# Respiratory System

```Respiratory Systems
• What is a respiratory system? How does it work?
• What are the functions of respiratory systems?
• What are the different respiratory strategies that animals
use?
Definitions
Respiration – sequence of events that result in the
exchange of oxygen and carbon dioxide between
the external environment and the mitochondria
External respiration – gas exchange at the
respiratory surface
Internal respiration – gas exchange at the tissues
Mitochondrial respiration – production of ATP via
oxidation of carbohydrates, amino acids, or fatty
acids. Oxygen is consumed and carbon dioxide is
produced
 Gas molecules move down concentration gradients
Mitochondrial respiration
Mitochondria
consume O2 to
produce ATP
Produce CO2 in
process
Organisms must
have mechanisms
to obtain O2 from
the environment
and get rid of CO2
→ External
respiration
Respiratory strategies of animals
• 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
Respiratory systems - physics
Diffusion
Diffusion is the movement
of molecules from a high
concentration to a low
concentration
• Slow over long distances
• Fast over short distances
Respiratory systems - diffusion
The Fick equation
J = rate of diffusion (moles/sec)
D = diffusion coefficient
A = area of the membrane
dx = diffusion distance
For gases, we usually use partial pressure rather than concentration
Respiratory systems - diffusion
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
For gases, we usually use partial pressure rather than concentration
Gas Pressure
• 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
P- total pressure; V- Volume; n – number of moles of gas
molecules; R – gas constant (8.314472 J · K-1 · mol-1)
T – temperature in Kelvin
Gas Pressure cont.
• 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
Gases Dissolve in liquids
 Gas molecules in air must first dissolve in liquid
(water or extra-cellular fluid) in order to diffuse into a
cell
Henry’s law: [G] = Pgas x Sgas
Gases Dissolve in liquids
CO2 is much more soluble in water than is O2. Thus, at
the same partial pressure, more CO2will be dissolved in a
solution than will oxygen
Diffusion Rates
Graham’s law
The relative diffusion of a given gas is
proportional to its solubility in the liquid and
inversely proportional to the square root of its
molecular weight:
Diffusion rate  solubility/MW
•
•
•
•
•
O2 32 atomic mass units
CO2 44 amu
In air “solubilities” are the same (1000 ml/L at 20oC)
Oxygen diffuses about 1.2 times faster than CO2
However, CO2 is about 24 times more soluble in aqueous
solutions than O2. So CO2 diffuses about 20 times faster than
O2 in water
Diffusion Rates at a constant temperature
Combining the Fick equation with Henry’s and
Graham’s laws:
Diffusion rate  dPgas x A x Sgas / X x (MW)
At a constant temperature the rate of diffusion is
proportional to
•Cross-sectional area (A)
•Solubility of the gas in the fluid (Sgas)
And inversely proportional to
•Diffusion distance (X)
•Molecular weight of the gas (MW)
Fluid Movement: Bulk flow
• Bulk flow: Mass movement of water or air as the result of
• Fluids flow from areas of high to low pressure
• Boyle’s Law: P1V1 = P2V2
Temperature and the number of gas molecules remain constant
Bulk flow and Boyle’s law
P1V1 = P2V2
P1V1 = P2V2
P2V2
P1 = P2
P2
P1 = P2 V2
Respiratory systems use changes in volume to cause
changes in pressure!
