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 surfacesFigure 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 waterFigure 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

availabilityof 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 systemA is the cross-sectional area of diffusionP1, P2 are the partial pressures of the

diffusing particle at a & bL 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 waterFigure 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 P1-P2/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 P1-P2/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 systemFigure 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 lamellaeFigure 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,

tracheaFigure 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

secondbreath 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 breathingFigure 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 thin-

walled 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

breathingFigure 48.11