water loss through breathing.
v Exoskeleton reduces water loss,
but also reduces gas exchange.
Insect Gas Exchange: Tracheae
v Large surface area for gas
exchange.
v Bring air close to tissues.
Insects have
tracheae: tubes
that carry air
close to tissues.
Insect Gas Exchange: Spiracles
v Close to limit water loss; open for
gas exchange.
Cockroach
trachea
spiracles
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Insect Gas Exchange
silkworm
spiracle &
tracheae
dung beetle
spiracle
Insect Gas Exchange: Tracheae
Brian McCauley
Insect Gas Exchange: Tracheae
v Some insects pump air in & out.
v 2-way air flow reduces water loss.
Insect Gas Exchange: Size Limits
v  Insect treacheae deliver O2 directly to
tissues.
v  As size increases, tracheal area must
increase even faster...
Insect size limited by O2
v  Maximum size may be limited by the
ability to deliver air to tissues.
Insect Size Limits
leg outline
trachea outline
250 µm
v  Leg tracheae occupy greater
percentage of the leg in bigger beetles.
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Insect Size Limits
Brian McCauley
More O2, bigger insects
v In paleozoic era ( mya), O2 level
was 30%...
v ... and insects were bigger!
v  Leg tracheae occupy greater
percentage of the leg in bigger beetles.
Lungless salamanders
v Gas exchange through
skin.
v Small, slender body.
v Must stay wet.
v Cold body; low O2 use.
Frogs: positive-pressure breathing
forces air into lungs
Amphibians with lungs
v Frogs and larger
salamanders do gas
exchange through
skin and lungs.
v Usually most O2 is
absorbed in the lungs,
but most CO2 is
eliminated through
the skin.
Reptiles
Reptiles have impermeable skin;
gas exchange happens in lungs.
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Reptiles
v Reptiles have impermeable skin;
gas exchange happens in lungs.
Brian McCauley
Mammals: higher metabolic
rate; more O2.
v Most have fairly low oxygen
requirements.
v Body temperature is usually near
ambient; this keeps water loss
low compared to warm-blooded
animals.
Mammals
Mammalian lungs
v Higher metabolic rate;
v Increased gas exchange.
v Greater lung surface area.
v More control of ventilation.
v More control of the flow of
oxygenated blood.
v More water loss.
Human lungs
amphibian:
v Large surface area:
100 m2.
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Mammalian alveoli, SEM
Brian McCauley
Human lungs
v Large surface area:
100 m2.
v Minimal diffusion
distance: 0.2 µm
between blood, air
Mammalian alveoli, SEM
Negative pressure breathing
Human breathing
Human lungs
v Dead space: 150 ml
v Tidal volume (resting):
500 ml
v Countercurrent air/
blood flow not
possible in alveoli...
v Tidal volume (exercise):
3000 ml
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Human lungs
v Countercurrent air/
blood flow not
possible in alveoli...
v But countercurrent
air flow in nasal
passages catches
H2O vapor.
Reptiles lose less water to evaporation
v  Don’t breathe as much
v  Body close to ambient temperature
Birds
v Like mammals, birds have high
metabolic rates.
Brian McCauley
Breathing Air
v Air has plenty of O2...
v But it dries you out.
Anything that speeds up
gas exchange in air will
also speed up evaporation.
Mammals: high MR, warm body,
more evaporation.
Moisture-catching turbinate bones
reduce water loss.
Birds: air sacs create 1-way flow in lungs
They still
have dead
space!
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Bird lung (cast)
Birds have a water-recycling system
analogous to that of mammals.
Brian McCauley
Bird lung: parabronchi
Animal
Circulation:
transporting gases
& other things
Circulatory systems supplement
diffusion with convection.
read Ch. 42.1-42.3
Circulatory System Functions
v Transport: O2 & CO2
Circulatory System Functions
v Transport: O2 & CO2
v Transport: food & waste
v Regulation: hormones
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Circulatory System Functions
v Transport: O2 & CO2
v Transport: food & waste
v Regulation: hormones
v Regulation: temperature
Brian McCauley
Circulatory System Functions
v Transport: O2 & CO2
v Transport: food & waste
v Regulation: hormones
v Regulation: temperature
v Protection: immune system
v Protection: clotting
Circulatory System Designs
Circulatory System Designs
v Gastrovascular cavity
is digestive tract &
circulatory system
Jellyfish have a gastrovascular cavity,
lined with ciliated cells.
