Lecture #17

Respiration and Gas Exchange

Partial Pressure

• each gas in a mixture of gases exerts its own pressure = partial pressure – partial pressures denoted as “p”– applies to gases in air and gases dissolved in liquids

• total pressure is sum of all partial pressures– atmospheric pressure (760 mm Hg) = pO2 + pCO2 +

pN2 + pH2O– to determine partial pressure of O2-- multiply 760 by

% of air that is O2 (21%) = 160 mm Hg

Respiratory Media• respiratory media – either air or water• conditions for gas exchange depend on this media

– air is less dense and easier to move over respiratory surfaces – it is easy to breathe air – but humans only extract 25% of the O2 out of the air they breathe

• O2 is plentiful in air – is always 21% of the earth’s atmosphere by volume

• gas exchange from water is much more demanding– amount of O2 dissolved in water varies with the conditions of the

water• warmer and saltier – less O2

– but it is always less than what is found in air• 40 times more O2 in air than in water!!

– water is also more dense and viscous – requires considerably more energy to move over the respiratory surface

Respiratory Surfaces• ventilation = movement of the respiratory medium

over the respiratory surface• O2 and CO2 exchange is by diffusion and occurs

across a moist surface• rate of diffusion determined by three things:

– 1. surface area– 2. thickness of respiratory membrane (e.g. alveolar

wall + capillary wall)– 3. diffusion coefficient – CO2 20X higher vs. O2– i.e. diffusion is faster when the area for diffusion is

large and the distance is short

Respiratory Surfaces• simple animals – every cell is close enough to the external

environment – gases diffuse quickly across the body surface– sponges, cnidarians and flatworms

• some animals have modified their skin to act as a respiratory organ – dense network of capillaries below the surface– earthworms and some amphibians like frogs

• however this is not true for larger animals – development of more complex structures like gills and lungs

• fish gas exchange– to exchange enough O2 – fish must pass large

quantities of water across the gill surface– water flows in the mouth and out the

operculum (slit-like opening in the body wall)– flows over the gills– most fishes have a pumping mechanism to

move water into the mouth and pharynx and out through the opercula

– some elasmobranchs and open ocean bony fishes (e.g. tuna) – keep their mouth open during swimming – ram ventilation

– gills are supported by gill arches – contain larger arteries and veins (branchial artery and vein)

– 2 gill filaments extend from each arch and are made up of plates called lamellae

– each lamella contains extensive capillary beds

Gills

Gillarch

OperculumWaterflow

BloodvesselsGill arch

Gill filaments

O2-poor blood

Water flowBlood flow

Lamella

O2-rich blood

– gas exchange across the lamellae – countercurrent or parallel exchange depending on the fish• parallel exchange – the blood flows in the same direction as the water

through the gills– exchange will stop once the difference between water and blood O2 levels

disappears• countercurrent exchange – the blood and water flow in opposite

directions– there always exists a small gradient so that oxygen flows into the blood from

the water

Counter-current exchange

Parallel exchange

• amphibian gas exchange:– requires a moist surface– skin can function as a respiratory organ through cutaneous

respiration• the majority of its total respiration

– gas exchange also occurs along the moist surfaces of the mouth and pharynx – buccopharyngeal respiration

• amphibian gas exchange:– contribution of cutaneous and buccopharyngeal

respiration to total gas exchange is relatively constant• so their rate cannot be increased if metabolic rate goes up• an alternate means of increasing respiration is required

– so amphibians also possess lungs• pulmonary ventilation occurs through a buccal pump mechanism• muscles of the mouth and pharynx create a positive pressure to

force air into the lungs

Tracheal System of Insects• the most common terrestrial respiratory system• air tubes that branch throughout the body

– largest tubes are called tracheae – open to the outside– branch into smaller tubes = tracheoles – deliver air directly to the cells of the tissues

• passive movement of air into the tracheae and diffusion brings in enough O2 to support cellular respiration

• larger insects with higher energy requirements – must ventilate air and out of the tracheae – through body movements produced by muscles

Tracheae

Air sacs

External opening

Trachea

Airsac Tracheole

Bodycell

Air

Terrestrial Animals & the Lung

• lungs are localized, regional respiratory organs• subdivided into numerous lobes, lobules and broncho-pulmonary

segments• these divisions are supplied by a series of branching tubes• lungs are supplied by the circulatory system – blood comes from

the right side of the heart• the amphibian lung is quite small – most respiration is done by the

skin• most reptiles, all birds and all mammals – respiration done lungs

The Lung

• Primary bronchi supply each lung• Secondary bronchi supply each lobe of the lungs (3 right + 2 left)• Tertiary bronchi splits into successive sets of intralobular bronchioles that supply each

bronchopulmonary segment ( right = 10, left = 8)• IL bronchioles split into Terminal bronchioles -> these split into Respiratory

Bronchioles• each RB splits into multiple alveolar ducts which end in an alveolar sac

The Alveolus

• Respiratory bronchioles branch into multiple alveolar ducts

• alveolar ducts end in a grape-like cluster = alveolar sac

• each grape = alveolus Pharynx

Larynx(Esophagus)

