OverviewRespiration – 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 tissueGas molecules move down concentration gradients

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

Overview, Cont.

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

• 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

Gas Pressure

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

Diffusion Rates

Graham’s law• Diffusion rate solubility/square root (molecular mass)

Combining the Fick equation with Henry’s and Graham’s laws• Diffusion rate 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)

Fluid Movement• Fluids flow from areas of high to low

pressure• Boyle’s Law: P1V1 = P2V2

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

Respiratory StrategiesAnimals 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

VentilationVentilation of respiratory surfaces reduces the

formation of static boundary layersTypes 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

Ventilation and Gas ExchangeBecause 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

Ventilation and Gas Exchange in Water

Strategies• Circulate the external medium through an

internal cavity• Various strategies for ventilating internal and

external gills

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

MolluscsTwo 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

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

Lamprey and hagfish have multiple pairs of gill sacs

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

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

ElasmobranchsSteps 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

Teleost FishesGills are located in the opercular cavity protected by the

flaplike operculumSteps 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

Teleost Fishes, Cont.

Fish GillsFish gills are arranged for countercurrent flow

Ventilation and Gas Exchange in Air

Two major lineages have colonized terrestrial habitats• Vertebrates• Arthropods

Arthropods• Crustaceans• Chelicerates• Insects

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)

CheliceratesSpiders and scorpionsHave 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

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

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

Vertebrates• Fish• Amphibians• Reptiles• Birds• Mammals

FishAir breathing has evolved multiple times in fishesTypes 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

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

ReptilesMost have two lungs; in snakes one lung is reduced

or absentCan 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

Reptiles, Cont.

Birds• Lung is stiff and

changes little in volume

• Rely on a series of flexible air sacs

• Gas exchange occurs at parabronchi

Bird Ventilation

Requires two cycles of inhalation and exhalation

Air flow across the respiratory surfaces is unidirectional

MammalsTwo 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

Mammal VentilationTidal ventilationSteps

• 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