Phylum Cnidaria
Circulatory System Designs
v Gastrovascular cavity
is digestive tract &
circulatory system
Flatworms: Phylum Platyhelminthes
Hydra (Phylum Cnidaria)
Circulatory System Designs
Insects: open
circulatory
systems.
Phylum Arthropoda
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Circulatory System Designs
Brian McCauley
Vertebrate circulatory systems: fish
v  2-Chambered heart
Earthworms:
closed
circulatory
systems.
v  1 blood circuit: from
heart to gills to
systemic capillaries
v  Blood loses pressure
at each step
Phylum Annelida
Vertebrate circulatory systems: frogs
v  3-Chambered heart
v  2 blood circuits: pulmocutaneous and
systemic
v  Blood gets pumped
twice
v  Some mixing of
oxygenated and
de-oxygenated blood
Vertebrate circulatory systems
Vert. circulation: birds & mammals
v  4-Chambered heart
v  2 blood circuits:
pulmonary and systemic
v  Blood gets pumped
twice
v  No mixing of oxygenated
and de-oxygenated
blood
v  Lower pressure in
pulmonary circuit
Blood circulation pattern
v  Mammalian
heart has two
separate paths
for blood flow.
fish

amphibian
Ž
mammal

v  Arteries go
away from the
heart, veins go
back to the
heart.
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Blood circulation pattern
v  Oxygenated
blood from
pulmonary
veins enters
left atrium.
v  Left ventricle
pumps blood to
aorta and
systemic
circulation.
Arteries & veins
Brian McCauley
Blood circulation pattern
v  De-ox. blood
from systemic
circ. goes to right
atrium and right
vena cava;
v  pumped to lungs
via pulmonary
arteries.
Artery & vein
vein
artery
Skeletal muscles also
help pump blood.
vasodilation
vasoconstriction
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v  X-section area of
capillaries is large
& variable.
Brian McCauley
Blood pressure & osmotic pressure
v  Blood moves
slower in
capillaries.
v  Pressure is high in
arteries, low in
veins.
Net fluid loss from capillaries
The Lymphatic System:
the other circulatory system
v  Fluid leaks out from capillaries (4L/day).
v  Lymphatic vessels
are everywhere in
the body.
v  In our closed
circulatory system,
blood & lymph are
separate systems -but connected.
v  The lymph system collects it.
Lymph vessel with valve
The Human Lymphatic System
v  Lymph moves due
to pressure
gradient,
v  skeletal muscle
pumping,
v  and one-way
valves.
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A Lymph Node
Brian McCauley
Insects: Open Circulatory System
v  Lymph nodes filter
lymph, remove
bacteria & other
pathogens.
v  Hemolymph not
confined to vessels
v  In our closed
circulatory system,
blood & lymph are
separate systems -but connected.
v  Exoskeleton helps
maintain pressure.
v  Low pressure (small
body).
v  O2 delivered to tissues
by tracheal system; no
hemoglobin
Phylum Arthropoda
Mammals: Closed Circulatory System
v  Blood stays in vessels;
v  Lymph system returns fluid
to blood.
v  High pressure (large body).
v  Muscles maintain pressure
v  O2 delivered by blood;
hemoglobin.
Reptiles (other than birds)
v  3-chambered heart
v  Some mixing of
oxygenated & de-ox.
v  Some can control
pulmonary, systemic
pressure separately.
v  Crocodilians have 4chambers & can shut off
pulmonary flow.
Respiratory Pigments
Gas Exchange &
Circulation
respiratory
pigments
v O2 isn’t highly soluble in blood or
body fluids.
v Respiratory pigments increase the
solubility of O2 in blood.
