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HMM/SCM1414-Biology I

8.1 Gaseous Exchange in animal8.1.1 Respiratory Systems in Animals(a) Respiratory Surface

Respiratory surface - the part of ananimal where gases are exchanged with

environment. Movements of CO2 and O2 across

respiratory surface occur by diffusion.

F ick s Law o f D i f fus ion : "Rate oftransfer of a gas (dV/dt) through a sheet of

tissue isproportional to tissue area (A)anddifference in gas partial pressure between

the 2 sides(P1 P2)andinversely

proportional to tissue thickness (T)."

2

Volume of gas =

(per unit time)

Area x Diffusion constant x(Partial Pressure 1 Partial

Pressure 2) Thickness

dV = A * D * (P1 P2)

dt T

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Fick's Law governs transfer rate of gases

through tissue.

For respiration, law governs transfer

rate of O2 from alveoli to blood across

thin blood gas barrier, and CO2 in

opposite direction.

CO2 diffuses 20x more rapidly than

O2 through tissue sheets.

Reason:CO2 has higher solubility,thusincreasing diffusion constant (D).

Diffusion constant is proportional to

solubility divided by square root of

molecular weight.

Characteristics of respiratory surfaces thatmaximize rate of gas exchange:

(1) Large surface area

The larger the animal and the more active

it is, the larger the surface area required

for gaseous exchange. The greaterthearea exposed to the environment, the

greaterthe rate of diffusion.

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(2) Thin

One cell thick diffusion rapid over very

short distance only. Ifdistance is

doubled, diffusion takes 4x longer.

(3) Moist

Gases must dissolve in waterbefore

diffusing across respiratory surfaces.

(4) A good blood supply

Anefficient transportwillensurethat

gases will be taken away from

respiratory surfacequicklyso

maintaining a largeconcentration

gradient.

(5) A good ventilation gradient

A pumping system, which will

continuously, deliver gas to respiratory

surfaces. This willmaintaina large

diffusion gradient across respiratorysurface.

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(b) Types of Respiratory Surfaces Structure of respiratory surface depends

on size of organism, whether it lives in

water or on land, and on its metabolic

demands.

(i) Body Surface Protists & other unicellular organisms gas

exchange over entire surface area.

Simple animals (sponges, cnidarians, &

flatworms) - plasma membrane of every cell

in body is close to outside environment for

gas exchange. However, in most animals, bulk of body

lacks direct access to respiratory medium.

Respiratory surface is a thin, moist

epithelium that separatesrespiratory

medium from blood or capillaries, which

transport gases to and from rest of body.(Refer Figure 42.18, Campbell)

Some animals (earthworms and some

amphibians) use entire outer skin as a

respiratory organ.

Have dense net of capillaries below moist

skin.

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Since respiratory surface must be moist,

theirhabitats limited to water or damp

places.

Animals using moistskin as their only

respiratory organ are usually small and

are eitherlong and thin orflat in shape,

with high ratio of surface area to volume.

(ii ) Gil ls Respiratory adaptations of most aquatic

animals.

Gills - outfoldings of body surface thatare suspended in water.

Water as a respiratory medium: Advantage: Cell membranes ofrespiratory

surface are kept moist since gills are

surrounded by aqueous environment.

Disadvantage: LowO2 concentrations in

water, especially in warmer and saltier

environments. Ventilation - increases flow ofrespiratory medium over respiratory

surface, ensuringstrong diffusion gradient

exists between gill surface and

environment.

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Without ventilation, a region of low O2 &

high CO2 concentrations forms around gill

as it exchanges gas with environment.

Fish gills ventilated by a current of water

that enters mouth, passes through slits in

pharynx, flows over gills, and exits body.(Figure 42.20, Campbell)

Because water is dense and contains

little O2 per unit volume, fishes must

expend considerable energy in

ventilating their gills.

Gas exchange at gill surface enhanced by

opposing flows of water and blood at gills -

countercurrent exchange(Figure 42.21,Campbell).

As blood moves through gill capillary, it

becomes more and more loaded with O2,

but it simultaneously encounters water

with even higher O2 concentrations

because it is just beginning its passage

over the gills.

All along the gill capillary, there is a

diffusion gradient favoring transfer of O2

from water to blood.

The countercurrent exchange mechanism

is so efficient that gills can remove more

than 80% of O2 from water to blood.

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Gills unsuited for animal living on land:

A large surface of wet membrane

exposed to air would lose too much water

by evaporation.

