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Chapter 42
Circulation and Gas Exchange
Material Exchange
• The exchange of materials from inside
to outside is an important function for
organisms.
• It’s easy for unicellular organisms.
• It becomes more difficult for
multicellular organisms.
• Complex organ systems have evolved
to move materials throughout an
organism.
Diffusion
• Diffusion takes time.
• It is difficult to move substances more
than a few millimeters with diffusion.
• Circulatory systems evolved to
circumvent this problem.
• Bulk movement coupled with diffusion
across small distances.
Invertebrates
• Many animals have a simple body plan
and don’t require a circulatory system.
• These animals have a thin
gastrovascular system (2 cells thick).
• Materials enter and exit through a
single opening and nutrients/wastes
diffuse through the thin cell layer.
Other Animals
• Many animals consist of multiple cell
layers and diffusion becomes
inefficient.
• 2 circulatory systems have evolved in
invertebrates to accommodate this:
• 1. Open
• 2. Closed
1. Open Circulatory Systems
• Invertebrates.
• There is no distinction
between blood and
interstitial fluid.
• It is called hemolymph.
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2. Closed Circulatory
System
• Invertebrates.
• Blood is confined to vessels
and is distinct from
interstitial fluid.
• Both systems have 3 basic
components:
• 1. Circulatory fluid (blood)
• 2. A set of tubes (blood
vessels)
• 3. A muscular pump
(heart)
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The Pump of Circulatory
Systems
• 1. “Single” chambered hearts
• Example: earthworm-peristalsis pumps
blood.
• 2. 2 chambered hearts
• Example: fish
The Pump of Circulatory
Systems
• 3. 3 chambered hearts
• Example: amphibians and reptiles (not
birds), pulmocutaneous circuit picks up
O2, and the heart pumps the blood
through the system.
• 4. 4 chambered hearts
• Example: mammals and birds, a
pulmonary circuit picks up O2 and returns
it to the heart. The heart distributes the
oxygenated blood to the rest of the body.
Circulatory Systems
• The heart acts to increase the
hydrostatic pressure which flows down
a pressure gradient and back to the
heart.
• Blood pressure is the motive force that
moves the fluid through the circulatory
system.
• Both open and closed systems are
widespread throughout the animal
kingdom.
Circulatory Systems
• Both open and closed have their
advantages and disadvantages.
Circulatory Systems--Open
• Open systems have lower hydrostatic
pressure.
• They are less costly in terms of
metabolic energy expenditure for
construction and maintenance.
Circulatory Systems--Closed
• They can achieve a higher blood
pressure.
• They are more effective at transporting
materials.
• They can accommodate larger, more
active animals.
Vertebrate Circulation
• Vertebrates generally have 1 or 2 atria,
and 1 or 2 ventricles.
• It is a closed system.
• Cardiovascular.
• Atria receive blood
• Ventricles pump blood
3 Main Blood Vessel Types
• 1. Arteries--carry
blood away from the
heart.
• 2. Veins--carry blood
to the heart.
• 3. Capillaries--the
spots where arteries
and veins meet and
nutrient exchange
occurs.
Blood Vessel Types
• Networks of capillaries infiltrate each
tissue and exchange many nutrients
with the tissues.
• Arterioles are the sites where small
vessels convey blood to the capillaries.
• At the downstream end, capillaries
converge into venules which converge
into veins.
Different Circulation
Systems
• Single circulation-invertebrates. Blood
or hemolymph passes through one or
more hearts on its way through the
organism.
• Pulmocutaneous circulationamphibians. Blood passes through the
lungs and skin picking up O2 and giving
off CO2.
Different Circulation
Systems
• Double circulation-blood passes
through a 3 (reptiles) or 4 (mammals &
birds) chambered heart twice.
• The first time, as CO2 rich blood.
• The second time as O2 rich blood.
A Pulmocutaneous Circuit
• A pulmocutaneous circuit
leads to capillaries in the
gas exchange organs-lungs and skin.
• Most of the O2 rich blood
is pumped into the
systemic circuit which
supplies O2 to the rest of
the body.
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A Pulmocutaneous Circuit
• There is some mixing
of the O2 rich and CO2
rich blood.
• A ridge in the ventricle
diverts most of the O2
rich blood to the
systemic circuit.
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Double Circulation
• The organization called
double circulation
provides for vigorous
blood flow.
• Blood gets pumped a
second time after
leaving the capillaries.
• Reptiles have double
circulation with a
pulmonary circuit.
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Double Circulation
• Some of these circuits contain
a 3 chambered heart-amphibians, reptiles.
• The ventricle is partially
divided by a septum to reduce
blood mixing.
• Double circulation restores
pressure to the systemic circuit
after blood has passed through
the lung capillaries.
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Double Circulation
• Double circulation
contrasts with single
circulation seen in fish.
• In fish, the blood flows
from the respiratory organs
to the other organs under
pressure.
• In double circulation
pressure is restored to the
systemic circuit after blood
has passed through the
lung capillaries.
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Advantages of a 4
Chambered Heart
• The 4 chambered heart was an essential
adaptation.