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 strategies of animals
• 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
Respiratory Strategies
Animals 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
Circulating the external medium through the body
Parazoa and Cnidaria
Circulating the external medium through the body
Tracheal system
Series of narrow tubes leading from surface to deep within body
Gases move in the tubes via a combination of diffusion and bulk flow
Cricket spiracle
Most animals have a circulatory system
• Diffusion of gases across a specialized respiratory
surface accompanied by circulatory transport
O2
O2
External Respiratory
medium
surface
Circulatory
system
Tissue
Cutaneous respiration
Respiration through skin
Found in some aquatic invertebrates and a few vertebrates
Conflict between respiration and protection
Salamander
Annelid
Lake Titicaca frog
External gills
Gills originate as outpocketings (evaginations)
• Advantages: high surface area, exposed to
medium
• Disadvantages: easily damaged, not suitable in
air
Salamander
Polychaete
Internal gills
• Advantages: High surface area, protected
• Disadvantages: not usually suitable in air
Lungs
Originate as infoldings (invaginations)
• Advantages: High surface area, protected, suitable
for breathing air
• Disadvantages: not suitable in water
Ventilation
 The active movement of the respiratory medium (air or
water) across the respiratory surface
 Ventilation of respiratory surfaces reduces the formation
of static boundary layers i.e. improves efficiency of gas
exchange
Types 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
Nondirectional ventilation
Medium flows past the respiratory surface in an unpredictable pattern
PO2 = 160mmHg
O2
40 60 80 100 120 140 160 160
Flow
Effect of increased boundary layer
Tidal ventilation
Medium moves in and out
PO2 = 160mmHg
Lung
Blood vessel
PO2
100mmHg 100 100
40 40
60
Flow
95
80
90
Gas exchange – unidirectional ventilation
Medium enters the chamber at one point and exits at another
With unidirectional ventilation, the blood can flow in three ways
relative to the flow of the medium
Medium
Resp. surface
Cocurrent flow
Blood
Medium
Countercurrent
flow
Resp. surface
Blood
Medium
Resp. surface
Crosscurrent flow
Blood
Orientation of Medium and Blood Flow
PO2 of medium and
blood will equilibrate
Orientation of Medium and Blood Flow
PO2 of blood
approaches that of the
inhalant medium
Orientation of Medium and Blood Flow
Found in birds
Initial-parabronchial (PI; ) and End-parabronchial values
(PE). Mixed venous (Pv) blood; Arterial blood (Pa).
The PO2 of arterial blood is derived from a mixture of all
serial air-blood capillary units and exceeds that of PE.
Ventilation and Gas Exchange
Because 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
Ventilation in water- invertebrates
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
Molluscs
Two 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 (squid)
Crustaceans
• Barnacles (filter feeding) 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
copepod
Echinoderms – sea stars, sea urchins, sea cucumbers
• Most sea stars and sea
urchins use their tube feet
for gas exchange
• Water is sucked in and
exits through the
• Sea stars also have
external gill-like structures
(respiratory papulae); cilia
move water over the
surface
Echinoderms, cont.
• Brittle stars and sea
cucumbers have internal
invaginations
• Brittle stars used cilia to
move water into bursae
• Sea cucumbers use
muscular contractions of
the cloaca and the
respiratory tree to
breathe water tidally
though the anus
Jawless Fishes - Hagfish
Lamprey and hagfish have multiple pairs of gill sacs
Hagfish
• A muscular pump (velum) propels water through the respiratory cavity
• Water enters the median nostril!!!! and leaves through a gill opening
• Flow is unidirectional
• Blood flow is countercurrent
Ventilation - hagfish
Gills sacs are arranged for counter current flow
Water flow
Jawless Fishes - Lamprey
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
Elasmobranchs – sharks, skates and rays
Steps 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 Fishes
Water flows in via the mouth, out via the opercular opening
Teleost Fishes
V
P
Fish Gills
Fish gills are arranged for countercurrent flow
Ventilation and Gas Exchange in Air
Two major lineages have colonized terrestrial
habitats
• Vertebrates
• Arthropods (eeeekkkk)
• Unidirectional ventilation of gills common in
water-breathing animals
• Tidal ventilation of lungs common in airbreathing animals
Ventilation in air
Molluscs
Insects
Spiders
Vertebrates
Ventilation – terrestrial molluscs
•
•
•
•
The “pulmonate” molluscs lack gills (or have highly
reduced gills
Instead, mantle cavity is highly vascularized and acts as
a lung
Garden snails and terrestrial slugs are pulmonates
Pumping of the mantle cavity moves air in and out of
these lungs
Arthropods (eeeeeeek)
• Crustaceans (crabs, woodlice and sowbugs)
• Chelicerates (spiders and scorpions)
• 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