Trachea

Right lung

Bronchus

Bronchiole

Diaphragm

(Heart)

Capillaries

Leftlung

Dense capillary bedenveloping alveoli (SEM)

50 m

Alveoli

Branch ofpulmonary artery(oxygen-poorblood)

Branch ofpulmonary vein(oxygen-richblood)

Terminalbronchiole

Nasalcavity

Alveolus• one cell thick - site of gas exchange by simple diffusion• surrounded by a capillary bed fed by a pulmonary arteriole and collected by

a pulmonary venule• deoxygenated blood flows over the alveolus picks up O2 and the oxygenated

blood leaves the alveolus -> heart• Type I alveolar cells: simple squamous cells where gas exchange occurs• Type II alveolar cells (septal cells): secrete alveolar fluid containing

surfactant• Alveolar dust cells: wandering macrophages remove debris

Ventilation & Breathing• ventilation = movement of the respiratory medium over

the respiratory surface• amphibians – use positive pressure breathing

– inflate their lungs by forcing air into them• mammals – use negative pressure breathing

– change the volume of the lungs to either increase or decrease air pressure within it – moves the air in and out

• birds – unique mechanism involving negative pressure breathing

• respiratory system is designed to be efficient and to provide the flight muscles with enough oxygen

• external nares located in the bill – draws air in – eventually enters into the bronchii

• bronchi connect to air sacs that occupy much of the body & to the lungs

• lung does not contain alveoli – but contains parabronchii – tiny channels for gas exchange

• inspiration and expiration results from increasing and decreasing the volume of the thorax and from the expansion and compression of the air sacs

• bird actually uses two rounds of inhalation/exhalation to move a volume of air through its respiratory system

Anteriorair sacs

Posteriorair sacs

Lungs

1 mm

Airflow

Air tubes(parabronchi)in lung

Anteriorair sacs

Lungs

Second inhalationFirst inhalation

Posteriorair sacs 3

24

1

4

31

2 Second exhalationFirst exhalation

Birds

• 1st inhalation – air moves into the posterior/abdominal air sacs

• 1st exhalation – posterior air sac contracts – forces air into the lungs for additional gas exchange

• 2nd inhalation – air passes from the lungs into the anterior air sacs; new air moves into the posterior air sacs

• 2nd exhalation – anterior air sacs contract and air moves out of body; posterior air sacs contract and a new volume of air moves in to lung

• due to this arrangement – birds have a near continuous movement of O2 rich air over the respiratory surfaces of the lungs

Anteriorair sacs

Posteriorair sacs

Lungs

1 mm

Airflow

Air tubes(parabronchi)in lung

Anteriorair sacs

Lungs

Second inhalationFirst inhalation

Posteriorair sacs 3

24

1

4

31

2 Second exhalationFirst exhalation

Birds

Mammalian Breathing

• to understand mammalian ventilation - must understand the physical relationship between the lungs and the thoracic cavity

• Pleural cavity is potential space between ribs & lungs– the lungs do not fill the entire pleural cavity– pressure of air inside the lungs is greater than the pressure in the pleural cavity

• lungs and thoracic cavity are lined with membranes– Visceral pleura covers lungs – Parietal pleura lines ribcage & covers upper surface of diaphragm

Respiratory pressures

• two different pressures need to be considered– 1. atmospheric (barometric) pressure

• caused by the weight of air on objects on the Earth’s surface– 2. intrapulmonary (intra-alveolar) pressure

• pressure within the lungs (within each alveolus)• when not ventilating – pressure of air inside the lungs = pressure of air outside the

body• ventilation happens because of a pressure gradient between AP and IP

Mammalian Ventilation: Boyle’s law• Inhalation - the diaphragm drops and

the rib cage swings up and out – the thoracic cavity increases in volume

• fluid adhesion holds the visceral and parietal pleural membranes together

• so when the parietal the movement of the thoracic cavity “pulls” the lungs with it

• this expands the lungs in volume – air pressure in the lung (i.e. IP) drops below atmosphere (i.e. AP)

Boyle’s law: As the size of closed container decreases, pressure inside is increasedAs the size of a closed container increases, pressure decreases

Rib cageexpands.

Airinhaled.

Lung

Diaphragm

Mammalian Ventilation: Boyle’s law

• Exhalation – the diaphragm comes back up and the rib cage swings back down – the thoracic cavity decreases in volume

• PLUS – elastic recoil of the lung tissue decreases volume

• lung volume decreases and the air pressure within the lungs increases vs. atmospheric

• air moves out to equilibrate Airexhaled.

Rib cage getssmaller.

Mammalian Ventilation: Boyle’s law

• additional muscles can be used to increase and decrease the volume of the thoracic cavity more than normal

• other animals use the rhythmic movement of organs in their abdomen to increase breathing volumes

Airexhaled.

Rib cage getssmaller.

