Transcript Gas Exchange in Animals

Gas Exchange in Animals
Principles & Processes
Gas Exchange
• respiratory gases
– oxygen (O2)
• required as final electron acceptor for
oxidative metabolism
– carbon dioxide (CO2)
• discarded byproduct of oxidative
metabolism
Gas Exchange
• respiratory mechanisms
– system to deliver oxygenated/remove
deoxygenated medium
– membrane for gas exchange
– system to carry O2 to cells/CO2 from cells
Gas Exchange
• physical factors affecting gas exchange
– gases cross respiratory membranes by
diffusion
– diffusion occurs much faster in air than in
water (~8000 X)
– O2 content of air is greater than O2 content
of water (<20 X)
– air is less dense (~800 X) & less viscous (50
X) than water
 air is a better respiratory medium than water
Gas Exchange
• problems for water breathers
– cells must be near oxygenated medium
• solutions
–thin (2-D) body
–perfused body
–specialized external exchange surfaces
–specialized internal exchange surfaces
specialized external exchange surfaces
Figure 48.1
Gas Exchange
• problems for water breathers
– an ectotherm’s O2 demand increases with
increased temperature
– O2 content of water decreases with increased
temperature
– compensatory increase in breathing
increases O2 demand
the
problem
with warm
water
Figure
48.2
Gas Exchange
• problems for (adventurous) air breathers
– air pressure decreases with altitude
• O2 partial pressure decreases with altitude
• rate of O2 diffusion decreases with
decreased O2 partial pressure
increased
altitude
decreases
the
availability
of O2
Gas Exchange
• CO2 removal
– [CO2] in air is ~350 ppm
• gradient for outward diffusion is always
steep
– [CO2] in water varies depending on aeration
• gradient for outward diffusion may be
very shallow
Gas Exchange
• Fick’s law of diffusion indicates how to
increase diffusion rates
Q = D·A·(P1-P2)/L
Q is the rate of diffusion from a => b
D is the diffusion coefficient of a system
A is the cross-sectional area of diffusion
P1, P2 are the partial pressures of the
diffusing particle at a & b
L is the distance between a & b
Gas Exchange
• using Fick’s law of diffusion
Q = D·A·(P1-P2)/L
– increase diffusion (Q) by
• increasing D (use air instead of water?)
• increasing A (increase exchange surface)
• increasing P1-P2 (replenish fresh air)
• decreasing L (decrease thickness of
exchange surface)
Gas Exchange
• animal gas exchange surfaces (increase A)
– external gills
• large surface area
• no breathing system needed
• exposed to possible damage or predation
– internal gills
• same large surface area, plus
• protection against damage, but
• requires breathing mechanism
gas
exchange
with
water
Figure
48.3
Gas Exchange
• animal gas exchange surfaces (increase A)
– lungs
• internal, highly divided, elastic cavities
• transfer gases to transport medium
– tracheae (insects)
• internal, highly branched air tubes
• transfer gases to all tissues
gas
exchange
with
air
Figure
48.3
Gas Exchange
• animal gas exchange surfaces (increase P1P2/L)
– exchange membranes are very thin (L small)
– breathing ventilates external surface (O2 at
P1 is high; CO2 at P2 is low)
– circulatory system perfuses internal surface
(O2 at P2 is low; CO2 at P1 is high)
Gas Exchange
• animal gas exchange surfaces (increase P1P2/L)
– exchange membranes are very thin (L small)
– breathing ventilates external surface (O2 at
P1 is high; CO2 at P2 is low)
– circulatory system perfuses internal surface
(O2 at P2 is low; CO2 at P1 is high)
• specific systems vary in the details of
ventilation, perfusion & exchange surface
Gas Exchange
• insect tracheae
– spiracles open into tubes (tracheae)
– tubes branch into smaller tubes (tracheoles)
– network ends in dead end air capillaries
entering all tissues
• gases diffuse from cell to atmosphere
entirely in air
• rate of diffusion is limited by
–A = diameter of tubes
–L = length of tubes
spiracles and tubular system
Figure 48.4
Gas Exchange
• fish gills
– opercular flaps protect gills
– gill arches support gill filaments
– gill filament surfaces bear lamellar folds (L)
– oxygenated water flows
• in mouth
• through gill filaments
• over lamellae
• out opercula
filament
lamellae
Figure
48.5
Gas Exchange
• fish gills
– maximize diffusion gradient (P1-P2) by
countercurrent flow
• water flow is unidirectional and constant
• blood flows in lamellae in opposite
direction
–low O2 blood <=> low O2 water
–partially oxygenated blood <=>
partially depleted water
–high O2 blood <=> high O2 water
counter-current flow maximizes the
diffusion gradient
Figure 48.6
Gas Exchange
• bird lungs
– continuous airway without dead end spaces
– trachea delivers inhaled air to posterior air
sacs
– air moves from posterior air sacs through
lung to anterior air sacs
• air moves through parabronchi
• gases exchange in air capillaries (L)
– air moves out from anterior air sacs through
trachea
trachea,
posterior air sacs,
lung,
anterior air sacs,
trachea
Figure 48.7
Gas Exchange
• bird lungs
– unidirectional flow through lung
• inhalation moves air into posterior air
sacs
• exhalation moves air out of anterior air
sacs and air from posterior air sacs to
lung
• inhalation refills posterior air sacs and
moves air from lung to anterior air sacs
• exhalation moves air out of anterior air
sacs
first breath
cycle
second
breath
cycle
Figure 48.8
Gas Exchange
• bird lungs
– maximize diffusion gradient (P1-P2) by
providing a continuous flow of fresh air
Gas Exchange
• mammalian lungs
– tidal ventilation
• fresh air is inhaled (tidal volume)
• fresh air mixes with depleted air (tidal
volume + expiratory reserve volume +
residual volume)
• gas exchange occurs between blood and
mixed air
• depleted air is partially exhaled (tidal
volume)
tidal breathing
Figure 48.9
tidal
volume
residual
volume
expiratory
reserve
volume
Gas Exchange
• mammalian lungs
– tidal ventilation
• fresh air is introduced only during
inhalation
• fresh air mixes with depleted air
• lung dead space does not receive fresh air
• dead end exchange surfaces do not
provide countercurrent flow
diffusion gradient (P1-P2) is limited by low P1
Gas Exchange
• mammalian lungs - structure/function
– air enters through oral and nasal openings
– passages join at pharynx
– larynx (voice box) admits air to trachea
– trachea conduct air to two bronchi
– bronchi carry air to lungs
– bronchi branch into smaller tubes
(bronchioles)
– smallest bronchioles terminate in thinwalled gas exchange sacs (alveoli)
Gas Exchange
• mammalian lungs - structure/function
– large number of alveoli provides massive
gas exchange surface (A)
– thin membranes of alveoli & alveolar
capillaries minimizes diffusion path length
(L)
big A,
little L
Figure 48.10
Gas Exchange
• mammalian lung ventilation
– lungs are contained in thoracic cavity
– each lung is enclosed by a pleural
membrane
– thoracic cavity is contained by muscular
boundaries
• diaphragm
• rib cage
–external intercostal muscles
–internal intercostal muscles
Gas Exchange
• mammalian lung ventilation
– exhalation
• relaxation of diaphragm allows elastic
expulsion of air from lung
• internal intercostal muscles decrease
thoracic volume
mechanism
of tidal
breathing
Figure 48.11