Lecture 17
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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

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Lecture #17

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Lecture 17

Lecture #17

Respiration and Gas Exchange


Partial pressure

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

  • 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 O2in air than in water!!

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


Respiratory surfaces

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 surfaces1

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


Lecture 17

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 lamellacontains extensive capillary beds

O2-poor blood

Gills

O2-rich blood

Gill

arch

Lamella

Blood

vessels

Gill arch

Water

flow

Operculum

Water flow

Blood flow

Gill filaments


Lecture 17

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


Lecture 17

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 – buccopharyngealrespiration


Lecture 17

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

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

Body

cell

Tracheae

Air

sac

Tracheole

Air sacs

Trachea

Air

External opening


Terrestrial animals the lung

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

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 intralobularbronchioles 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

The Alveolus

Branch of

pulmonary vein

(oxygen-rich

blood)

Branch of

pulmonary artery

(oxygen-poor

blood)

  • Respiratory bronchioles branch into multiple alveolar ducts

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

  • each grape = alveolus

Terminal

bronchiole

Nasal

cavity

Pharynx

Left

lung

Larynx

Alveoli

(Esophagus)

50 m

Trachea

Right lung

Capillaries

Bronchus

Bronchiole

Diaphragm

(Heart)

Dense capillary bed

enveloping alveoli (SEM)


Alveolus

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 & 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


Lecture 17

Birds

respiratory system is designed to be efficient and to provide theflight muscles with enough oxygen

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

bronchi connect to air sacsthat 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

Anterior

air sacs

Posterior

air sacs

Lungs

Airflow

Air tubes

(parabronchi)

in lung

1 mm

Lungs

Anterior

air sacs

Posterior

air sacs

3

2

4

1

First inhalation

Second inhalation

1

3

First exhalation

Second exhalation

4

2


Lecture 17

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

Anterior

air sacs

Posterior

air sacs

Lungs

Airflow

Air tubes

(parabronchi)

in lung

1 mm

Lungs

Anterior

air sacs

Posterior

air sacs

3

2

4

1

First inhalation

Second inhalation

1

3

First exhalation

Second exhalation

4

2


Mammalian breathing

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

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

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)

Air

inhaled.

Rib cage

expands.

Lung

Diaphragm

Boyle’s law:

As the size of closed container decreases, pressure inside is increased

As the size of a closed container increases, pressure decreases


Mammalian ventilation boyle s law1

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 theair pressure within the lungs increases vs. atmospheric

  • air moves out to equilibrate

Air

exhaled.

Rib cage gets

smaller.


Mammalian ventilation boyle s law2

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

Air

exhaled.

Rib cage gets

smaller.


Lecture 17

Respiratory Volumes and Capacities

  • 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

  • (TV) = amnt of air that enters or exits the lungs

  • 500 ml per inhalation

  • 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

  • functional residual capacity =

  • ERV + RV,2300 ml


Control of breathing

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

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

Figure 42.29

CO2 level

decreases.

Stimulus:

Rising level of

CO2 in tissues

lowers blood pH.

Response:

Rib muscles

and diaphragm

increase rate

and depth of

ventilation.

medulla detects drop in CSF pH

neurons in

carotid and

aortic arch sense

drop in blood

pH

Carotid

arteries

Aorta

Sensor/control center:

Cerebrospinal fluid

Medulla

oblongata


Respiratory pigments

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

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

PO (mm Hg)

2

Hemoglobin & O2

100

100

O2 unloaded

to tissues

at rest

pH 7.4

80

80

pH 7.2

O2 unloaded

to tissues

during exercise

60

Hemoglobin

retains less

O2 at lower pH

(higher CO2

concentration)

O2 saturation of hemoglobin (%)

60

O2 saturation of hemoglobin (%)

40

40

20

20

0

0

20

40

60

80

100

0

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

Tissues

at rest

Tissues during

exercise

Lungs

0

20

40

60

80

100

PO (mm Hg)

2

(b) pH and hemoglobin dissociation

(a) PO and hemoglobin dissociation at pH 7.4

2


Co2 transport

Body tissue

CO2 transport

from tissues

CO2 transport

CO2 produced

Interstitial

fluid

CO2

Plasma

within capillary

CO2

Capillary

wall

  • 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

CO2

H2O

Hemoglobin (Hb)

picks up

CO2 and H+.

Red

blood

cell

H2CO3

Hb

Carbonic

acid

H+

HCO3

Bicarbonate

HCO3

To lungs

CO2 transport

to lungs

HCO3

H+

HCO3

Hemoglobin

releases

CO2 and H+.

Hb

H2CO3

H2O

CO2

CO2

CO2

CO2

Alveolar space in lung


Diving mammals

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


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