Download Chapter 44 Gas Exchange and Circulation

Chapter 44 Gas Exchange and Circulation
Important points…
•  Animals take in oxygen and expel carbon dioxide to
sustain cellular respiration.
•  Terrestrial animals and aquatic animals face
different challenges in performing gas exchange.
•  Gas exchange organs maximize the rate of oxygen
and carbon dioxide diffusion by presenting a large,
thin surface area to the environment.
•  Gas exchange organs maintain a steep partialpressure gradient that favors entry of oxygen and
elimination of carbon dioxide.
44.1 Air and Water as Respiratory Media
•  Air or Water is the medium. The medium contains oxygen & carbon
dioxide
•  Ventilation of gas exchanger (=breathing; but confused by use of term
“respiration rate” when correct would be “ventilation rate”)
•  Gas exchange at a surface – interface between medium & gas exchanger
•  “Respiration” is equivalent to oxidative metabolism
(and CO2)
Does Freeman need a geography lesson ;)
Sea level barometric pressure is 760 mmHg.
Since oxygen is 21% of air, the partial pressure of oxygen is
0.21 x 760 = 160 mmHg.
The amount of oxygen and carbon dioxide in water
varies also.
The source of gases in water is the atmosphere
(mostly), and gas diffuses from the atmosphere into
water at the air/water surface.
Factor 1 à surface/depth ratio
Factor 2 à mixing
AIR AIR AIR AIR
AIR
Deep water,
relatively small
surface area,
expect O2 & CO2
gradient with little at
bottom.
Shallow water,
relatively large
surface area, most
of water will have
O2 & CO2.
FACTOR 2: MIXING
Gas exchange in water is much more difficult than in
air.
Oxygen solubility in water is much
less:
O2 in air
100-130 ml O2/liter of air
O2 in water
0-10 ml O2/liter of water.
(But CO2 is 20X more soluble in water than O2 – this
is why elaborate aqueous transport mechanisms
evolved for O2 only, not for CO2.)
Aquatic: Just move more of the medium past your
gas exchanger to compensate for the reduced
amount of O2 in water?
…oops: water is 1000X more dense than air and
100X more viscous!!!!!
A terrestrial animal may use 1 – 2% of its energy
budget to ventilate its gas exchanger.
A fish may use 20% of its energy budget for
ventilation!
Warm water holds less O2 than cold water and salt
water holds less than fresh water.
So… tropical salt water fish have the greatest
burden.
Fick’s law controls the rate of diffusion
Diffusion occurs across the
medium ß à gas exchanger epithelium
This is the interface between:
water and gill (aquatic gas exchange organ)
or
air and lung (terrestrial gas exchange organ)
There are many adaptations
for ventilating gill gas
exchangers.
Counter-current
exchanger in fish
gills.
Two basic strategies in fish for ventilating gills:
l 
Ram jet ventilation
l 
Buccal pumping
Extreme ram jet ventilators: some species of
sharks. They will suffocate if they stop swimming.
Extreme buccal pumpers: sit-and-wait predators.
How the
buccal
pump
works.
Insect terrestrial gas exchange -Insects are relatively small, have relatively lower
metabolic rates. After invading land ‘discover’
that oxygen is relatively abundant.
So… a simple system of internal tubing, with
spiracles opening to the outside, is sufficient to
keep open circulatory system (hemocoel)
perfused with oxygen.
This system is limiting, though, and has
prevented larger (modern) insects from evolving.
Hmm, “some sort of
breathing mechanism”.
What could it be?
Of course you would jump
right to the hypothesis that
flying is the answer. Not!
Note: No antagonist muscle: exoskeleton
provides restoring force à faster operation
How about vertebrates invading land?
Amphibians are the first to do so…
Amphibians inherit the fish buccal pump – replace oral
valve with a nasal valve!
Buccal pumping uses positive pressure to inflate lungs.
Amphibians also use highly vascularized dorsal skin as
a gas exchange organ.
Reptile invention:
Replace positive pressure system with a
negative pressure mechanism to inflate
lungs.
Muscles move back liver, this expands the
body cavity, and creates a vacuum.
From reptiles…
•  Birds evolve a unique ventilatory mechanism
•  Mammals expand on reptile system:
l 
l 
Divide body cavity into abdominal and
thoracic sections (diaphragm separates the
two)
Use intercostal muscles and diaphragm to
expand thoracic cavity and create negative
pressure.
First mammals, then birds…
Basic mammalian lung plan
More detail next slide
?
Note extremely
thin wall - this
improves the
speed of
diffusion
Surface tension of the water film
on the inside of alveolus tries to
collapse the alveolus. This forces
out air when muscles, expanding
the body cavity, relax.