• It helps support the endothermy.
• It delivers the high quantity of O2 and fuel
necessary for metabolism.
• It removes wastes produced via metabolism.
• This movement of substances is made possible
by the separate systemic and pulmonary
circuits and the 4 chambered heart.
The 4 Chambered Heart
• Consists mostly of
cardiac muscle.
• It goes through a
series of rhythmic
contractions and
relaxations to move
blood.
• When the chambers
relax they fill; when
they contract, they
empty.
• Systole-contraction
• Diastole-relaxation
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The 4 Chambered Heart
• Ventricles are
usually larger
because they move
more blood.
• The AV valve is
between the atria
and ventricles.
• They prevent the
backflow of blood
during contraction.
• Semilunar valves
prevent backflow of
blood while it’s in
the aorta.
Heart Rhythm
• Maintaining rhythm
is important because
O2 is very important.
• Some cardiac
muscles are selfexcitable and do not
need any input from
the nervous system
to contract.
• The heart’s
pacemaker is the SA
node.
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The SA Node
• It sets the rate and timing of heart
contractions.
• It is located in the wall of the right
atrium and is made of specialized
cells.
• Myogenic hearts have a pacemaking ability
that originates in the heart.
• Neurogenic hearts have contractions that
originate from motor nerves.
The SA Node
• The SA node
generates electrical
impulses similar to
those produced by
nerve cells.
• The cardiac muscle
contains the SA node
that sets the tempo
for the entire heart.
• The impulse is
quickly passed
through the muscles
and passes a relay
point called the AV
node.
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The AV node
• The AV node is
located in the wall
between the right
atrium and right
ventricle.
• The AV node slightly
delays the signal
impulse.
• The delay ensures
that the atria
empty before the
ventricles contract.
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Purkinje Fibers
• These are specialized muscle fibers
that conduct signals to the apex of the
heart and throughout the ventricles.
The Heart
• Although the SA node sets the tempo
for the heart, it is innervated by 2
nerves that affect heart rate.
• One speeds it up.
• The other slows it down.
• Hormones, body temperature, and
physical activity also have an effect on
heart rate.
Blood Vessels
• Arteries and veins.
• They are all built from
similar tissues:
• 1. The outside layer
consists of connective
tissue with elastic
fibers.
• This allows for
functional stretch and
recoil.
Blood Vessels
• 2. The middle layer
consists of smooth
muscle and more
elastic fibers.
• 3. The inner layer is
lined with a smooth
layer of flattened
endothelial cells.
Arteries
• Arteries have thicker
walls and are more
elastic.
• They are for pumping
quickly at high
pressure.
• Their elastic walls
allow for pressure to
remain high even when
the heart isn’t
contracting.
Capillaries
• These are different
from arteries and
veins.
• They lack the 2 outer
layers (connective
tissue and smooth
muscle) and consist of
only endothelial cells.
• This facilitates the
exchange of materials
between blood and
interstitial fluid.
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m
Capillaries
• Generally, the smaller
the diameter of the
pipe, the faster liquid
has to flow.
• Due to the large crosssectional area of
capillaries, blood
moves very slowly
through them.
• Blood pressure is low
at the capillary due to
the resistance
encountered.
Capillaries
• As the blood leaves
the capillary, it
speeds back up as it
enters the vein.
Vein
• Veins have thinner
walls.
• The blood moves
slower through them
and is under low
pressure.
• The movement is
facilitated by muscle
contraction.
• Veins have valves
that prevent the
backflow of blood
down the vein due to
low pressure.
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Veins
• Blood returns to the heart via action of
smooth muscles in the venules and veins.
• Contraction of skeletal muscle greatly
assists movement of the blood.
Blood Flow Through
Capillaries
• There are 2 mechanisms that regulate
the distribution of blood flow through
the capillaries:
• 1. Smooth muscle constricts,
decreasing the diameter of the
arteriole reducing blood flow through
it.
Blood Flow Through
Capillaries
• 2. Precapillary sphincters that control
the flow of blood through the
arterioles and sphincters.
In The Capillary Beds
• Substances move in and out by
diffusion, endocytosis, and exocytosis.
• There are many clefts between the
endothelial cells--diffusion occurs
here.
• Blood pressure is largely responsible
for movement of materials through the
clefts however.
In The Capillary Beds
• Many substances are
too large to cross the
endothelium.
• Osmotic pressure
between the arteriole
and venule remains
constant.
• However, blood
pressure drops
significantly between
the arteriole and
venule.
In The Capillary Beds
• At the arteriole
end, blood
pressure is high
compared to the
osmotic pressure.
• Fluids flow out of
the arteriole and
into the interstitial
fluid.
In The Capillary Beds
• At the venule end, the
blood pressure is low
compared to the
osmotic pressure.
• Fluid flows into the
venule from the
interstitial fluid.
• 85% of the fluid lost
from the arteriole end
is recovered.
• 15% of the fluid is
recovered by the
lymphatic system.
Lymphatic System
• The fluid of the lymphatic
system is called lymph.
• It has roughly the same
composition as interstitial
fluid.
