Respiratory Systems:
Ventilation & Gas Exchange
Ventilation of Respiratory Surfaces

Non-directional ventilation:
◦ Medium flows past gas exchange surface in an
unpredictable pattern.

Tidal Ventilation
◦ External medium moves in and out of respiratory
system in a back and forth movement.

Unidirectional ventilation:
◦ Respiratory medium flows in at one point, and
exits via another.
Perfusion of Respiratory Surfaces

The circulatory system allows oxygen
from the respiratory surface to be
transported long distances by bulk flow.

The movement of blood through the
respiratory surface can effect the
efficiency of gas exchange.
Ventilation & Perfusion of
Respiratory Surfaces

Non Directional Ventilation:
(1) skin breathers
(2) tidal ventilators

Unidirectional Ventilators
(1) Concurrent
(2) Countercurrent
(3) Crosscurrent
Non-Directional Ventilation

Partial pressure of oxygen (PO2) in the
blood leaving the gas exchanger can
approach the PO2 in the medium.

Anything that increases diffusion distance,
will decrease oxygen exchange efficiency
and reduce the PO2 in the blood leaving
the gas exchanger.
Non-Directional Ventilation

If ventilation is inefficient, an oxygen
depleted boundary layer will form at
the respiratory surface.

In animals that tidally ventilate, PO2 in the
respiratory cavity is lower than the
outside medium.
Tidal Ventilation

Respiratory cavities do not
fully empty.

Fresh air mixes with
oxygen-depleted residual air
 PO2 of
blood equilibrates
with the PO2 of the
respiratory cavity.
Tidal Ventilation
Unidirectional Ventilation

Blood can flow in one of 3 ways relative
to the flow of the medium:
(1)
Same Direction = Concurrent
(2)
Opposite Direction = Countercurrent
(3)
At an angle = Crosscurrent
Concurrent Flow

PO2 of the blood to
equilibrate with the PO2
of the respiratory
medium.
Countercurrent Flow

PO2 of blood leaving the
gas exchange surface can
approach that of the
incoming medium.
Crosscurrent Flow

PO2 is usually higher than
what would be seen for
concurrent, but lower
than countercurrent.
Concurrent Flow
Countercurrent Flow
Ventilation of Respiratory Surfaces

Animals respond to changes in
environmental O2 or metabolic demands
by altering the rate or pattern of
ventilation rather than its direction.
Ventilation in Air & Water

Water:
◦ Unidirectionally ventilated gills

Air:
◦ Tidally ventillated lungs
◦ Unidirectionally ventillated lungs
Ventilation in Water

Oxygen content of air nearly 30x water

Water is more dense and viscous than air

Unidirectional ventilation is less energetically
costly than tidal ventilation

Countercurrent arrangement of blood flow
improves oxygen extraction efficiency.
Elasmobranchs

Use buccal pump for ventilation:
◦ Expand buccal (mouth) cavity volume
◦ Water rushes into the buccal cavity via the
mouth and spiracles.
◦ Muscular contraction forces water past the
gills and out via external gill slits.

Buccal cavity acts as both a suction pump
and a force pump.
Buccal Pump
http://www.youtube.com/watch?v=HeI
UySBQJUQ&feature=related
Teleost (Bony) Fish

Gills are located in opercular cavities
and protected by the operculum.
Buccal-Opercular Pump
Ram Ventilation

Fish swims forward with mouth open:
◦ water flows across gills without active pumping.
Ram Ventilation

Obligate ram ventilators = lost ability
to actively pump ater over their gills and
must rely soly on ram ventilation

Must swim to maintain oxygen levels in
blood
Fish Gills
Fish Gills

4 gill arches in each opercular cavity.
◦ Provided structural support

2 rows of gill filaments project from
each gill arch.

Each filament is covered with rows of
secondary lamellae.
◦ Perpendicular to filament
Fish Gills

Each gill arch contains an afferent & efferent
blood vessel.
◦ Afferent blood vessels carry deoxygenated
blood to the capillaries in the secondary lamellae.
◦ Efferent blood vessels carry oxygenated blood
from the capillaries back to the gill arch.

Secondary lamellae:
◦ Thin-walled & highly vascularized
◦ Primary respiratory surface
Fish Gills
Fish Gills

Counter current exchange.

Blood flow through capillaries in
secondary lamellae is opposite the flow of
water through the gills.

Oxygen extraction from water can be as
high as 70 - 80%.
Fish Gills
Ventilation in Air

Oxygen availability high

Density of medium is low

Face evaporation across respiratory
surface, therefore internally located.
Amphibians

Use cutaneous respiration, external gills,
lungs, or some combination of these 3.
◦ Depends if they are extracting oxygen from
water or from air.

Ventilate lungs using a buccal force pump.
Amphibians
Reptiles

Most have two lungs – tidal ventilation

Air comes into the organism via the
mouth and trachea, and each lung has a
bronchus that allows airflow into the
chambers of the lungs.
Reptiles

Rely on suction pumps to ventilate lungs.

Ventilatory cycle is triphasic –
divided into 3 phases:
1. Inspiration (suction pump)
2. Breath-hold
3. Expiration (passive)
Reptiles

Changing volume of chest cavity:

Snakes and Lizards:
◦ Intercostal muscles

Turtle and tortises:
◦ Pair of sheet-like abdomen muscles & movement
of forelimbs.

Crocodilians:
◦ Hepatic septum, liver, & diaphragmaticus muscles.
Reptiles
Reptiles
Birds

Unidirectionally ventilate their lungs.

Lung is stiff and undergoes little change in
volume during ventilatory cycle.

Series of air sacs associated with lungs:
◦ Posterior airs sacs
◦ Anterior air sacs
Birds
Birds

Bird ventilation requires two cycles of
inhalation and exhalation.

Airflow across the respiratory
surfaces of the lungs is unidirectional
and almost continuous.
Birds
Birds

At syrinx the trachea divides into 2 primary
bronchi.

Primary bronchi split into secondary bronchi,
termed dorsobronchi.

Dorsobronchi further divide into parabronchi.

Parabronchi lead into secondary bronchi, termed
ventrobronchi, and back to primary bronchi.
Birds
Birds

Parabronchi
◦ smallest airways of a bird lung.
◦ are folded, forming hundreds of blind-ended
structures called air capillaries.

Air capillaries
◦ Primary site of gas exchange
◦ Thin walls = minimal barrier for gas exchange
Birds
Nares &
Mouth
Trachea
Syrinx
Trachea
Primary Bronchi
(2)
Anterior Air
Sacs
Posterior
Air Sacs
Ventrobronchi
Parabronchi
“air capillaries”
Dorsobronchi
Birds: Inspiration
Birds: Expiration
Birds: Parabronchi