How Organisms Exchange Gases: Simple Diffusion

• Gas is exchanged between respiratory medium and body fluids through diffusion across a respiratory surface

• To effectively exchange gases, the surface must be

1. thin2. wet

How Organisms Exchange Gases: Simple Diffusion

• Some animals have no specialized respiratory organs or circulatory systems– O2 obtained through simple diffusion

• O2 tension must be high enough at the surface for O2 to reach the center of the organism

How Organisms Exchange Gases: Simple Diffusion

• With radius, the greater [O2] at the surface must be to supply oxygen to the core– Example: radius = 1 mm, VO2 = 0.001 ml/g*min

PO2 needed= 0.15 atm

– Example: radius = 1 cm, VO2 = 0.001 ml/g*min

PO2 needed = 15 atm

• Few animals thicker than 1 mm rely on simple diffusion for gas exchange

How Organisms Exchange Gases: Respiratory Organs

• Larger animals possess specialized respiratory surfaces

– regions with large surface area/volume ratio• branches, flattened areas, etc. - SA• thin walls - diffusion distance

– allow easy passage of gas into a circulatory system

• Convection of respiratory medium over the respiratory surfaces (ventilation) typically required

Types of Respiratory Surfaces

• Integument– use skin for gas exchange

– requires thin, moist, permeable integument

• Evaginations (gills)– specialized respiratory organ

– increases external surface area

• Invaginations (lungs)– increase respiratory surface area

– protect respiratory surface

Respiratory Surface Ventilation• Unidirectional Flow

– Medium flows over respiratory surfaces in one direction

– New medium continuously flows over surfaces

• Bidirectional (Tidal) Flow– Medium flows into respiratory

surfaces then out in the opposite direction

– Incoming medium mixed with “used” medium

Gas Exchange Between Body Fluids and the Environment

• Occurs through diffusion– Dependent on difference in PO2 and PCO2

between the body fluids and respiratory medium

• The flow of body fluids relative to the flow of the respiratory medium influence pressure gradients for gas exchange

Patterns of Flow at Exchange Surfaces

• Concurrent Flow– Body fluid and respiratory medium flow in

same direction– Gradient reduced with distance

• Countercurrent Flow– Body fluid and respiratory medium flow in

opposite directions– Gradients sustained over distance

• Crosscurrent Flow– Body fluid and respiratory medium flow at

nonparallel angles to each other– Gradient slowly decreases with distance

Respiration in Water: Integument

• Small Animals– High SA/V ratio

• Large Animals– Often elevated surface area

– Often used in conjunction with other respiratory systems

• Requires permeable integument– Elevated water intake, ion loss, etc.

Respiration in Water: Lungs

• Not very practical– Requires animal to generate tidal flow of

water• Energetically expensive• Low efficiency of O2 uptake

• Sea cucumber– Respiratory tree derived from anal canal

Respiration in Water: Gills

• Evaginations of the respiratory surface– large surface area– thin cuticle

• Used primarily for respiration in water– external exposure helps increase

circulation of medium across respiratory surface

– water supports weight of the gills without need for structural support

Respiration in Water: Gill Ventilation

• Flow of water over gills is necessary for supplying oxygen

– Move gill through the water (practical only for small animals)

– Move water over the gill:• ciliary action (bivalves)

• pumping devices (teleost fish and arthropods)

• ram ventilation (sharks, tuna)

Teleost Fish Gills: Structure

• Gills positioned on either side of buccal cavity underneath the operculum

• Four brachial arches, each carrying two rows of gill filaments

• Each filament carries rows of parallel lamellae

• Capillary circulation is countercurrent to water

Teleost Fish Gills: Ventilation

• Water flows into mouth, over the gills, and out the gill slits

• Water is driven across the gills by two pumps:

– Buccal pressure pump

• forces water from mouth over the gills

– Opercular suction pump

• sucks water from the mouth over the gills

Buccal Pump Function• Mouth opens, buccal cavity floor depressed

– Water drawn into buccal cavity• Mouth closes, floor raises

– drives water over gills into opercular cavities– tissue flaps prevent backflow of water back out mouth

• Expansion of opercula draws water into opercular cavity from oral cavity

– flaps prevent water from being pulled in through gill slits• Compression of opercula forces water out through the gill slits• Synchronization of the two pumps allows flow over the gills

through most of the respiratory cycle

Respiration in Air

• Higher oxygen content

• Higher gas diffusion rates– can get O2 from less volume

• Lower density and viscosity– easier to move

• Loss of water problematic

Respiration in Air: Integument

• Use skin for gas exchange

• Limited surface area

• Must keep surface moist

• Often used in conjunction with other respiratory organs

Respiration in Air: Integument

• Integumental exchange often supplements that of other respiratory organs

• Relative contribution of different surfaces to overall gas exchange varies among species and among conditions