• Movements of the gill bailer propels air in and out of the
branchial chamber
Terrestrial isopods (woodlice and sowbugs)
• Have a thick layer of chitin on one side of the gill for support and
the other side is thin walled and used for gas exchange
• Anterior gills contain air-filled tubules (pseudotrachea). Oxygen
diffuses down the pseudotrachea and dissolves in the interstitial
fluid
Chelicerates - Spiders and scorpions
Have 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
• Oxygen diffuses into the
trachea and dissolves in
the interstitial fluid before
diffusing into the tissues
Insects
• Have an extensive tracheal system - series of air-filled tubes
• Tracheoles – terminating ends of tubes that are filled with
hemolymph
• Oxygen dissolves in the hemolymph
• Open to outside via spiracles
• Gases diffuse in and out
• High diffusion coefficient of oxygen in air allows oxygen to
diffuse through the tracheal system
Ventilation in insects
• Some species can
expand and compress
the trachea
• Changes in tracheal
volume cause changes
in pressure, which
causes air to flow
through the system
Images of insect trachea obtained
Insect Ventilation
3 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
Ventilation in 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
Aquatic insects
• Most aquatic
insects
breathe air
• Mosquito
larvae have
“snorkel”
Hydrofuge hairs
How insects
keep their
snorkels dry
Water beetle (Dytiscus)
• Water beetles
carry scuba tanks
(air bubbles)
Air breathing vertebrates
Air breathing evolved in fishes
Aquatic habitats can become hypoxic
Under these conditions, the ability to breathe air
is a substantial benefit
Vertebrates
•
•
•
•
•
Fish
Amphibians
Reptiles
Birds
Mammals
Evolution of air breathing
Some fish use “aquatic surface respiration”
when hypoxic
Swim to the surface and ventilate gills with
water from the thin well-oxygenated water layer
near surface
Some fish can gulp air into mouth (buccal
cavity)
Buccal cavity highly vascularized for gas
exchange
Fish
Air breathing has evolved multiple times in fishes
Types of respiratory structures
•
•
•
•
•
Reinforced gills that do not collapse in air
Mouth or pharyngeal cavity for gas exchange (highly vascularized)
Vascularized stomach
Specialized pockets of the gut
Lungs
Ventilation is tidal using buccal force similar to other fish
Amphibians - ventilation
• Amphibians
have simple
sac-like lungs
• Form as
outpocketings
of the gut
Amphibians
Types of respiratory structures
• Cutaneous respiration
• External gills
• Simple bilobed lungs; more complex in terrestrial frogs
Ventilation is tidal using a buccal force pump
Amphibians – external gills
Polychaete
Salamander
surface area, exposed
to medium
damaged, not suitable
in air
Amphibians
Reptiles
* Most have two lungs; in snakes one lung is reduced or
absent
* Can 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: mechanisms to change the volume
•Snakes and lizards: use intercostal muscles.
Contraction of the intercostals moves the ribs
forward and outward, increasing the volume
•Turtles and tortoises: Use abdominal muscles that
expand and compress the lungs
•Crocodilians: Hepatic septum is attached to the
anterior side of the liver. Paired diaphramaticus
muscles run from the hepatic septum to the pelvic
girdle. Diaphramaticus muscles contract which
decreases the volume in the abdominal cavity and
increases the volume of the lungs. As a result
pressure in the lungs decreases
Reptiles
Ventilation in birds and mammals
• Birds use unidirectional
ventilation
• Mammals use tidal
ventilation
Birds
• Lung is stiff and
changes little in
volume
• Rely on a series of
flexible air sacs
• Gas exchange
occurs at
parabronchi
Bird lungs – crosscurrent flow
Oxygen extraction efficiency high (up to 90%)
Bird Ventilation
Requires two cycles of inhalation and
exhalation
Air flow across the respiratory surfaces is
unidirectional
Bird Ventilation
Mammals
Two 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
Mammalian lungs
Mammalian lungs - alveoli
Type I cells
gas exchange
Type II cells
surfactant
secretion
Mammalian lungs
Airways:
Larynx
Trachea
Bronchii
Bronchioles
Alveoli
Its Friday 
Physical Properties of the Lungs
Compliance:
• Distensibility (stretchability):
• Ease with which the lungs can expand.
• 100 x more distensible than a balloon.
• Compliance is reduced by factors that produce resistance
to distension.
Elasticity:
• High content of elastin proteins.
• Very elastic and resist distension.
• Recoil ability.
Physical Properties of the Lungs
•Pulmonary ventilation involves different pressures:
• Atmospheric pressure
• Transpulmonary pressure
• Intraalveolar (intrapulmonary) pressure
• Intrapleural pressure
• Atmospheric pressure is the pressure of the air outside the body.
•Transpulmonary pressure is the pressure difference across the wall of
the lung. Keeps the lungs against chest wall.
• Intraalveolar pressure is the pressure inside the alveoli of the lungs.
•Intrapleural pressure is the pressure within the pleural cavity.
