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Overview. Respiration – sequence of events that result in the exchange of oxygen and carbon dioxide between the external environment and the mitochondria

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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


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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



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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


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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


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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


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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)


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Fluid Movement

  • Fluids flow from areas of high to low pressure

  • Boyle’s Law: P1V1 = P2V2


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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


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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


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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


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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


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Ventilation and Gas Exchange in Water

  • Strategies

    • Circulate the external medium through an internal cavity

    • Various strategies for ventilating internal and external gills


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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


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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


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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


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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


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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


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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


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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



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Fish Gills

  • Fish gills are arranged for countercurrent flow


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Ventilation and Gas Exchange in Air

  • Two major lineages have colonized terrestrial habitats

    • Vertebrates

    • Arthropods


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Arthropods

  • Crustaceans

  • Chelicerates

  • Insects


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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)


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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


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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


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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


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Vertebrates

  • Fish

  • Amphibians

  • Reptiles

  • Birds

  • Mammals


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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


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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


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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



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Birds

  • Lung is stiff and changes little in volume

  • Rely on a series of flexible air sacs

  • Gas exchange occurs at parabronchi


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Bird Ventilation

  • Requires two cycles of inhalation and exhalation

  • Air flow across the respiratory surfaces is unidirectional


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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


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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


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