From amphibians on, surfactant is
produced in lungs to control water
surface tension. The surface tension
is potentially so great it would cause
alveolar collapse and no reasonable
positive or negative pressure could
inflate lungs.
The surfactant produced in human lungs is
very effective.
Normal water surface tension is 70 mN/m.
Laundry detergent reduces this to 25 mN/m
Lung surfactant reduces it to 1 mN/m !!
Specialized Bird Lungs
Ventilation of the Bird Lung
(Caudal sacs)
(Cranial sacs)
1
Cycle 1 inhalation 1:
New air fills caudal sacs.
2
3
Cycle 2 inhalation 2:
Expansion of anterior sac
pulls new air across and
out of lungs
4
Cycle 1 exhalation 1:
New air forced out of
caudal sacs into lungs.
Cycle 2 exhalation 2:
Contraction of cranial sacs
forces “new” air out of
bronchus.
Each cycle is similar but valves control flow direction.
Flow always same direction across lungs (not tidal)
Cross-current exchange improves performance.
Recent research alert!
Alligators and Crocodiles – sister groups to
birds – and large Varanids also have one-way
lung airflow!!
Elaborate
branching patterns
in lungs.
Use probe to
measure airflow
during artificial
inhalation and
exhalation.
Crocs & Gators
Birds
One way flow in monitor lizards too
CIRCULATION
CIRCULATION - topics
1. Blood cells & gas transport
2. Circulatory systems
l  Blood Vessels
l  Hearts
3. Control of circulation
After diffusion at lungs, oxygen carried by
hemoglobin (Hb), and hemoglobin
contained within red blood cells.
Less diffusion distance without RBCs, but
the high concentration of Hb free in blood
would create a huge osmotic pressure
problem!
Sigmoidal curve
“cooperativity”
Due to interaction of peptide chains in hemoglobin (Hb)
One (1) Hb
molecule
Four (4)
polypetide chains
One polypeptide chain:
l  150 aa globin
l  One heme group
3+ atom
l  Contains one Fe
l  Binds 1 O molecule
2
So four (4) O2 molecules bound per Hb molecule
Extra delivery due to
sigmoidal function
Increased oxygen delivery to tissues if there is a
greater demand.
How do tissues express their increased need?
Reduced oxygen means slower
mitochondrial function
Build up of lactate and pyruvate
Drop in pH
Due to the higher solubility of carbon
dioxide, no special transport proteins
needed.
LUNGS
TISSUES
Blood Pumping
Don’t memorize all the numbers, just
understand the concepts: where is the PO2
higher, lower etc.
Many animals have
hearts, not just
vertebrates.
There may be blood
vessels, but also
large open spaces
where blood flows à
lower pressure
system
ß This minimizes the diffusion
distance
Note: one cell layer only,
thus only very small
hydrostatic pressures
are tolerated
Variable-width gaps between endothelial cells
allow some plasma to escape
Is there diffusion of gas here?
Thick walls of arteries tolerate very high
pressures.
A portion of plasma forced out under hydrostatic
pressure, returns via osmotic pressure.
Pressure drop
Hearts need a
pressurizing
element and at
least one valve to
prevent backflow.
Muscle/tendon provide constant small
tension to keep valves open during filling
phase.
Hearts & circulation pattern increase in
complexity with increased aerobic needs
SV = SINUS VENOSUS
CA = CONUS ARTERIOSUS
Improved performance by total separation of
pulmonary and systemic circuits.
Too complicated – just learn next slide!
Heart Contraction sequence
Atria contract – move about 20% of the
blood that will ultimately be ejected to
systemic circulation, into ventricles.
Ventricles contract à pressure builds
à A-V valves forced closed à
pressure builds à ventricle pressure
exceeds artery backpressure à
semilunar valves open
Systolic pressure
during ejection
Diastolic pressure
during filling
The cardiac cycle as a loop, independent of time.
Abbreviations:
AVVC = atrioventricular valve
closure
AVVO = atrioventricular valve
opening
EDV = end-diastolic volume
ESV = end-systolic volume
IVVC = Isovolumic ventricular
contraction
IVVR = isovolumic ventricular
relaxation
SLVC = semi-lunar valve
closure
Note that TIME is not on the graph!
SLVO = semi-lunar valve
opening.
Stroke Volume (SV) is the amount of blood
ejected on each beat.
CO = S.V. x H.R.
Where S.V. = stroke volume,
H.R. = heart rate
Starling’s law – heart pumps all the blood
that fills it… so only control over CO is HR!