Pressure is negative, due to lack of air in the intrapleural space
Insert fig. 16.15
Lungs, pleura, and chest wall
Airway resistance
• Flow = DP/R
• If resistance increases, a greater DP
is needed to maintain the same flow
• Airway resistance is inversely
proportional to airway radius to the
4th power (1/r4)
• Bronchoconstriction – reduction in
• Bronchodilation – increase in radius
Bronchoconstriction and Bronchodilation
Bronchoconstriction:
stimulation of parasympathetic nervous system
Histamine
Irritants
Bronchodilation:
stimulation of sympathetic nervous system
Circulating epinephrine
(binds to beta-2 receptors)
High alveolar PCO2
Mammal Ventilation
Tidal ventilation
Steps
• 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
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
Mammals
Mammalian lungs - ventilation
Air moves into and out of
the lungs along pressure
of volume changes
Surfactants
• Surfactants – reduce surface tension by
disrupting the cohesive forces between
water molecules
• Results in an increase in lung compliance
and a decrease in the force needed to
inflate the lungs
• In humans, surfactant synthesis does not
begin until late gestation
Tidal volume – total volume of air
moved in one ventilatory cycle
Dead space – air that does not
participate in gas exchange
• Two components
volume of the trachea and
bronchi
volume of any alveoli that is
not being perfused with
blood
Spirometry
Method for measuring pulmonary function
Lung Volumes and Capacities
Emphysema
• In emphysema, the
walls of the alveoli
break down
• Increases lung
compliance, but
reduces lung
elastance
Gas Transport
• Sponges, cnidarians, and insects circulate
external fluid past almost every cell in their
bodies and can rely on diffusion
• Larger animals use circulatory systems
Ventilation-perfusion matching
• Ventilation of the respiratory surface must
be matched to the perfusion of the
respiratory surface
• VA/Q quantifies this. Should be close to 1
• Humans: VA ~ 4-5 L/min and Q ~ 5L/min
Gas transport
Bulk flow
Ventilation
Diffusion
Bulk flow
Circulation
Diffusion
Gas transport
Diffusion
Bulk flow
Ventilation
Bulk flow
Circulation
Diffusion
Oxygen Transport
• Solubility of oxygen in aqueous fluids is
low
• Metalloproteins contain metal ions which
reversibly bind to oxygen and increase
oxygen carrying capacity by 50-fold
• By binding oxygen to carriers, PO2 in the
blood remains low and results in improved
oxygen extraction
Amount of oxygen that can dissolve in plasma is limited at
physiological PO2 (Henry’s Law [G] = Pgas * Sgas)
Respiratory pigments
• Respiratory pigments help to increase the amount of O2 in
blood
• Oxygen-binding molecules
• Contain metal ions
• Gives them a strong colour (e.g. hemoglobin – red)
• Oxygen binds reversibly to the metal ion
• Bind to the pigment at the lungs
• Releases from the pigment at the tissues
Types of respiratory pigment
• Hemoglobin - vertebrates,
nematodes, some annelids, some
crustaceans, some insects
• Hemerythrin - sipunculids, priapulids,
brachiopods, and one family of
annelids
• Hemocyanin - arthropods and
molluscs
Respiratory Pigments
Metalloproteins are referred to as respiratory pigments
Three major types
• Hemoglobins
• Most common
• Vertebrates, nematodes, some annelids, crustaceans, and
insects
• Consist of a protein globin bound to a heme molecule
containing iron
• Usually located within blood cells
• Appears red when oxygenated
• Myoglobin is a type of hemoglobin found in muscles
• Hemocyanins
•
•
•
•
Arthropods and molluscs
Contain copper
Usually dissolved in the hemolymph
Appears blue when oxygenated
• Hemerythrins
•
•
•
•
Sipunculids, priapulids, brachiopods, some annelids
Contains iron directly bound to the protein
Usually found inside coelomic cells
Appears violet-pink when oxygenated
Hemoglobin
•
•
•
•
•
•
Vertebrate hemoglobins are tetramers
Two alpha chains
Two beta chains
Each contains a heme group
Each heme group can bind 1 molecule of oxygen
Therefore 1 Hb molecule can bind 4 oxygen molecules
Myoglobin (Mb)
• Type of hemoglobin
found in vertebrate
muscle
• Monomer
• Each Mb molecule
binds one molecule
of oxygen
Hemocyanin
• Arthropods & molluscs
• Contain copper instead of iron
