Overview

1. Overview • Respiration – sequence of events that result in the exchange of oxygen and carbon dioxide between the external environment and the mitochondria • Mitochondrial respiration – production of ATP via oxidation of carbohydrates, amino acids, or fatty acids; oxygen is consumed and carbon dioxide is produced • External respiration – gas exchange at the respiratory surface • Internal respiration – gas exchange at the tissue • Gas molecules move down concentration gradients

2. Overview, Cont. • Unicellular and small multicellular organisms rely on diffusion for gas exchange • Larger organisms must rely on a combination of bulk flow and diffusion for gas exchange, i.e., they need a respiratory system

3. Overview, Cont.

4. The Physics of Respiratory Systems • Rate of diffusion (Fick equation) • dQ/dt = D x A x dC/dx • Rate of diffusion will be greatest when the diffusion coefficient (D), area of the membrane (A), and energy gradients (dC/dx) are large, but the diffusion distance is small • Consequently, gas exchange surfaces are typically thin, with a large surface area

5. Gas Pressure • Total pressure exerted by a gas is related to the number of moles of the gas and the volume of the chamber • Ideal gas law: PV = nRT • Air is a mixture of gases: nitrogen (78%), oxygen (21%), argon (0.9%), and carbon dioxide (0.03%) • Dalton’s law of partial pressures: in a gas mixture each gas exerts its own partial pressure that sum to the total pressure of the mixture

6. Gases Dissolve in liquids • Gas molecules in air must first dissolve in liquid in order to diffuse into a cell • Henry’s law: [G] = Pgas x Sgas

7. Diffusion Rates • Graham’s law • Diffusion rate a solubility/square root (molecular mass) • Combining the Fick equation with Henry’s and Graham’s laws • Diffusion rate aPgas x A x Sgas / X x square root (MW) • At a constant temperature the rate of diffusion is proportional to • Partial pressure gradient (Pgas) • Cross-sectional area (A) • Solubility of the gas in the fluid (Sgas) • Diffusion distant (X) • Molecular weight of the gas (MW)

8. Fluid Movement • Fluids flow from areas of high to low pressure • Boyle’s Law: P1V1 = P2V2

9. Surface Area to Volume Ratio • As organisms grow larger, their ratio of surface area to volume decreases • This limits the area available for diffusion and increases the diffusion distance

11. Respiratory Strategies • Animals more than a few millimeters thick use one of three respiratory strategies • Circulating the external medium through the body • Sponges, cnidarians, and insects • Diffusion of gases across the body surface accompanied by circulatory transport • Cutaneous respiration • Most aquatic invertebrates, some amphibians, eggs of birds • Diffusion of gases across a specialized respiratory surface accompanied by circulatory transport • Gills (evaginations) or lungs (invaginations) • Vertebrates

12. Ventilation • Ventilation of respiratory surfaces reduces the formation of static boundary layers • Types of ventilation • Nondirectional - medium flows past the respiratory surface in an unpredictable pattern • Tidal - medium moves in and out • Unidirectional - medium enters the chamber at one point and exits at another • Animals respond to changes in environmental oxygen or metabolic demands by altering the rate or pattern of ventilation

13. Ventilation and Gas Exchange • Because of the different physical properties of air and water, animals use different strategies depending on the medium in which they live • Differences • [Oair] 30x greater than [Owater] • Water is more dense and viscous than air • Evaporation is only an issue for air breathers • Strategies • Unidirectional: most water-breathers • Tidal: air-breathers • Air filled tubes: insects

14. Ventilation and Gas Exchange in Water • Strategies • Circulate the external medium through an internal cavity • Various strategies for ventilating internal and external gills

15. Sponges and Cnidarians • Circulate the external medium through an internal cavity • In sponges flagella move water in through ostia and out through the osculum • In cnidarians muscle contractions move water in and out through the mouth

16. Molluscs • Two strategies for ventilating their gills and mantle cavity • Beating of cilia on gills move water across the gills unidirectionally • Blood flow is countercurrent • Snails and clams • Muscular contractions of the mantle propel water unidirectionally through the mantle cavity past the gills • Blood flow is countercurrent • Cephalopods

17. Crustaceans • Filter feeding (barnacles) or small species (copepods) lack gills and rely on diffusion • Shrimp, crabs, and lobsters, have gills derived from modified appendages located within a branchial cavity • Movements of the gill bailer propels water out of the branchial chamber; the negative pressure sucks water across the gills

18. Jawless Fishes • Hagfish • A muscular pump (velum) propels water through the respiratory cavity • Water enters the mouth and leaves through a gill opening • Flow is unidirectional • Blood flow is countercurrent Lamprey and hagfish have multiple pairs of gill sacs