• inspiratory capacity (IC) = max. amnt of air taken in after a normal exhalation, 3500 ml • vital capacity = max. amnt of air capable of inhaling,IRV + TV + ERV = 4600 ml• total lung capacity = VC + RV = 6000ml

Respiratory Volumes and Capacities

• (TV) = amnt of air that enters or exits the lungs 500 ml per inhalation

• functional residual capacity =

ERV + RV, 2300 ml

• inspiratory reserve volume(IRV) = IC + TV, 3000 ml

• residual volume (RV) = amnt of air left in lungs after forced expiration = 1200 ml

• expiratory reserve volume (ERV) = amnt of air forcefully

exhaled, 1100 ml

Control of Breathing• controlled by three clusters of

neurons that make up the Respiratory Center

• 1. medullary rhythmicity area – in the medulla oblongata– controls the rate and depth of

breathing• 2. pneumotaxic area – in the pons

– shortens the breath

• 3. apneustic area – in the pons– prolongs the breath

• detects changes in the pH of the CSF surrounding the brain

CO2 is the major determinant for breathing rate

• the major determinant of CSF pH is the blood’s pH

• the major determinant of blood pH is the dissolution of CO2 into the plasma

• CO2 combines with the water of the plasma to create carbonic acid

• carbonic acid dissociates into H+ ions (pH) and bicarbonate ions (HCO3-)

Figure 42.29

Homeostasis:Blood pH of about 7.4

CO2 level

decreases. Stimulus:Rising level ofCO2 in tissues

lowers blood pH.Response:Rib musclesand diaphragmincrease rateand depth ofventilation.

Carotidarteries

AortaSensor/control center:Cerebrospinal fluid

Medullaoblongata

neurons incarotid andaortic arch sensedrop in bloodpH

medulla detects drop in CSF pH

Respiratory pigments• CO2 dissolves in the water of the plasma• but O2 dissolves poorly in plasma

– reduces the amount of O2 that the blood can carry• so there is the need for a respiratory pigment to bind oxygen• hemocyanin – respiratory pigment of molluscs, arthopods,

annelids– has copper as it’s oxygen binding element

• hemoglobin used by most other animals– uses iron to bind oxygen– acts as an “oxygen sponge”– allows for the transport of significant amounts of O2 in the blood

Hemoglobin

• comprised of 4 proteins called globin • each globin has a heme group• each heme group has an iron-containing pigment at its

core• each iron atom binds one O2 molecule

– as one heme binds one O2 – the other three increase their affinity for their O2 “partners”

– as one heme releases its O2 – the other three lose their affinity for their O2

• so each Hb can carry four O2 molecules

Hemoglobin & O2

2

(a) PO and hemoglobin dissociation at pH 7.4

Tissues duringexercise

Tissuesat rest

Lungs

PO (mm Hg)2

0 20 40 60 80 1000

20

40

60

80

100

O2 unloaded

to tissuesduring exercise

O2 unloaded

to tissuesat rest

O2 s

atu

rati

on

of

he

mo

glo

bin

(%

)

(b) pH and hemoglobin dissociation

PO (mm Hg)2

0 20 40 60 80 1000

20

40

60

80

100

Hemoglobinretains lessO2 at lower pH

(higher CO2

concentration)

pH 7.2pH 7.4

O2

satu

rati

on

of

hem

og

lob

in (

%)

Bohr shift: low pH decreases the affinity of Hb for O2

CO2 transport• CO2 produced by tissue cells & diffuses into the

plasma • over 90% of CO2 then diffuses into the RBC• some CO2 combines with Hb• most CO2 reacts with the cytosol inside the RBC to

form carbonic acid – catalyzed by the enzyme carbonic anhydrase

• dissociation of carbonic acid into H+ and HCO3-• Hb binds the H+ ions and prevents the Bohr shift• most of the HCO3- diffuses out of the RBC into the

plasma• in the lungs – Hb releases the H+ ion – it combines

with the HCO3- to reform carbonic acid• carbonic acid breaks up into H2O and CO2; CO2 is

released by Hb• CO2 diffuses into the alveolar air

Body tissue

Capillarywall

Interstitialfluid

Plasmawithin capillary

CO2 transport

from tissuesCO2 produced

CO2

CO2

CO2

H2O

H2CO3 HbRedbloodcell Carbonic

acid

Hemoglobin (Hb)picks up

CO2 and H+.

H+HCO3

Bicarbonate

HCO3

HCO3

To lungs

CO2 transport

to lungs

HCO3

H2CO3

H2O

CO2

H+

HbHemoglobin

releases

CO2 and H+.

CO2

CO2

CO2

Alveolar space in lung

Diving Mammals• humans can hold their breath for no more than 3 minutes• seals – can dive to 200-500m and can hold their breath for

close to 20 minutes• some whales can reach depths of 1500m and stay submerged

for close to 2 hours• evolutionary adaptations:

– 1. ability to store large amounts of O2 in their muscle mass– 2. adaptations to conserve O2 – little effort to swim and their

buoyancy allows them to change depths easily– 3. regulatory mechanisms routes blood to the brain, spinal cord, eyes,

adrenal glands – shut off in other areas during a dive