• Copper is complexed directly to amino acids in the
protein
• Multimeric (up to 48 subunits)
• Blue when oxygenated
Hemerythrins
• Sipunculids, priapulids, brachiopods, and one family of
annelids
• Do NOT contain heme
• Iron is bound directly to amino acids in the protein
subunits (usually 2 iron molecules per subunit)
• Molecules are usually trimeric or octomeric
• Very pretty violet colour when oxygenated, colorless
when deoxygenated
Oxygen carrying capacity of blood
• Carrying capacity = the maximum amount of oxygen that
can be carried in blood
• Total O2 in blood = dissolved O2 + O2 bound to
respiratory pigment
• Increased amount of respiratory pigment = increased
capacity for carrying oxygen
Oxygen carrying capacity of blood
• Because of the low
solubility of oxygen in
aqueous solutions, only
a small amount of
oxygen can dissolve in
blood
• PO2 is equal in plasma
and lungs, but oxygen
content of plasma is
much lower
Oxygen carrying capacity of blood
• If an oxygen carrier
such as hemoglobin
is present, some of
the oxygen will bind
to the pigment
• This oxygen no
longer contributes to
PO2
• PO2 is the same as in
the previous
example, but oxygen
content is higher
Oxygen carrying capacity of blood
• At low environmental PO2,
the PO2 of the plasma is low
• Less oxygen dissolves in
plasma
• The amount of oxygen
bound to the pigment may
also decrease somewhat (if
the plasma PO2 is low
enough)
• But, the total oxygen
content of the blood is still
higher than if no pigment
were present
Oxygen Equilibrium Curves
• Shows the relationship
between partial pressure of
oxygen in the plasma and
the percentage of
oxygenated respiratory
pigment in a volume of
blood
• As partial pressure
increases, more and more
pigment molecules will bind
oxygen, until the saturation
point
• P50 – oxygen partial
pressure at which the
pigment is 50% saturated
Oxygen Equilibrium Curves
• Obey the law of mass
action
• Hb + O2 HbO2
• If oxygen concentration
increases, reaction
shifts to the right
• If oxygen concentration
decreases, reaction
shifts to the left
Shapes of Oxygen Equilibrium Curves
• Can be either hyperbolic or
sigmoidal
• Each molecule of myoglobin
binds oxygen independently
and therefore has a hyperbolic
shape
• Hemoglobin exhibits a
sigmoidal curve because of
cooperativity – hemoglobin has
a higher affinity for oxygen
when more of its heme groups
are bound to oxygen
Oxygen Equilibrium Curves
Log[Y/(1-Y)] vs. Log(PO2)
Shapes of Oxygen Equilibrium Curves
Factors that can affect the shape of the oxygen
equilibrium curve
• Molecular structure of the respiratory pigment
• Environmental factors such as pH, CO2,
allosteric modifiers, temperature
Structure of the pigment
Fetal hemoglobin has a lower P50 than maternal hemoglobin
Structure of the pigment
• Myoglobin has a
lower P50 than
Hemoglobin
• Myoglobin has
higher oxygen
affinity
• Also note the
difference in the
shapes of the curve
• The hemoglobin
curve is sigmoidal
• Myoglobin curve is
hyperbolic
Conditions That Affect Oxygen Affinity
pH and PCO2
• Bohr effect or shift – a decrease in pH or increase in
PCO2 reduces oxygen affinity - right shift
• This facilitates oxygen transport to active tissues and
facilitates oxygen binding at the respiratory surfaces
Conditions That Affect Oxygen Affinity
Temperature
• Increases in temperature decrease oxygen affinity;
right shift
• Promotes oxygen delivery during exercise
Organic modulators (e.g., 2,3-DPG, ATP, GTP)
• Increases in these modulators decrease oxygen
affinity; right shift
Carbon monoxide and Hb
• Carbon monoxide is a byproduct of
combustion
• Carbon monoxide (CO) can bind to
hemoglobin
• CO has 250 times the affinity for
hemoglobin than O2
• Hb becomes 100% saturated with CO at
PCO = 0.6 mm Hg
• Hb becomes 100% saturated with O2 at
PO2 = 600 mm Hg
Conditions That Affect Oxygen Affinity
pH and PCO2
•Root effect – a Bohr effect with a reduction in the oxygen carrying
capacity
• Seen in hemoglobin of many teleost fishes
• Helps in oxygen delivery to eye and swim bladder
• Fish
• Many bony fish have a swimbladder that helps
to maintain neutral buoyancy
• Gas-filled sac
• Fill with gas to increase buoyancy
• Remove gas to reduce buoyancy