No hemoglobin: 0.3 ml O2/100 ml blood
Hemoglobin: 20 ml O2/100 ml blood
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Brian McCauley
Gases diffuse down gradients
Respiratory Pigments
v Hemoglobin is the main
respiratory pigment in mammals;
it’s carried in erythrocytes.
human
toad
Partial pressure (mm Hg)
Figure 42.30b
160
PO2
PCO 2
Inhaled
air
Exhaled
air
120
v Increase PO2
80
v Increase O2
solubility.
40
0
How to get more O2 to tissues
1
2
3
4
5
6
7
8
(b) Partial pressure of O2 and CO2 at different points in the
circulatory system numbered in (a)
Hemoglobin
binds to O2 in the
lungs, then
releases O2 in the
tissues.
Binding affinity of Hemoblobin for O2
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Brian McCauley
Fetal vs. adult hemoglobin
saturation
v Myoglobin:
another O2 carrier,
found in muscle.
v Single subunit; no
cooperativity.
Body tissue
CO2 transport
from tissues
CO2 produced
Globin gene evolution
Interstitial
fluid
Plasma
within capillary
CO2
CO2
Capillary
wall
CO2
H 2O
Red
blood
cell
H2CO3
Carbonic
acid
HCO3- +
Bicarbonate
HCO3-
Hb
H2CO3
CO2 Transport in Blood
H+
To lungs
CO2 transport
to lungs
HCO3HCO3- +
Hemoglobin (Hb)
picks up
CO2 and H+.
v Dissolves, converts to
carbonic acid.
H+
Hb
Hemoglobin
releases
CO2 and H+.
v Binds to hemoglobin.
H2O
CO2
CO2
CO2
CO2
Alveolar space in lung
Bohr effect
Dissolving CO2 makes
blood more acidic:
H2O + CO2 ó H2CO3 ó H+ + HCO3The carbonate buffer system
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Brian McCauley
Control of breathing
Gas Exchange
diving
v Diving: How to die, with
or without SCUBA.
v Why seals can dive so
long (and not die).
Free diving hazard:
Shallow-water blackout
v Hyperventilate and dive deep.
v Diving increases PO in your lungs,
2
so you exctract more O2 from air.
v With low PCO in blood, you don’t
2
feel the need to breathe.
v When you come back to the
surface, PO2 in your lungs
decreases dramatically.
Scuba hazards:
Pulmonary barotrauma
v As you ascend, ambient pressure
decreases.
v If you hold your breath, the
expanding air in your lungs forces
bubbles across the epithelium…
v pulmonary barotrauma and air
embolism.
Scuba hazards:
Decompression Sickness
v High pressure causes N2 to
dissolve in tissues.
v When pressure decreases, N2
forms bubbles in tissues,
potentially blocking blood flow.
v Decompression stops allow N2 to
diffuse out slowly.
Diving hazards:
Free diving & SCUBA
v Shallow-water blackout
PO2 decreases while
ascending; free diving only.
v Decompression sickness
N2 comes out of solution while
ascending; SCUBA only.
v Pulmonary barotrauma.
Air in lungs expands while
ascending; SCUBA only.
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Marine Mammal
Diving Physiology
Brian McCauley
Elephant Seals
Why can they dive so
much better than us?
California sea lion
Elephant seal dives:
Swim 90 km/day
Elephant seal dives:
v Underwater 90% of time
Elephant seal dives:
Avg dive 24 min, max 2 hr
2.5 min surface
Elephant seal dives:
Dives avg. 400+ meters;
max 1500 meters
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It’s not in the lungs
v Lung volume ≈ 4.6% of
body volume for all
mammals -- including
marine mammals.
Brian McCauley
Marine mammals have 2
kinds of tricks:
v Use oxygen slowly.
v Store a lot of oxygen.
v Marine mammals don’t
rely on the air in their
lungs while underwater.
Use oxygen slowly:
The Diving Reflex:
v Heart rate slows
v Blood pressure decreases
v Peripheral circulation
reduced
v Spleen shrinks: more blood
into circulation
Store more oxygen
v Large blood volume
v High concentration of
erythrocytes
v High hemoglobin
concentration per cell
v High myoglobin
concentration
Deep dives -- no problem?
v Shallow-water blackout
v Decompression sickness
v Pulmonary barotrauma
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