Respiration in Air: Integument

• Anurans– Use both lungs and skin

for gas exchange

– Usage of each depends on gas and on metabolic demands and developmental stage

Respiration in Air: Gills

• Uncommon– poorly suited for gas exchange in air

• Thin, branched structures require support– if too thin, collapse under own weight and stick

together due to water surface tension– if too thick, lose effectiveness as respiratory surface

• External exposure increases evaporative water loss

– Covering reduces passive ventilation

Respiration in Air: Gills

Terrestrial Crabs and Isopods• Smaller gills w/ fewer, shorter branches than

aquatic spp.• Thicker cuticles on branches (more rigid)• Chambers are larger and more highly

vascularized – more lung-like

Modified Gill Structures of Air-Breathing Fish

• Hundreds of fish species can breathe air

• Various structures– Vascularized buccal and opercular cavities– Suprabranchial chambers– Modified swim bladders– Modified digestive tract

• Possible adaptation to low PO2 water

Respiration in Air: Tracheae

• Network of air-filled tubes (tracheae) extending throughout body of the animal

• Connected to exterior by spiracles (gated)

• Gas transport independent of circulatory system

• Work by passive ventilation or by active ventilation

Insects, Arachnids, Isopods

Respiration in Air: Tracheae

• Spiracles regulate gas exchange and water loss

• Discontinuous gas exchange

– CO2 released in bursts accompanied by H2O loss

– Reduce H2O loss

– Avoid oxygen toxicity

Respiration in Air: Lungs

• Invaginations of the respiratory surface

– increase surface area

• Used primarily for air breathing– supports and protects respiratory surface– isolates volumes of air from the atmosphere

• reduces evaporative water loss

• requires pumping action for circulation of medium

Examples of Lungs

• Gastropods - simple cavity in mantle– highly vascularized epithelium– single opening (pneumostome)– passive or active ventilation

Examples of Lungs

• Arachnids: Book Lung– multiple lamellar folds– typically passive air exchange

Examples of Lungs

• Alveolar Lungs – Most terrestrial vertebrates– formation of numerous

partitions or sacs (alveoli) within the lungs

– walls of sacs very thin and highly vascularized

– Tidally ventilated

Examples of Lungs

• Parabronchial Lungs (Birds)– lungs connected to a series of

air sacs

– allows continuous, unidirectional flow of air through the lungs

How is Air Circulated in Lungs?

Two methods in vertebrates:

• Positive Pressure Pump– push air out of oral cavity into the lungs

• Negative Pressure Pump– pull air into lungs from oral cavity

Positive Pressure Lungs

1. Glottis closed, buccal cavity expanded, air drawn in through nares

2. Glottis opens, air in lung passes out through nares

3. Nares close, oral cavity compresses, driving fresh air into lungs

Lungfish, Amphibians, Some Reptiles

Negative Pressure Lungs

• Expansion of thoracic cavity pulls air into lungs from oral/nasal cavities

• Relaxation of muscles compresses thoracic cavity, pushing air out

Reptiles, Mammals, Birds

Air Flow in Parabronchial Lungs

• Avian lungs are linked to several air sacs– cranial group

– caudal group

• Sacs not directly involved in gas exchange

• Allow unidirectional flow of air through the lungs

Air Flow in Parabronchial Lungs

• Requires two lung cycles for air to move fully through the lungs

a Inspiration 1 - air drawn down bronchus into caudal sacs

b Expiration 1 - air pushed from caudal sacs into lungs

c Inspiration 2 - air pulled into cranial sacs from lungs

d Expiration 2 - air pushed from cranial sacs out bronchus

http://www.sci.sdsu.edu/multimedia/birdlungs/

Air Flow in Parabronchial Lungs

• PO2 blood leaving lungs is higher than that of the exhaled air

• Blood flows cross-current to the flow of air

– similar to countercurrent, but not quite as effective

Regulation of RespirationAir Breathers vs. Water

Breathers• PCO2 has greater effect on respiration frequency air

breathers– O2 plentiful

– CO2 levels can build up ( pH)

• PO2 has greater effect on respiration frequency water breathers– O2 in short supply

– CO2 levels low and readily soluble in water