Respiratory
Exchange
Chris Elliott
cje2@york.ac.uk
Learning Objectives
1. To understand the factors influencing gas
exchange in water and air
2. To be aware of the diversity of patterns of
air/water and blood flow in gas exchange
at the respiratory surface of lungs and gills
3. To be familiar with the insect tracheal
system in which gas exchange is
independent of the circulatory system
References
 Schmidt Nielsen, K (1997) Animal
Physiology 5th ed  Library
 Amazon
 Copy of Powerpoints
 Via
vle.york.ac.uk
 Direct http://biolpc22.york.ac.uk/303/
Aerobic metabolism
Most animals respire aerobically
O2 + glucose
CO2 + H2O + 36 ATP
acquire
dispose
availability
in air
in water
Solubility
in air vs water
Anaerobic metabolism
 glycolysis
 glucose
→ 2 lactate + 2 ATP
 goldfish under ice
 convert
lactate to ethanol + release it
 seals etc. diving
Diffusion not enough?
2

Vr
Fo 
6K
for a spherical organism, radius 1 cm,
using 1 ml O2 /kg/min external
pressure needs to be 15 atm
Ok for
•protozoa
•jellyfish
Diffusion rates for O2 and CO2:
approx 10,000 x more in air than in H2O
Pumping needed
 why?
 Blood
 External fluid
 Air
 water
Diffusion not enough?
Major physiological problems:
In aqueous environments: low O2 availability outside
(increases with decreasing temperature,
decreases with increasing salinity)
In terrestrial environments: high CO2 content in blood
So move fluids…
bulk flow
bulk flow
• Linkage between circulatory and
ventilatory systems:
• arrangements different for
different animal groups
Lungs/gills allow…
Development of specialized surfaces for gas
exchange
 Low
general permeability of organism
surface
 Protects from mechanical damage and
pathogens
 Reduces water exchange
Specialized respiratory surfaces are shaped by the
physiology of gas exchange in the medium in which they live
O2
CO2
lungs
(invaginations)
impermeable
gills
(evaginations)
Issues of H2O loss reduced
Pool arrangement
Uniform pool (e.g. vertebrate lungs)
Evenly ventilated pool of air in lung with equilibrated O2 at lower partial
pressure internally (A) to externally (I). Blood flows along pulmonary
surface and acquires O2 by diffusion until concentration in blood equals A
Dead space
Snorkel increases dead space
from ~140 to 300 ml
•doubles [ CO2 ]
lung volume ~ 5 L
tidal volume 500 ml
water pushes on lungs,
max pressure change 11kPa,
~1.2m
Concurrent (cephalopod gills)
Streams of blood and water in same direction until O2 concentration
Equilibrates at intermediate concentration. Inefficient, blood O2 concentration
Never reaches initial level of X.
Transfer O2 from water to blood
Concurrent flow
→→
Fish gills
Counter current arrangement (v. efficient)
Blood and water in opposite direction, leads to complete transfer of O2.
Good design, no more demanding in terms of energy or material to
concurrent system.
Transfer O2 from water to blood
Countercurrent flow
→←
Cross current bird lungs
Capillary blood flows at an angle to respiratory medium to give variable
Transfer of O2. Intermediate effectiveness between concurrent and
Countercurrent.
Lung/gill size with body size
Summary so far
 Limit to O2 and CO2 by diffusion
 respiratory
surface
 pumping
 countercurrent
 Next : Lungs
most efficient
Lungs
Lungs: Evolved many times (e.g. land snails) esp. in fish.
From air-filled pocket? For bouyancy? Air breathing? Sound
production?
Bimodal lifestyles in fish and amphibians
Breathing through skin (eels), modified gills (catfish)
Specialisation in gut (lungfish)
Lung diversity
Efficiency
cf metabolic rate?
Lungfish
offered a choice of water/air
Neoceratodus
(Australian, river)
Lepidoserin
(S. american, stagnant pools)
Protopterus
(African, stagnant pools)
Lungfish
moved from water to air
Neoceratodus
(Australian, river)
Lepidoserin
(S. american, stagnant pools)
Protopterus
(African, stagnant pools)
Wildebeest lung
c
~1 mm
c - capillary
bronchioles
alveoli
Force pump in amphibia
•Air forced into
lungs by +ve
pressure
3 Elastic recoil +
lowered buccal
cavity - air out
1 Air into buccal cavity
via nares
2 Close nares, raise floor
of buccal cavity - forces
air into lungs
frog needs 60-70% of O2 via lungs
Suction pump
Reptiles, birds
and mammals
Crocodile lung
Bird lungs
open ended tubes for
unidirectional flow
Bird lung
straight tubes
0.5mm
How birds breathe
air sac
lung
air sac
Air sacs not directly involved in O2 uptake; act as bellows
Do crocs also breathe one way ?
in
out
Summary so far
 Limit to O2 and CO2 by diffusion
 respiratory
surface
 pumping
 countercurrent
 Lungs
 force
air down
 pull air down
 push and pull
 Next: Gills
most efficient
Gills in fish
Flow of water over fish gills
Operculum cover
Branchial arch
Double pump
Not relevant for mackerel and tunny
Ram ventilation
 most fish switch from opercular pump to
ram ventilation at 0.5-1m/s
 tuna, marlin etc. always use ram ventilation
 transfers ventilation work from opercular to
body muscles
Other gills
axolotl
crab
•reverse flow to keep gill
clean
Summary so far
 Limit to O2 and CO2 by diffusion
 respiratory
surface
 pumping
 countercurrent
 Lungs
 Gills
 Next: Insects
most efficient
Insect tracheae
 gas exchange independent of circulatory
system:
a
branching system of fine air filter tubes
spiracles
(on thorax
& abdomen)
tracheae
Spiracles and tracheoles
Directional flow
• Some air exchange can be facilitated in larger insects
• abdominal pumping
Compression of trachea
 use a synchrotron
Book lung
•Arachnids and chelicerates
•used in combination with trachea
muscle
Summary to end
 Limit to O2 and CO2 by diffusion
 respiratory
surface
 pumping
 countercurrent
most efficient
 Lungs
 Gills
 Insect trachea
 spiracles
& tracheoles
 low basal rate
 extra pumping in response to stress