19. Jawless Fishes, Cont. • Lamprey • Ventilation is similar to that in hagfish when not feeding • When feeding the mouth is attached to a prey (parasitic) • Ventilation is tidal though the gill openings

20. Elasmobranchs • Steps in ventilation • Expand the buccal cavity • Increased volume sucks fluid into the buccal cavity via the mouth and spiracles • Mouth and spiracles close • Muscles around the buccal cavity contact forcing water past the gills and out the external gill slits • Blood flow is countercurrent

21. Teleost Fishes • Gills are located in the opercular cavity protected by the flaplike operculum • Steps in ventilation • With the mouth open, the floor of the buccal cavity lowers • Volume increases • Pressure decreases and sucks water in from outside • Concurrently, with the operculum closed, the opercular cavity expands • Volume increases • Pressure decreases and suck water in from the buccal cavity • Mouth closes • Floor of buccal cavity raises • Volume decreases • Pressure increases and pushes water into the opercular cavity • Operculum opens and water leaves through the opercular slit • Active fish can also use ram ventilation

22. Teleost Fishes, Cont.

23. Fish Gills • Fish gills are arranged for countercurrent flow

24. Ventilation and Gas Exchange in Air • Two major lineages have colonized terrestrial habitats • Vertebrates • Arthropods

25. Arthropods • Crustaceans • Chelicerates • Insects

26. Crustaceans • Terrestrial crabs • Respiratory structures and the processes of ventilation are similar to marine relatives, but • Gills are stiff so they do not collapse in air • Branchial cavity is highly vascularized and acts as the primary site of gas exchange • Terrestrial isopods (woodlice and sowbugs) • Have a thick layer of chitin on one side of the gill for support • Anterior gills contain air-filled tubules (pseudotrachea)

27. Chelicerates • Spiders and scorpions • Have four book lungs • Consists of 10-100 lamellae • Open to outside via spiracles • Gases diffuse in and out • Some spiders also have a tracheal system – series of air-filled tubes

28. Insects • Have an extensive tracheal system - series of air-filled tubes • Tracheoles – terminating ends of tubes that are filled with hemolymph • Open to outside via spiracles • Gases diffuse in and out

30. Insect Ventilation • Types • Contraction of abdominal muscles or movements of the thorax • Can be tidal or unidirectional (enter anterior spiracles and exit abdominal spiracles) • Ram ventilation (draft ventilation) in some flying insects • Discontinuous gas exchange • Phase 1 (closed phase): no gas exchange; O2 used and CO2 converted to HCO3-;  in total P • Phase 2 (flutter phase): air is pulled in • Phase 3: total P  as CO2 can no longer be stored as HCO3-; spiracles open and CO2 is released

31. Vertebrates • Fish • Amphibians • Reptiles • Birds • Mammals

32. Fish • Air breathing has evolved multiple times in fishes • Types of respiratory structures • Reinforced gills that do not collapse in air • Mouth or pharyngeal cavity • Vascularized stomach • Specialized pockets of the gut • Lungs • Ventilation is tidal using buccal force similar to other fish

33. Amphibians • Types of respiratory structures • Cutaneous respirations • External gills • Simple bilobed lungs; more complex in terrestrial frogs and toads • Ventilation is tidal using a buccal force pump

34. Reptiles • Most have two lungs; in snakes one lung is reduced or absent • Can be simple sacs with honeycombed walls or highly divided chambers in more active species • More divisions result in more surface area • Ventilation • Tidal • Rely on suction pumps • Results in the separation of feeding and respiratory muscles • Two phases: inspiration and expiration • Use one of several mechanisms to change the volume of the chest cavity

35. Reptiles, Cont.

36. Birds • Lung is stiff and changes little in volume • Rely on a series of flexible air sacs • Gas exchange occurs at parabronchi

37. Bird Ventilation • Requires two cycles of inhalation and exhalation • Air flow across the respiratory surfaces is unidirectional

38. Mammals • Two main parts • Upper respiratory tract: mouth, nasal cavity, pharynx, trachea • Lower respiratory tract: bronchi and lungs • Alveoli are the site of gas exchange • Both lungs are surrounded by a pleural sac

39. Mammal Ventilation • Tidal ventilation • Steps • Inhalation • Somatic motor neuron innervation • Contraction of the external intercostals and the diaphragm • Ribs move outwards and the diaphragm moves down • Volume of thorax increases • Air is pulled in • Exhalation • Innervation stops • Muscle relax • Ribs and diaphragm return to their original positions • Volume of the thorax decreases • Air is pushed out via elastic recoil of the lungs • During rapid and heavy breathing, exhalation is active via contraction of the internal intercostal muscles