Gills would collapse as their fine

filaments, no longer supported by water,

cling together, reducing surface area for

exchange.

(iii) Tracheal System Tracheal systems & lungs - respiratory

adaptations of terrestrial animals.

Advantages of air over water as

respiratory medium: Higher O2 concentration.

O2 and CO2 diffuse much faster in air -

respiratory surfaces do not have to be

ventilated as thoroughly as gills.

If ventilation is required, less energy

needed because air is lighter and easierto pump and much less volume needs to

be breathed to obtain an equal amount of

O2.

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Disadvantages:

Respiratory surface, which must be large

and moist, continuously loses water to air

by evaporation.

Problem greatly reduced by having

respiratory surface folded into body.

Tracheal system of insects composedof air tubes that branch throughout body. Largest tubes (tracheae) open to outside

while finest branches (tracheoles)extend to surface of nearly every cell

where gas is exchanged by diffusion

across moist epithelium that lines the

terminal ends. (Figure 42.22, Campbell) The open circulatory system does not

transport O2 and CO2 since all cells are

located close to respiratory medium.

For small insect, diffusion through

trachea brings in enough O2 and removes

enough CO2 to support cellular respiration. Larger insects with higher energy

demands ventilate their tracheal systems

with rhythmic body movements that

compress and expand the air tubes.

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An insect in flight has very high metabolic

rate, consuming 10 to 200 times more O2

than it does at rest.

Alternating contraction and relaxation of

flight muscles compresses and expands

body, rapidly pumping air through

tracheal system.

Flight muscles are packed with

mitochondria, supplied with O2 bytracheal tubes.

(iv) Lungs Lungs restricted to one location.

Since respiratory surface of lung is not indirect contact with all other parts of

body, circulatory system transports gases

between lungs and rest of body.

Have dense net of capillaries under

epithelium that forms respiratory surface.

In spiders, terrestrial snails, &vertebrates.

Among vertebrates, amphibians have

relatively small lungs that do not provide a

large surface, and many lack lungs

altogether.

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Rely on diffusion across other body

surfaces, especially moist skin, for gas

exchange.

In contrast, most reptiles (including all

birds) and all mammals rely entirely on

lungs for gas exchange.

Turtles may supplement lung breathing

with gas exchange across moist

epithelial surfaces in mouth and anus. Lungs and air-breathing have evolved in a

few fish species (lungfishes) as

adaptations to living in O2-poor water or

to spending time exposed to air.

Besides lungs, birds have eight or nine air

sacs that increase respiratory efficiency. In general, size and complexity of lungs

are correlated with an animals metabolic

rate (and hence rate of gas exchange).

For example, lungs of endotherms have a

greater area of exchange surface than

lungs of similar-sized ectotherms.

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8.1.2 Human Respiratory System Lungs located in thoracic (chest) cavity.(Figure 42.23, Campbell)

Have spongy texture and are lined with a

moist epithelium that functions as

respiratory surface.

A system of branching ducts conveys air

to the lungs.

Air enters through nostrils and is thenfiltered by hairs, warmed and humidified,

and sampled for odors as it flows through

nasal cavity. Nasal cavity leads to pharynx, an

intersection where the paths for air &

food cross.

Pharynx leads to larynx, a cartilaginousstructure adapted for sound production.

During swallowing, epiglottis coversentrance to larynx, the glottis.

From larynx, air passes into trachea,whose shape is maintained by rings of

cartilage.

Trachea branches into two bronchi, oneleading into each lung.

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Within lung, each bronchus branches

repeatedly into finer and finer tubes,

bronchioles. Epithelium lining major branches of

respiratory tree covered by cilia and thin

film of mucus.

Mucus traps dust, pollen, and other

particulate contaminants, and beating

cilia move mucus upward to pharynx,where it is swallowed.

Bronchioles lead to a cluster of air sacs,

the alveoli. Gas exchange occurs across thin

epithelium of alveoli.

Total surface area of alveoli in human 100 m2 - sufficient to carry out gas

exchange for whole body.

O2 in air entering alveoli dissolves in

moist film and rapidly diffuses across

epithelium into web of capillaries that

surrounds each alveolus. CO2 diffuses in opposite direction.

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8.1.3 Role of Partial Pressure Gradient Diffusion of gas depends on differences in

partial pressure, the concentration of aparticular gas to overall total.

Atmospheric pressure at sea level = 760

mm Hg.