The mean pressure is estimated like this:
M.A.P. = 0.67*DP + 0.33*SP
Suppose DP = 60 mm Hg, SP = 120 mm Hg.
M.A.P. = .67*60 + .33*120 = 80 mm Hg
(60 mmHg is about the minimum to sufficiently
perfuse all tissues in humans)
Why is M.A.P. not just the average of DP and
SP?
Starling:
•  As resting muscle is stretched, the
tension increases exponentially so…
•  …increasing venous return to the heart
stretches the ventricle, which in turn
results in more forceful ejection of
blood at the very next heart beat.
Typical intrinsic heart rate is 70 beats per
minute.
Typical stroke volume is 70 mls
So… typical CO is 70 x .07 = 5 liters per
minute.
Human blood volume (all of it) is 5 liters, so
your entire blood volume circulates every
minute!
Stroke volume is less than the total volume filled.
SV = 60-70 mls
Healthy human ejection fraction = 55 – 70%
So 30+ % remains in heart each stroke.
Weakened heart, early pathology EF = 40 – 55%
Heart failure < 40%
How the heart
synchronizes
contraction
Regulation of Blood Pressure
and Blood Flow
Pressure = F/A. For
given force of
contraction, as x.s.
area increases,
pressure decreases
Flow = vA.
Flow must
stay the
same, so as
x.s. area
increases
velocity decr.
Blood shunting is critical because vascular
volume much greater than blood volume.
Too much vasodilation à blood pressure drops
to zero!!!
Why do you feel faint (perhaps) when you
stand up quickly (or raise head up quickly)?
Shunting takes time and gravity drains blood
from brain in less time!
How do giraffes do it?
Giraffe up
Giraffe down
1.  Heart weighs 25 lbs (vs ¾ lb for human) &
has much thicker walls.
2.  Jugular vein (drains brain) has a valve that
blocks outflow due to gravity.
3.  Fast blood shunting of blood in head away
from facial muscles, tongue, etc and just to
brain.
Blood shunting particularly important for diving
mammals (but humans have diving reflex too)
For mammals adapted for diving, the change in
heart rate can be dramatic.
Fur seals go from 120 to 18 beats per minute!
By closing off all unnecessary capillary beds, mean
arterial pressure remains high even with low C.O.
M.A.P. = C.O. * total peripheral resistance
So as heart rate drops, C.O. drops, but TPR
goes up.
Human clinical hypertension control
1. Based on controlling salt
Normal: Eat extra salt (any prepared food!) à drink more
à increase blood volume à increase blood pressure. Don’t
care if BP not too much.
Hypertensive: Give diuretics à dump waterà lower blood
volume à lower BP
Normal: Eat extra salt à renin mech. conserve H20 à
hypervolemic à higher BP. Don’t care if BP not too much.
Hypertensive: Remove salt in diet à renin system can’t
work, reduces blood volume à decr. BP
Human clinical hypertension control
2. Based on preventing BP increase
Normal: If low BP à renin à angiotensin II à pump more
salt out of filtrate à drag more water out of filtrate à incr.
blood pressure.
Hypertensive: Block angiotensin converting enzyme (ACE
inhibitor) à block mech. to increase BP à keep BP low.
Normal: Stress, etc à adrenalin release à binds to β type
“adrenergic” receptors on heart à increase contraction
strength & rate à incr. BP.
Hypertensive: Take β-blocker à prevent incr. in rate à
decr. BP
Congestive Heart Failure -- 1. Early stage
Decr. heart
pumping
strength
Decr. arterial
pressure
Neg. Feedback control
loop: Restores pressure
BUT there will be excess
blood volume and excess
tissue fluid.
Incr. arterial
pressure
Activate renin/
angiotensin,
aldosterone,
sympathetic n.s.
Sodium
retention
water
retention
Congestive Heart Failure -- 2. Late stage
Much decr.
heart pumping
strength
Decr. arterial
pressure
Little/no urine flow, edema
due to water retention.
Heart stress greater due
to greater blood volume.
Can’t incr. arterial
pressure – heart too
weak
Activate renin/
angiotensin,
aldosterone,
sympathetic n.s.
Sodium
retention
water
retention
Congestive Heart Failure -- 3. End stage
Much decr.
heart pumping
strength
Positive feedback: Ever
increasing load on heart
à imbalance of O2 need
vs O2 delivery
à successive minor heart
attacks
à  weaker heart
à more blood volume
Hypervolemia
Lung fluid
build-up
Incr. diffusion
distance
Less O2
Incr. load
on heart
Incr.
heart O2
demand