Gas Exchange

Part I

Respiration –

taking up O2 giving up CO2

Photosynthesis –

taking up CO2, giving up O2.

Respiratorymedium(air or water)

Organismallevel

Cellular level

Energy-richfuel molecules

from food

Respiratorysurface

Circulatory system

Cellular respiration

CO2O2

ATP

What is diffusion?

epswww.unm.edu/.../eps462/graphics/diffusion.gif

Depends on partial pressure, surface area

A gas always diffuses from an area of high partial pressure to low partial pressure.

What is equilibrium?

Partial pressure of gases: pressure exerted by a particular gas in a mixture of gases.

We need to know: Pressure that is exerted by mixture Fraction of mixture represented by the particular

gas Atmosphere is 21% by volume O2. At sea level

atmospheric pressure is 760mm Hg. PO2 is 760mm Hg X 0.21 = 160mm Hg

What happens in water?

Amount of gas dissolved in water is proportional to partial pressure in air solubility in water.

At equilibrium partial pressure of a gas in air (PO2 of 160mm Hg) = partial pressure of that gas in solution (PO2 of 160mm Hg)

Concentration of a gas depends on the solubility of the gas. Solubility decreases with increase of temperature

and dissolved solids. Concentration of O2 [O2] is about 40 times more in

air than water.

Comparison of the two respiratory media:

  Air Water

density less more

viscosity less more

[O2] higher lower

Aquatic animals have had to evolve very effective and efficient gas exchange strategies.

Respiratory surfaces are plasma membranes which must be moist. Gas exchange takes place by diffusion.

Rate of diffusion is directly proportional to the surface area across

which it occurs inversely proportional to the square of the

distance the molecules have to travel. To speed up the rate of diffusion, respiratory

surfaces have to be LARGE and THIN.

In unicellular and simple animals diffusion occurs between all cells and environment.

If body surface is enough then skin can be a respiratory organ. Earthworm – surface is moist, supplied

richly by capillaries

LE 42-3

A closed circulatory system.

Auxiliary hearts Ventral vessels

Dorsal vessel(main heart)

If body surface area is insufficient – need for specialized respiratory organs

Larger animals have respiratory organs consisting of respiratory surfaces and other structures.

Size of respiratory surface depends on Size of organism Metabolic demands

To accommodate large respiratory surfaces inside the body – Folded Branced

Examples: gills, trachea, lungs

Gills: outfoldings of the body that are suspended in water; surface area much larger than the rest of the body.

There are a large variety of gills

Gill

Parapodia

Marine wormMarine Worm

LE 42-20d

Gills

CrayfishCrayfish

LE 42-20a

Gills

Coelom

Tube foot

Sea starSea Star

Gillarch

Waterflow Operculum

Gillarch

Bloodvessel

Oxygen-richblood

Water flowover lamellaeshowing % O2

Gillfilaments

O2

Oxygen-poorblood

Lamella

15%40%

70%

100%

90%

60%

30% 5%

Blood flowthrough capillariesin lamellaeshowing % O2

Countercurrent exchange

Ventilation: movement of respiratory medium over respiratory surface.

Promoted by moving the gills moving water over the gills swimming

Countercurrent exchange: exchange of substance between two fluids (blood and water) flowing in opposite directions and thereby maximizing gas exchange efficiency (about 80%)

Gills are unsuitable for land: water supports the filaments and keep them

separate gills would dry up

Tracheal systems: Most common respiratory structure. Consists of: Large tubes (trachea – supported by chitin rings) branch into… Smaller tubes, tracheoles (fluid at terminal end); bring enough O2

to the tissues and removes enough CO2 from the tissues. Air sacs: supply air to organs with higher O2 needs.

Air sacs Tracheae

Spiracle

Bodycell

TracheoleAirsac

Trachea

Air Body wall

MyofibrilsTracheoles Mitochondria

2.5 µm

O2 demand can go up during flight by up to 200X.

The demand is met by: Contraction and relaxation of the flight muscles

pumps air through the tracheal system

Flight muscles rich in mitochondria.

Withdrawal of fluid from tracheole into body increases surface area.

Lungs: localized respiratory organs; inflodings of the body surface separated

consisting of numerous small pockets.

Circulatory system transports O2 to the body from the lungs and CO2 from the body to the lungs

Most reptiles, all birds and mammals use lungs for gas exchange

Amphibians and some reptiles (turtles) supplement lungs with parts of their skin.

Some aquatic animals (lungfishes) use lungs for gas exchange

  Gills Trachea Lungs

habitat of organisms water land land

involves circulatory system

yes no yes

location in bodyhangs outside

in localized areas

through out the body

localized organs

inside the body

For animals with gills or lungs –

endotherms have greater surface area than ectotherms.