Since atmosphere is 21% O2 (by volume),

partial pressure of O2 is 0.21 760, orabout 160 mm Hg.

Partial pressure of CO2 is 0.23 mm Hg.

Gas diffuses from region of higher to

region of lower partial pressure. (Figure 42.27,Campbell)

Blood arriving at lungs via pulmonary

arteries has lower partial pressure of O2

and higher partial pressure of CO2 than air

in alveoli.

As blood enters alveolar capillaries, CO2

diffuses from blood to air within alveoli,

and O2 in alveolar air dissolves in fluidthat coats epithelium and diffuses across

surface into blood.

By the time blood leaves lungs in

pulmonary veins, its partial pressure of O2

has been raised and its partial pressure

of CO2 has been lowered.

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In tissue capillaries, gradients of partial

pressure favor diffusion of O2 out of blood

and CO2 into blood.

Cellular respiration removes O2 from and

adds CO2 to interstitial fluid by diffusion.

After blood unloads O2 and loads CO2, it is

returned to heart and pumped to lungs

again, where it exchanges gases with air

in alveoli.

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8.1.4 Respiratory pigments A diversity of respiratory pigments has

evolved in various animal taxa to support

their normal energy metabolism:

Hemocyanin: In hemolymph ofarthropods and many molluscs, - has

copper as its O2-binding component,

coloring the blood bluish. Hemoglobin: In red blood cells -

respiratory pigment of almost all

vertebrates.

Consists of four subunits, each with a

heme group (cofactor) that has an iron

atom at its center. (Figure 5.23, Campbell) Because iron actually binds the O2,

each hemoglobin molecule can carry

four molecules of O2.

Hb + 4O2 HbO8 (Oxyhemoglobin)

Hemoglobin binds O2 reversibly, loading

O2 at lungs or gills and unloading it in other

parts of body.

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8.1.5 Oxygen Transport and BohrEffect

Low solubility of O2 in water is a

fundamental problem for animals that rely

on circulatory systems for O2 delivery.

Thus, most animals transport most of O2

bound to respiratory pigments instead of

dissolved in solution.

(a) Oxygen Dissociation Curve forHemoglobin

(Figure 42.28, Campbell)

O2 saturation of hemoglobin (%) plotted

against different values of partial pressure

of O2 (PO2) (mmHg)

Shows how readily hemoglobin

acquires and releases O2molecules from

its surrounding tissue.

Where dissociation curve has a steepslope, even a slight change in PO2 causes

hemoglobin to load or unload a substantial

amount of O2.

Steep part corresponds to range of partial

pressures found in body tissues.

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A slight drop in PO2 causes relatively large

increase in amount of O2 unloaded by

blood.

When pO2 is high (alveoli) - hemoglobin

almost fully saturated.

Significance - hemoglobin picks up O2

as it passes through lungs.

When the pO2 falls at first (plateau) - littleeffect on % saturation.

Significance - as blood passes

through heart and arteries, pO2 drops

slightly but hemoglobin does not lose

much O2.

When there is a relatively small change inpO2 (steep part of curve) - large change in

% saturation of hemoglobin.

Significance - when blood reaches the

respiring tissues, hemoglobin gives up

most of its O2.

At low pO2 hemoglobin is unsaturated - ithas given up most of its O2.

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(b) The Bohr Shift As in all proteins, hemoglobins

conformation is sensitive to a variety of

factors.

For example, a drop in pH (increase in

CO2/pCO2) lowers affinity of hemoglobin

for O2, an effect called Bohr shift - curveshifts to the right.

Increase in temperature also shifts curve

to right.

Because CO2 reacts with water to form

carbonic acid, an active tissue will lower

the pH of its surroundings and induce

hemoglobin to release more O2. Significance - hemoglobin more efficient

at releasing O2 (more oxyhemoglobin

dissociates).

As tissues become more active, rate of

respiration increases, more CO2 is

released. Thus, tissues receive more O2 and can

continue aerobic respiration at same

partial pressure of O2.

Hemoglobin more efficient at taking up O2

when CO2 levels are low i.e. in lungs.

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8.1.6 Carbon dioxide transport Hemoglobin also helps transport CO2 and

assists in buffering blood pH. (Figure 42.29,Campbell)

7% of CO2 transported in solution.

23% binds to amino groups of

hemoglobin.

70% transported as bicarbonate ions.

CO2 from respiring cells diffuses intoblood plasma and then into RBCs.

