How Organisms Exchange Gases: Simple Diffusion

1. 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 • thin • wet Fig 21.1

2. 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

3. 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 PO2needed= 0.15 atm • Example: radius = 1 cm, VO2 = 0.001 ml/g*min PO2needed= 15 atm • Few animals thicker than 1 mm rely on simple diffusion for gas exchange

4. 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

5. 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 Fig 21.2

6. 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 Figs 21.10a and 21.3

7. 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

8. 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 Figs 21.4 and 21.5

9. 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.

10. 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

11. 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 Figs 21.25 - 21.27, 21.10, and 21.13

12. 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)

13. 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 Fig 21.10

14. 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 Figs 21.10 - 21.11

15. 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 Fig 21.10

16. 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

17. Respiration in Air: Integument • Use skin for gas exchange • Limited surface area • Must keep surface moist • Often used in conjunction with other respiratory organs

18. 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 Fig 21.8

19. 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: Integument Fig 21.15

20. 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

21. 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

22. 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

23. 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 Fig 21.28 Insects, Arachnids, Isopods

24. 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

25. Respiration in Air: Lungs • Invaginations of therespiratory 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

26. Examples of Lungs • Gastropods - simple cavity in mantle • highly vascularized epithelium • single opening (pneumostome) • passive or active ventilation Fig 21.26d

27. Examples of Lungs • Arachnids: Book Lung • multiple lamellar folds • typically passive air exchange Box 21.3

28. 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 Figs 21.13 and 21.16 - 21.18

29. Examples of Lungs • Parabronchial Lungs (Birds) • lungs connected to a series of air sacs • allows continuous, unidirectional flow of air through the lungs Figs. 21.23 - 21.24

30. 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

31. Positive Pressure Lungs Lungfish, Amphibians, Some Reptiles • Glottis closed, buccal cavity expanded, air drawn in through nares • Glottis opens, air in lung passes out through nares • Nares close, oral cavity compresses, driving fresh air into lungs Fig 21.14

32. 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

33. 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 Figs 21.24 and 21.22a

34. Air Flow in Parabronchial Lungs • Requires two lung cycles for air to move fully through the lungs • Inspiration 1 - air drawn down bronchus into caudal sacs • Expiration 1 - air pushed from caudal sacs into lungs • Inspiration 2 - air pulled into cranial sacs from lungs • Expiration 2 - air pushed from cranial sacs out bronchus Fig 21.22 http://www.sci.sdsu.edu/multimedia/birdlungs/

35. 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, butnot quite as effective Fig 21.23

36. 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