• In most species this gas is oxygen
• Gas gland excretes lactic
acid
• Acidity causes hemoglobin of
the blood to lose its oxygen
• Oxygen diffuses into the
through a complex structure
known as the rete mirabile
Carbon Dioxide Transport
• Carbon dioxide is more soluble in body
fluids than oxygen
• However, little CO2 is transported in the plasma
• Some CO2 binds to proteins
(carbaminohemoglobin)
• Most CO2 is transported as bicarbonate
• CO2 + H2O  H2CO3 (carbonic acid) 
HCO3- (bicarbonate) + H+
• Carbonic anhydrase catalyzes the formation of
HCO3-
Carbon Dioxide Equilibrium Curve
• Shows the
relationship between
PCO2 and the total
CO2 content of the
blood
• The shape of the
curve depends on
the kinetics of HCO3formation
• Deoxygenated blood
can carry more CO2
than oxygenated
blood (Haldane
effect)
Haldane effect
• Removal of oxygen
from hemoglobin
increases
hemoglobin’s affinity
for carbon dioxide
• Allows CO2 to be
carried bound to
hemoglobin
Vertebrate Red Blood Cells and CO2 Transport
• Carbonic anhydrase is located within RBCs
• Reactions to synthesize HCO3- occur in the
RBCs even though most of this HCO3- is carried
in the plasma
Carbon dioxide transport – at tissues
•
•
•
•
•
•
•
•
CO2 is produced by aerobic metabolism
Rapidly diffuses out of tissues and into red cell
Carbonic anhydrase catalyzes formation of bicarbonate
the H+ formed by this reaction binds to Hb
the bicarbonate ions are moved out the the RBC by a transporter protein
(band III)
Bicarbonate does not readily diffuse through membranes
if it were not removed, build up within red
cell would inhibit CA reaction
Band III exchanges HCO3- for Cl- (Chloride shift)
Carbon dioxide transport – at respiratory surface
• PCO2 of air/water is lower than blood
• CO2 diffuses out of plasma across respiratory
surface
• CO2 diffuses out of RBC into plasma
• Equilibrium of CO2-bicarbonate reaction is shifted
• Bicarbonate ions move from plasma into RBCs
(reverse-chloride shift)
• Bicarbonate and H+ from carbonic acid and then
CO2
• CO2 diffuses out of RBC into plasma and
then across respiratory surface
Regulation of Respiratory Systems
• Respiratory systems are closely regulated
• Respond to changes in external and internal environment
• Must be able to supply sufficient oxygen to meet metabolic
demands
• Must be able to remove carbon dioxide to prevent pH
disturbance
Vertebrate respiratory and circulatory
systems work together to regulate gas
delivery by
• Regulating ventilation
• Altering oxygen carrying capacity and affinity
• Altering perfusion
pH homeostasis
Buffer systems:
Buffer: moderates changes but does not prevent changes in pH
Proteins, Phosphate Ions and bicarbonate
Lungs:
Via respiratory compensation
Kidneys:
Use ammonia and phosphate buffers
Acid Base disturbances – Metabolic acidosis
Disturbance
Acidosis
Metabolic
H+
pH


HCO3-

CO2 + H2O  H2CO3  HCO3 +  H
-
+
Step by step:
Causes include lactic acid accumulation, ketoacids (from breakdown
of fats or amino acids ). Can also be due to loss of bicarbonate
1.
2.
3.
4.
Hydrogen concentration increases, pH decreases
Equilibrium shifts to the left
HCO3- buffer is used up and CO2 increases
Response of body: CO2 can be blown off at the lungs.
Also, renal compensation
Acid Base disturbances – Respiratory acidosis
Disturbance
Acidosis
Respiratory
H+
pH


HCO3-

 CO2 + H2O  H2CO3  HCO3- + H+
Step by step:
Hypoventilation results in an increase of carbon dioxide and elevated
PCO2.
1.
2.
3.
4.
5.
Plasma CO2 levels increase; H+ increases
pH decreases
Equilibrium shifts to the right
Hydrogen and bicarbonate concentrations increase
Response of body: Renal compensation
Acid Base disturbances – Metabolic alkalosis
Disturbance
Alkalosis
Metabolic
H+
pH


HCO3-

CO2 + H2O  H2CO3  HCO3 + H
-
+
Step by step:
Loss of protons due to vomiting of acid stomach contents orexcessive
Intake of bicarbonate-containing antacids
1. H+ decreases
2. pH increases
3. Equilibrium shifts to the right, CO2 decreases and bicarbonate goes
up
4. Response of body: Adjust respiration and renal compensation
Acid Base disturbances- Respiratory alkalosis
Disturbance
Alkalosis
Respiratory
H+
pH


HCO3-

CO2 + H2O  H2CO3  HCO3- + H+
Step by step:
Due to hyperventilation.