CO2 first reacts with water, assisted by

enzyme carbonic anhydrase, to form

H2CO3.

H2CO3 dissociates into hydrogen ion (H+)

and bicarbonate ion (HCO3)

H+ attaches to hemoglobin (forming

hemoglobinic acid, HHb) and other

proteins, and act as buffer, minimizing

change in blood pH.

HCO3 diffuses into plasma.

As HCO3 diffuses out of RBCs, chloride

ions (Cl)diffuse into RBCs to maintain

electrical neutrality process is known

as chloride shift. As blood flows through lungs, process is

reversed as diffusion of CO2 out of blood

shifts chemical equilibrium in favor of

conversion of HCO3 to CO2.

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8.1.7 Control of breathing(Figure 42.26, Campbell)

Breathing control centers - in medullaoblongata and pons.

Medullas centers set basic breathing

rhythm inspiratory center increasesinspiratory rate; expiratory center cutsoff inspiratory activity & promotes

expiration.

Inspiratory center send impulse via

phrenic nerves to diaphragm and viathoracic nerves to rib muscles,stimulating them to contract and making

us inhale.

Pons helps control transition from

inhalation to exhalation.

A negative-feedback mechanism via

stretch receptors prevents our lungsfrom over-expanding by discharging

inhibitory impulses via vagus nerves toexpiratory center in medulla.

Medullas control center monitors CO2

level of blood and regulates breathing

activity appropriately.

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Chemoreceptors in medulla detectchanges in pH (increase in CO2) of blood

and cerebrospinal fluid bathing brain.

CO2 reacts with water to form carbonic

acid, which dissociates into HCO3 and

H+, which lowers pH.

Chemoreceptors in walls of aorta &

carotid arteries also detect changes in

pH. Nerves impulses relay changes to

medulla.

Medullas control center increases depth

and rate of breathing, and excess CO2 is

eliminated in exhaled air.

O2 concentrations in blood usually have

little effect on breathing control centers.

But, if O2 level falls markedly, example, at

high altitudes, chemoreceptors in aorta

and carotid arteries in neck send signals

to medulla to increase breathing rate.

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Figure: Control of Ventilation

From: Biological Science: Green, N.P.O., Stout, G.W., &Taylor, D.J., Cambridge University Press.

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BREATHINGCENTER

CO2

from

respiratoryactivity

1. Stimulation

Diaphragm &

intercostalsmuscles contract

2. Nerve

impulse fired

Lungs inflated

3. Inspiration

Inhibitory impulse

fired from stretchreceptors in lungs

4. Inhibition of

inspiration

Diaphragm &intercostals

muscles relax

Lungs deflated

5. No impulse

Inhibitory impulseno longer fired

6. Expiration

FOREBRAINVoluntary

control

Step 1 begins

again

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8.1.8 Lung Volumes Volume of air inhaled and exhaled can be

measured using a spirometer.

Terms to describe volume changes of lungs

during breathing:

a) Tidal Volume (TV) - The averagevolume of gas inspired and exhaled during

normal breathing.

b) Inspiratory Reserve Volume (IRV) -The maximum amount of gas that can be

inspired from the inspiratory point of a

normal tidal volume.

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Vitalcapacity

Tidalvolume

Inspiratoryreserve volume Respiratory

inspiratory level

Maximum expiratorylevel

Maximum inspiratorylevel

Residualvolume

Expiratoryreserve volume

Functionalresidualcapacity

5.0

3.45

3.0

1.5

0

Lungv

ol u

me/dm

3

Time

Inspiratorycapacity

Restingexpiratory level

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c) Expiratory Reserve Volume (ERV) -The maximum volume of gas that can be

exhaled from the resting end-expiratory level.

d) Residual Volume (RV) - The volume ofgas that remains in the lungs after a

maximum exhalation.

e) Inspiratory Capacity (IC) - Themaximum volume of gas that can be inspired

from the resting end-expiratory level.f) Vital Capacity (VC) - The maximum

amount of gas that can be exhaled after a

maximum inspiration.

g) Functional Residual Capacity (FRC) -The volume of gas that remains in the lungs

at the end of a normal exhalation.h) Tidal Lung Capacity (TLC) - The total

volume of gas contained in the lungs after a

maximum inspiration.