Gas Exchange

Part II

Pathway of air to the gas exchange surface in mammals:

Nasal cavity

Pharynx

Larynx

Glottis (covered by epiglottis during swallowing)

Trachea

Bronchi

Bronchioles Alveoli

Nasalcavity

Leftlung

Heart

Larynx

Pharynx

Esophagus

Trachea

Rightlung

Bronchus

Bronchiole

Diaphragm

Mucus traps dust, beating cilia move the mucus to esophagus

Millions of alveoli in lungs, total area about 100 m2.

Alveoli are surrounded by capillaries.

Surface is coated by moist fluid that helps in gas exchange.

Surfactants keep alveoli from collapsing.

Branchfrompulmonaryvein(oxygen-richblood)

Terminalbronchiole

Branchfrompulmonaryartery(oxygen-poorblood)

Alveoli

50 µ

m

Colorized SEMSEM

50 µ

m

Breathing: process to ventilate lungs.

Amphibian breathing: positive airflow.

Mammalian breathing: negative pressure breathing.

Mammalian breathing During inhalation - expand thoracic cavity, causes

lower air pressure in thoracic chamber, air rushes in; opposite process for exhalation.

Rib muscles, diaphragm, double layered membrane between lungs and thoracic cavity participate.

During exercise muscles of neck, back and chest are also involved.

LE 42-24

Rib cageexpands asrib musclescontract

Airinhaled

Lung

Diaphragm

INHALATIONDiaphragm contracts

(moves down)

Rib cage getssmaller asrib musclesrelax

Airexhaled

EXHALATIONDiaphragm relaxes

(moves up)

Tidal volume: volume of air inhaled and exhaled at each breath (~ 500ml)

Vital capacity: maximum volume of air that a person can exhale after

maximum inhalation, OR maximum volume of air that a person can inhale after

maximum exhalation. 3.4L in college age women, 4.8L in college age men.

decreases with age.

Residual volume: Air that remains after forced exhalation.

Avian breathing:

Ventilation is more efficient and more complex.

Maximum PO2 is higher than that of mammals.

Birds are better adapted to higher altitudes than humans.

Airflow over gas exchange surface is in one direction only

No mixing of fresh and used air. 8 – 9 pairs of air sacs that act as bellows. Parabronchi in the lungs, no alveoli 2 sets of inhalation and exhalation are needed to

completely pass air through the system.

LE 42-25

Anteriorair sacs

LungsPosteriorair sacs

Trachea

Air

Lungs

Air

Air tubes(parabronchi)in lung 1 mm

EXHALATIONAir sacs empty; lungs fill

INHALATIONAir sacs fill

Breathing is controlled (involuntarily) to ensure

Gas exchange coordinates with circulation

Metabolic needs are met

Breathing is controlled by two regions at the base of the brain – pons and medulla oblongata

Breathingcontrolcenters

Cerebrospinalfluid

Medullaoblongata

Pons

During respiration cells produce CO2.

CO2 concentration in blood goes up.

CO2 diffuses from blood to cerebrospinal fluid (CSF).

In CSF

CO2 + H2O H2CO3 HCO3- + H+

Increased metabolic activity (exercise) – [CO2] increases

Results in increase in [H+]

Results in decrease in pH.

pH in CSF is an indicator of blood [CO2]. Decrease in pH is an indicator of increased

[CO2] Decreased pH in cerebropspinal fluid results

in control centers of the brain increasing the rate and depth of breathing.

When CO2 is exhaled, pH increases and breathing is returned to normal.

Breathingcontrolcenters

Cerebrospinalfluid

Medullaoblongata

Pons

Carotidarteries

Aorta

Diaphragm

Rib muscles

CO2 concentration is primarily used to control breathing

O2 concentration influences breathing only when it is very low. Aorta and carotid arteries have O2 sensors which

signal the brain ti increases breathing Increased breathing is always coupled with

increased cardiac output.

Coordination of circulation and gas exchange.

Heart is a dual pump.

Circulatory system is divided into pulmonary circuit and systemic circuit.

Blood with higher PCO2 and lower PO2 comes from the heart to the lungs.

LE 42-5

Anteriorvena cava

Pulmonaryartery

Capillariesof right lung

Aorta

Pulmonaryvein

Right atrium

Right ventricle

Posteriorvena cava

Capillaries ofabdominal organsand hind limbs

Pulmonaryvein

Left ventricle

Left atrium

Aorta

Pulmonaryartery

Capillaries ofhead andforelimbs

Capillariesof left lung

Air in the alveoli has higher PO2 and lower PCO2 than blood in the capillaries.

O2 in the alveoli dissolves in the fluid coating the alveolar epithelium and diffuses into the blood.

CO2 dissolves from blood to the air in the alveoli.

Blood leaving the lungs and going to the heart has higher PO2 and lower PCO2 than the blood entering the lungs.

From the heart the blood goes into the systemic circulation.

