Air-breathing fishes
• Hundreds of convergent cases of air-breathing
using accessory structures.
– Adaptations
Ad
i
to oxygen d
depletion
l i and
d periodic
i di
droughts.
– Most facultative.
– Some obligatory.
• Most species are tropical freshwater.
• Oxygen-deficient
d f
water more common in tropics.
–
–
–
–
Decaying organic matter.
High temperature.
temperature
Shaded jungle waters.
Little temperature variation.
Gas bladder
Gas bladder and lungs
• Gas-filled sac:
– Forms as diverticulum from anterior gut (endoderm).
– Pressure regulated by countercurrent exchange
system (gas gland, =rete mirabile, =rete).
• Connection to gut may be present or absent.
absent
– Physostomous vs. physoclistous (=physocleistous)
• Functions:
– Original function:
• Respiration: in lungfishes and many primitive bony
fishes
fishes.
– Derived functions:
y
y
• Buoyancy.
• Resonator: for producing or detecting sounds.
Connections of
gas bl
bladder
dd
with gut
Countercurrent flow of arterial and venous blood
Gas gland (rete mirabile)
Teleosts
Gars, bowfin
Bichir
Lungfishes
A hibi
Amphibians
T l
Teleosts
t
Tetrapods
Osteichthyes
Sarcopterygians
Lungfish lungs
Homologies of respiratory
p
y systems
y
• Gills and lungs are not homologous:
– Developmental
p
patterns
p
very
y different.
• Anterior vs. posterior pharynx.
– Some species have both.
• Lungfishes,
f h
early
l amphibians.
hb
• Lungs of tetrapods are homologous with gas
bladders of
f fishes:
– First observed in early placoderms.
– Derived function in modern fishes: buoyancy.
buoyancy
Increase in surface area
of
f gasgas-exchange
h
surface
f
Lung development
Lungs
g
• Develop from lung buds:
– Ventral
V
l floor
fl
of
f pharynx.
h
– Posterior to pharyngeal pouches.
– Endoderm.
E d d
• Trachea:
– Connects
C
t llungs to
t pharynx
h
via
i bronchi.
b
hi
– Single duct, supported by tracheal rings
(cartilages)
(cartilages).
– Glottis: opening of trachea into pharynx.
Mammalian lungs
Glottis and larynx
• Glottis: muscular slit, closes off trachea to
prevent aspiration of food or liquids.
• Proximal end of trachea modified into larynx
(“voice box”) in most tetrapods.
• Larynx
L
st
strengthened
th
d by
b hyoid
h id bone
b
and
d
cartilages:
– Homologous with pharyngeal arches.
– Size and number of cartilages varies in different
tetrapod groups.
• Vocal
V
l cords:
d bands
b d of
f elastic
l ti cartilage.
til
– Vibrate when air passes over them.
– Size and tension controlled by branchiomeric
muscles.
Human larynx
The larynx
y in vertebrates
• Fishes: absent.
• Amphibians
hibi
– Caecilians: absent.
– Salamanders: poorly developed.
• Pair of small lateral cartilages derived from
l t pair
last
i of
f pharyngeal
h
l arches.
h
– Anurans (frogs & toads): well developed.
• Lateral,
L t
l arytenoid,
t
id cricoid
i id cartilages.
til
s
• Well developed vocal cords.
resonators
• Vocal sacs in mouth cavity act as resonators.
The larynx
y in vertebrates
• Reptiles:
– Most
M st resemble
s mbl ssalamanders.
l m d s
– Some lizards (e.g., geckos): well developed larynx
and
n vocal cords.
r .
– Crocodilians: loud bellowing calls.
• Arytenoid and cricoid cartilages, as in anurans.
• Thyroid cartilage.
– Dinosaurs: some had well developed larynx.
• Birds:
Bi d
– Larynx poorly developed, small cartilages.
– Syrinx: uniquely derived cartilaginous structure.
structure
Bird sryinx
The larynx in vertebrates
• Mammals: well developed in most species.
– Epiglottis closes glottis during swallowing.
– Large thyroid cartilage (“Adam’s apple”).
• Two fused plates.
• Joined to hyoid bone by ligaments.
ligaments
– Arytenoid cartilages move vocal cords.
– Cricoid cartilage
g attached to first tracheal
cartilage.
– Two sets of vocal cords: “true” and “false”:
• One or other well developed in most species.
species
• Humans use both, but true cords are larger.
• Felines use true cords to vocalize, false to purr.
Lungs
• Lungs paired:
– Usually symmetric in amphibians, reptiles, birds.
– Usually asymmetric in mammals.
• Typically right lung is “lobed”, left lung is not.
• Humans: 3 lobes in right lung,
lung 2 in left
left.
• Trachea divides into “tree” of branches:
– Paired primary bronchi subdivide into:
• Mesobronchi (secondary bronchi).
• Bronchioles.
• Alveoli: blind sacs, highly vascularized.
Human
pulmonary
circulation
Countercurrent respiratory flow
i gills
in
ill and
d llungs
Lungs
• Contained within pleural region of pleuroperitoneal cavity.
• Bidirectional (tidal) ventilation driven by skeletal muscles.
–
–
–
–
M
Many
thoracic
th
i and
d abdominal
bd i l muscles
l involved.
i
l d
Complex rib movements.
Inhalation
Inhalat
on muscles dom
dominant
nant in
n mammals.
Exhalation muscles dominant in reptiles and birds.
• Most mammals have a diaphragm:
– Set of large muscles.
– Made air-tight by epithelial and connective tissues.
– Completely divides body cavity (coelom) into:
• Pleural and pericardial cavities: lungs and heart.
• Peritoneal cavity: gut and associated organs.
• Crocodilians use liver as a diaphragm.
S bdi isi n of
Subdivision
f th
the coelom
l m
Pleural cavity pressure is slightly
less than lung pressure
Breathing is
controlled by the
autonomic
t
i nervous
system
Crocs use the liver as a diaphragm
Bird lungs
g
• Adaptations for flight:
– High metabolic rates.
rates
– Bones lightweight and fragile.
– Efficient ventilation.
ventilation
• Bronchus subdivides into afferent parabronchi.
– Afferent parabronchi  air capillaries
 efferent parabronchi.
– Unidirectional air flow through air capillaries.
• Ends of bronchi branch into air sacs.
– Distributed widely throughout body, bone cavities.
Bird lungs
Evolutionary trends in the
vertebrate respiratory system
• Modification of the location of the
g surface:
environmental exchange
– From outer surface of the body (evaginations).
• Aquatic and semi-aquatic vertebrates.
– To inner surface of the body (invaginations).
(invaginations)
• Aquatic vertebrates and amniotes.
• Elaboration of lungs:
– From auxiliary exchange surface in aquatic and
semi-aquatic
q
vertebrates.
– To primary exchange surface in amniotes.
Evolutionary trends in the
vertebrate respiratory system
• Control of breathing
g via visceral receptors:
p
– Water :
• CO2 highly soluble, O2 relatively insoluble.
pattern
• [O2] primarily regulates breathing pattern.
– Air :
• O2 and CO2 both soluble.
• [CO2] primarily
i
il regulates
l t b
breathing
thi pattern.
tt