1. CO2 decreases, pH increases
2. Equilibrium shifts to the left, protons and bicarbonate decrease
3. Response of body: Renal compensation
Responses to Acid-base disturbances
Metabolic Acidosis:
1. Hyperventilate (blow off CO2) PCO2 decreases
2. Renal System - Secretion of H+ and reabsorption of
bicarbonate
Respiratory Acidosis:
1. Renal System: Secretion of H+ and reabsorption of
bicarbonate
Metabolic Alkalosis:
1. Hypoventilation PCO2 increases. Corrects pH problem but
increases bicarbonate (only short term)
2. Renal System: bicarbonate excreted and H+ reabsorbed
Respiratory Alkalosis:
1. Renal System: bicarbonate excreted and H+ reabsorbed
Regulation of Ventilation
• Rhythmic firing of
central pattern
generators within the
medulla initiate
ventilatory movements
• Pre-Botzinger complex
is an important
respiratory rhythm
generator in mammals
Ventilation is automatic
• Ventilation is an
automatic process
• Continues even when
we are unconscious
• Central pattern
generator in medulla
• Exact location within
the medulla varies
among species
Regulation of Ventilation
• Chemosensory input helps
modulate the output of the
central pattern generators
• Chemoreceptors detect
changes in CO2, H+, and O2
• Oxygen is the primary
regulator in water-breathers
while CO2 is the primary
regulator in air-breathers
Regulation of ventilation
• Rhythm generation neurons
of the preBotzinger complex
send output via motor
neurons
• Motor neurons innervate
intercostal muscles,
diaphragm, and abdominal
muscles
• Cause muscle contraction,
resulting in either inspiration
or expiration
Regulation of ventilation
• Ascending sensory input comes from chemosensory neurons
in carotid and aortic bodies, in vasculature of lungs, and
chemoreceptors in the medulla
• Modulates the rate and depth of breathing
• Negative feedback loop to maintain blood PO2 and PCO2
within a narrow range
Regulation of ventilation
Homeostatic feedback
loop for the regulation of
ventilation
Carotid and aortic chemoreceptors
Glomus cells = chemosensory cells
Mechanisms in carotid body
• Glomus cells contain oxygengated K+ channels
• Oxygen sensor detects low
PO2
• Closes K+ channels
• Cell depolarizes
• Causes release of dopamine
• Stimulates sensory neuron
Mechanisms in central chemoreceptors
• Most important
controllers in
mammals
• Sensitive to changes
in PCO2 and pH
• CO2 crosses
blood/brain barrier
• Carbonic anhydrase
converts CO2 to
HCO3- and H+
• H+ stimulates receptor
• Stimulates ventilation
Chemoreceptor reflex
Chemoreceptor reflex
•Most of the response
is mediated by central
chemoreceptors
•Increased PCO2
stimulates increased
ventilation
Irritant receptors/stretch receptors
• Trigger bronchoconstriction
• Trigger coughing
• Trigger sneezing
Irritant receptors/stretch receptors
• Hering-Breuer inflation reflex
• Reduces ventilation when
lungs are over-inflated
• Usually only experienced
during/after intense exercise
Regulation of ventilation in water breathers
• Water-breathers such as fish are typically more
sensitive to PO2 than PCO2
• Decreases in PO2 increase ventilation
• Most species are sensitive to blood PO2
• A few species are thought to be sensitive to
environmental PO2
Environmental Hypoxia
• Hypoxia – lower than normal levels of
oxygen
• Can be caused by environmental hypoxia,
hemoglobin content
• Hyper-, hypocapnia – higher or lower than
normal levels of CO2
High Altitude Physiology
High altitude
8,000 - 12,000 feet
[2,438 - 3,658 m]
Very high altitude
12,000 - 18,000 feet
[3,658 - 5,487m]
Extremely high altitude
18,000+ feet
(Everest 29,000ft)
[5,500m]
Oxygen at Altitude
• Pressure declines as altitude increases
• Oxygen delivery to body dependent on partial pressure of
oxygen
• Concentration of oxygen doesn’t change but you get less
oxygen per breath
• At 12,000ft you get ~40% less oxygen per breath
Physiological Response to Altitude
Moderate Altitude – initial symptoms
•
•
•
•
•
•
Nausea
Fatigue
Loss of appetite
Difficulty sleeping
Frequent Urination
Physiological Response to Altitude
High Altitude – initial symptoms
•
•
•
•
•
•
Confusion