In the tissues cellular respiration removes the O2 from the cells and adds CO2.

PO2 is higher and PCO2 is lower in the blood in the tissue capillaries than in the tissues.

O2 diffuses out of the blood and enters the cells and CO2 diffuses out of the cells and enters the blood.

This blood is returned to heart and sent to lungs.

Inhaled air

Bloodenteringalveolar

capillaries

Alveolarepithelialcells

Alveolar spaces

Alveolarcapillaries

of lung

Exhaled air

Bloodleavingalveolar

capillaries

Pulmonaryveins

Pulmonaryarteries

Tissuecapillaries

HeartSystemicveins

Systemicarteries

Bloodleavingtissue

capillaries

Bloodenteringtissue

capillaries

Tissuecells

CO2O2

CO2O2

O2 CO2

CO2O2

< 40 > 45

40 45

CO2O2

100 40

CO2

O 2

CO2O2

40 45

CO2O2

104 40

O2

CO 2

CO2O2

CO2O2

CO2 O2

104 40

120 27160 0.2

Diffusion of O2 in the blood alone is inadequate for meeting metabolic needs.

O2 transport is done to a large degree by respiratory pigments.

During exercise cardiac output is

12.5L of blood per minute with respiratory pigment

555L without the pigment

Respiratory pigments: protein bound to metal, have distinctive color

Hemoglobin: protein and iron (vertebrates)

Hemocyanin: protein and copper (some arthropods and molluscs)

Hemoglobin (Hb): 4 polypeptide subunits each with an iron atom cofactor.

Found in red blood cells.

Polypeptide chain

O2 unloadedin tissues

O2 loadedin lungs

Iron atomHeme group

Functions of hemoglobin:

carries O2,

carries C O2, acts as a buffer in blood

Binds to oxygen reversibly.

Subunits show co-operativity in binding and release.

O2 unloaded fromhemoglobinduring normalmetabolism

O2 reserve that canbe unloaded fromhemoglobin totissues with highmetabolism

PO2 and hemoglobin dissociation at 37°C and pH 7.4P (mm Hg)O2

Tissues duringexercise

Tissues at rest

Lungs

1008060402000

20

40

60

80

100

O2

satu

rati

on

of

hem

og

lob

in (

%)

Cellular respiration increases CO2 production.

CO2 production lowers pH

Lower pH decreases Hb affinity for oxygen. (Bhor shift)

Bohr shift:additional O2 released fromhemoglobin atlower pH(higher CO2

concentration)

pH and hemoglobin dissociation

P (mm Hg)O2

1008060402000

20

40

60

80

100

O2

satu

rati

on

of

hem

og

lob

in (

%)

pH 7.2

pH 7.4

When cellular respiration is higher Hb releases more O2.

CO2 transport.

In solution (7%) Bound to Hb (23%) Bicarbonate (HCO3

-) 70%

CO2 transport from tissues to alveolar space

CO2 transportfrom tissuesCO2 produced

Tissue cell

CO2

CO2

CO2

Interstitialfluid

Blood plasmawithin capillary

Capillarywall

Hemoglobinpicks up

CO2 and H+

CO2 transportto lungs

To lungs

H2CO3

Carbonic acid

H2O

Hb

HCO3–

Bicarbonate

Redbloodcell

H++

HCO3–

HCO3–

Hemoglobinreleases

CO2 and H+

H++HCO3–

CO2

H2CO3

H2O

CO2

CO2

CO2

Hb

Alveolar space in lung

From tissue and interstitial fluid to plasma

Large part (~90%) diffuses into the red blood cells

Some picked up by Hb CO2 and water in red blood

cells react forming carbonic acid

Carbonic acid dissociates into bicarbonate and hydrogen ions.

Hb binds most of the H+; this helps maintain pH, preventing Bhor sift.

CO2 transportfrom tissuesCO2 produced

Tissue cell

CO2

CO2

CO2

Interstitialfluid

Blood plasmawithin capillary

Capillarywall

Hemoglobinpicks up

CO2 and H+

To lungs

H2CO3

Carbonic acid

H2O

Hb

HCO3–

Bicarbonate

Redbloodcell

H++

HCO3–

HCO3- diffuses into the

plasma. At the lungs HCO3

- diffuses back into the red blood cells.

Combines with H+ to form CO2 and water

CO2 is unloaded from Hb. Diffuses from plasma into

interstitial fluid. CO2 diffuses into alveolar

space, exhaled out.

CO2 transportto lungs

To lungs

HCO3–

Hemoglobinreleases

CO2 and H+

H++HCO3–

CO2

H2CO3

H2O

CO2

CO2

CO2

Hb

Alveolar space in lung

Animals like cheetah, pronghorned antelope have been selected enhancement normal physiological mechanisms at every stage of O2 metabolism.

Diving mammals have myoglobin that have higher affinity for O2 than human myoglobin.