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. thin 2. wet How Organisms Exchange Gases: Simple Diffusion Some animals have no specialized respiratory organs or circulatory systems O 2 obtained through simple diffusion O 2 tension must be high enough at the surface for O 2 to reach the center of the organism How Organisms Exchange Gases: Simple Diffusion With radius, the greater [O 2 ] at the surface must be to supply oxygen to the core Example: radius = 1 mm, V O2 = 0.001 ml/g*min P O2 needed= 0.15 atm Example: radius = 1 cm, V O2 = 0.001 ml/g*min P O2 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 1
Gas Exchange Between Body Fluids and the Environment Occurs through diffusion Dependent on difference in P O2 and P CO2 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 O 2 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) 2
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 O 2 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 3
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 P O2 water Respiration in Air: Tracheae 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 Spiracles regulate gas exchange and water loss Discontinuous gas exchange CO 2 released in bursts accompanied by H 2 O loss Reduce H 2 O loss Avoid oxygen toxicity 4
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 Gastropods - simple cavity in mantle highly vascularized epithelium single opening (pneumostome) passive or active ventilation Arachnids: Book Lung multiple lamellar folds typically passive air exchange 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 How is Air Circulated in Lungs? Two methods in vertebrates: Parabronchial Lungs (Birds) lungs connected to a series of air sacs allows continuous, unidirectional flow of air through the lungs Positive Pressure Pump push air out of oral cavity into the lungs Negative Pressure Pump pull air into lungs from oral cavity 5
Positive Pressure Lungs Lungfish, Amphibians, Some Reptiles 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 Negative Pressure Lungs Reptiles, Mammals, Birds Expansion of thoracic cavity pulls air into lungs from oral/nasal cavities Relaxation of muscles compresses thoracic cavity, pushing air out 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 P O2 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 Respiration Air Breathers vs. Water Breathers P CO2 has greater effect on respiration frequency air breathers O 2 plentiful CO 2 levels can build up ( ph) P O2 has greater effect on respiration frequency water breathers O 2 in short supply CO 2 levels low and readily soluble in water 6