Reduced mental acuity
Loss of coordination
Cerebral edema
Pulmonary edema
death
Response to Altitude - Respiratory
* Low inspired oxygen (PaO2 low)
* CO2 production normal
• Arterial chemoreceptors sense low O2
• Increase rate and depth of breathing
Response to Altitude - Respiratory
*
*
*
*
Hypocapnia
Reduces drive to breathe
Intermittent breathing (especially at night)
Causes difficulty sleeping
O2 Dissociation Curve
• Respiratory alkalosis shifts curve to left
• Increased DPG shifts curve to right
Acclimatization to Altitude
Problems:
* Hypoxia
* Secondary hypocapnia
•
•
•
Increased EPO
Increased hematocrit
Peripheral vasodilation
Acclimatization to Altitude
*
*
Hypoxia
Secondary hypocapnia
•
•
•
•
Hypoxia persists
Therefore hyperventilation continues
Respiratory alkalosis
Compensate by increasing renal excretion of
HCO3-
Pulmonary Blood Flow
•
•
•
•
•
•
Inspired air low pO2
Sensors in lung detect low oxygen
Causes vasoconstriction in lung
Reduces blood flow
Increases blood pressure
High-Altitude Hypoxia
Llamas
• Moderate hematocrit
• High affinity Hb
• Very small RBCs
Deer Mice
Live at a variety of altitudes
Deer Mice
High Altitude mice have reduced DPG
• Migrates over Everest
• Hb sequence differs by 1bp from all other geese
• Greatly increases O2 affinity
Physiology of Diving
Physiology of Diving
Boyle’s Law
P1V1 = P2V2
• As pressure increases, volume decreases
• As we dive, pressure increases
Breath-hold Diving
“The dive response”
• Apnea
• Peripheral vasoconstriction
• Redistribution of cardiac output
• Limit breath-hold diving ~1 minute for humans
•
•
•
•
•
•
Modified body form
Increased oxygen stores
Ability to modify blood distribution
Ability to collapse lung
Regional hypothermia
Ability to buffer CO2
Limits to diving
Oxygen stores
Exercise Physiology
Initiation of muscle contraction
•
•
•
•
•
Nervous signal initiates muscle contraction
ACh binds to receptor on motor end plate
Causes an action potential
Causes release of Ca2+ from SR
Initiates contraction
Fueling muscle contraction
•
•
•
•
ATP
Phosphocreatine
Carbohydrates
Lipids (and proteins)
Exercise intensity and duration
• For short duration high-intensity exercise can use
anaerobic pathways with PCr and carbohydrates as a
fuel
• Results in lactate production
• For longer duration exercise must switch to aerobic
pathways and lipids as a fuel
• This requires oxygen
• Blood flow to the exercising muscle must increase
Active hyperemia
• Increased muscle oxygen
consumption
• Causes decrease in local oxygen
concentration
• Release of local vasodilatory
factors (e.g. nitric oxide)
• Causes vasodilation
• Decreases resistance in arteriole
• Causes increased flow
• Increases supply of oxygen to
working muscles
Blood flow during exercise
• Misconception: Blood flow to brain increases during exercise
• This is incorrect: Global cerebral blood flow to the brain is constant,
approximately 750 ml/min, regardless of mental or physical activity
• At rest: brain gets ca. 15% of total cardiac output per beat
• As exercise intensity increases, cardiac output is redistributed (mostly
to muscles) and the brain receives a lower percentage per beat.
• At maximal intensity, the brain gets ca 4% of cardiac output per beat
• However, this reduction is precisely offset by the overall increase in
total cardiac output (the heart beats ca 4 times faster), resulting in a
• Global oxygen and glucose uptake is also constant
Cardiac output
CO = HR x SV
• In untrained individuals CO increases from
5L/min - 20L/min; trained athletes up to
~35L/min during exercise
• Contribution of both heart rate and stroke
volume
Blood flow
•
•
•
•
•
Sympathetic nervous system causes generalized vasoconstriction
Reactive hyperemia causes increased flow to skeletal muscles
Net effect is a decrease in total peripheral resistance
Cardiac output (total blood flow) increases greatly
So what happens to blood pressure?
Blood pressure
MAP = CO x TPR
Ventilation during exercise
Exercise oxygen consumption
Ventilation during exercise
Have a great weekend!
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