Summary of Lung Capacities IC = VT + IRV

FRC = ERV + RV

VC = VT + IRV + ERV

TLC = VT + IRV + ERV + RV

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Normal values for lung volumes &capacities (For normal adult male)

Volumes Capacities (ml)1. IRV

2. TV

3. ERV

4. RV

5. IC

6.VC

7. FRC

8. TLC

3100

500

1200

1200

3600

48002400

6000

http://www.mededsys.com/courses_online/208/208.html#lungvol

Lungs hold more air than the vital capacity

- some air, the residual volume, remainsin lungs because alveoli do not completely

collapse.

Since lungs do not completely empty and

refill with each breath cycle, newly inhaled

air is mixed with O2-depleted residual air.

Thus, maximum O2 concentration in

alveoli is considerably less than in

atmosphere.

Although this limits effectiveness of gas

exchange, CO2 in residual air is critical

for regulating pH of blood and breathing

rate in mammals.

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Respiratory diseases

Disease Respiratory failureAsthma Constriction of smooth muscles in

the bronchiolar and bronchial wall,

excess mucus secretion and

insufficient recoil of the alveoli.

Caused by allergy and emotional

upset. Results in difficulty in

breathing.

Pneumonia Alveoli filled of fluid, caused by

chemical, bacteria (Streptococcus),

viruses, protozoa or fungi.

Tuberculosi

s

Mycobacterium tuberculosis,

water-borne bacteria causes lung

damage in variety of ways. Theinfectious bacteria are normally

spread through air by coughing and

sneezing.

Lung cancer

( pulmonary

carcinoma)

Any inhaled irritant stimulates cell

to grow abnormally. Individual has

difficulty to breathe; chest pains

and spitting blood. Cigarette

smokers have 20 times more risk

than non smokers of having this

disease.

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8.2.2 Mechanism of stomatal openingand closing(Figure 36.13, Campbell)

Changes in water potential ( w) that open

and close stomata result from reversible

uptake and loss of K+ by guard cells.

Stomata open when guard cells actively

accumulate K+ into vacuole.

Water potential in guard cells decreases,

leading to inflow of water by osmosis.

Stomata close due to exodus of K+ from

guard cells, leading to osmotic loss of

water.

Regulation of aquaporins may also be

involved in swelling and shrinking of

guard cells by varying permeability of

membranes to water.

K+ fluxes across guard cell membranes

are coupled to generation of membrane

potentials by proton pumps. Stomatal opening correlates with active

transport of H+ out of guard cells.

The resulting voltage (membrane

potential) drives K+ into cell through

specific membrane channels.

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( a ) S tomata l open ing Membrane proton pump is activated -

pumps H+ out of cell:

Generates a stronger membrane potential

(gets more negative, originally -100 mV, it

goes to -150 or -180 mV), i.e., membrane

is hyperpolarized.

Triggers inward-specific K+ channels to

open. K+diffuses in down its electrochemical

gradient.

K+concentration can increase from 100

mM to 400 - 800 mM

Cl- also diffuses in to balance positive

charge of K+

. Guard cells also make malate2-to

balance the K+and lower the pH.

Accumulation of ions makes water

potential (w) of guard cells more negative.

Water enters cells, moving down water

potential gradient, causing guard cells toswell.

Stomata open due to changes in volume of

guard cells.

Radial arrangement of cellulose

microfibrils in guard cell walls forces

pore to open when guard cells swell.

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(b ) S tom ata l c losure Closure initiated by:

Shutting down proton pump.

Opening anion (like Cl-) channels,

allowing anions to flow out.

This dissipates much of the membrane

potential (charge difference becomes

less negative, going back to original

membrane potential of -100 mV)

Inward-specific K+channels close,

outward-specific K+ channels open.

K+diffuses out of cell, again down its

electrochemical potential.

Water potential in guard cells becomes

less negative.

Water flows out of cells & they shrink.

Reduced volume of guard cells causes

stomatal pore to collapse shut.

http://www.esf.edu/efb/course/EFB530/lectures/stomata.htm

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Factors (cues) that trigger stomatal

opening:

(i) Blue-light receptors in guard cells

stimulate activity of ATP-powered proton

pumps in plasma membrane, promoting

uptake of K+.

(ii) Depletion of CO2 within air spaces of leaf

as photosynthesis begins.

(iii) Circadian rhythm - internal clock

located in guard cells that regulate

cyclic processes.

Factors that trigger stomatal closing:

(i) Darkness.

(ii) High internal CO2 concentration.

(iii) Abscisic acid - produced by mesophyll

cells in response to water deficiency/

stress.

(iv) Circadian rhythm.

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