• Lymph capillaries are
interspersed throughout
the cardiovascular
capillaries.
• These assist with the
reabsorption of fluid.
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Lymphatic System
• The lymph vessels are very similar to
veins.
• They have valves that prevent the
backflow.
• Rhythmic contractions of smooth
muscle assist in lymph flow.
• Skeletal muscle contractions are the
main sources of lymph movement.
Lymphatic System
• Lymph nodes are lymph
vessel organs.
• They contain WBC’s
(lymphocytes) and cells
specialized for defense
(macrophages).
• They filter the lymph
and attack foreign
invaders (antigens).
• They often swell when
we are ill.
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Blood Cells
• Blood cells and cell
fragments occupy about
45% of the blood
volume.
• 55% is plasma.
• Plasma is 90% water, it
contains electrolytes.
• Plasma proteins help to
maintain pH, osmotic
balance, and blood
viscosity.
• Some of these proteins
are immunoglobulins
that function in
defense.
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Plasma
• Blood plasma suspends 3
elements:
• 1. RBC’s--oxygen transport,
most numerous.
• 2. WBC’s--defense of body.
• 3. Platelets--fragments of
cells which help in the
clotting process.
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This image is copyright Dennis Kunkel
1. Red Blood Cells
• Shape is related to its
function.
• Biconcave increases its
surface area.
• Small size and number
increases surface area-related to function.
• Mammalian lack nuclei-allows for more
hemoglobin.
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1. Red Blood Cells
• RBC production is stimulated by a
negative feedback mechanism.
• When the amount of O2 reaching body
tissues decreases, this stimulates the
kidney to synthesize EPO.
• When more O2 reaches the tissues, EPO
levels fall and erythrocyte production
slows.
Heart Disease
LDL’s are bad.
HDL’s are good.
Cholesterol is bad in large quantities.
It sticks to the inside walls of arteries
and results in a narrowing of the
arteries, a stiffening of their walls
(called atherosclerosis).
• This increases the risk of heart attack
and stroke.
•
•
•
•
2. Leukocytes (WBC’s)
• These are white blood cells and there
are 5 types:
•
•
•
•
•
1.
2.
3.
4.
5.
Monocytes
Neutrophils
Basophils
Eosinophils
Lymphocytes
• Collectively, these fight infection.
The Contents of Blood
Lymphocyte
Neutrophil
Platelets
Basophil
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Monocyte
Erythrocytes
Eosinophil
2. Leukocytes (WBC’s)
• These spend most of their time in the
interstitial fluid where they fight
invaders.
Stem Cells
• Recall that they are pluripotent.
• They can develop into many different
types of things:
• 1. Erythrocytes
• 2. Leukocytes
• 3. Platelets
• They are found in bone marrow.
3. Platelets
• These plug wounds and prevent blood
loss.
• Wounds release factors that make
platelets sticky and enable them to
adhere to collagen fibers in connective
tissue slowing blood loss.
Respiratory System
• Respiratory surfaces allow for the
exchange of gases.
• They are always thin and bathed in
water.
• In most animals, the respiratory
medium is a thin, moist epithelium.
• This separates the respiratory medium
from the blood.
Respiratory System
• In animals that don’t respire through
their skin, there are three common
respiratory surfaces:
• 1. Gills
• 2. Trachea
• 3. Lungs
1. Gills
• Are out-foldings of
the body surface
suspended in water.
• They are loaded with
capillaries.
• Animals with gills
ventilate them which
moves water with a
high concentration of
O2 over them.
1. Gills
• Blood moves in an opposite direction to
the movement of water past the gills.
• The O2 transfer is highly efficient.
• This is called counter-current exchange,
and loads the blood with O2.
• It keeps a diffusion gradient over the
entire length of the capillary.
2. Tracheal System
• Found in insects.
• It is made up of tubes that branch through
the body which is a variation on a folded,
internal respiratory surface.
• The trachea branches smaller and smaller
and contacts nearly every cell.
3. Lungs
• These are respiratory organs found in
one spot of the body.
• They have a dense net of capillaries
immediately below the epithelium on
the respiratory surface.
• They are connected to a closed system
that transports gases to and from other
regions of the body.
Air Pathway
• Nares pharynx, larynx, trachea,
bronchi, bronchioles, alveoli
• It is like a tree tipped upside down.
• The epithelial lining of the three major
branches of the respiratory system are
covered by cilia and a thin film of
mucus.
Air Pathway
• The mucus traps particulate matter
and the cilia sweeps it out.
• The thinnest bronchioles are dead end
sacs called alveoli, have a high SA.
• O2 dissolves in the moist film covering
the epithelium and quickly diffuses
into the web of capillaries surrounding
the alveolus.
• CO2 diffuses in the opposite direction.
Breathing
• The diffusion of a gas depends on
partial pressures.
• When water is exposed to air, the
amount of gas dissolved in the water is
proportional to the partial pressure in
the air, and its solubility in water.
• Gases always diffuse from regions of
high partial pressure to regions of low
partial pressure.
Breathing
• Positive pressure breathing-amphibians
• Negative pressure breathing-humans
• Tidal volume is the volume of air
inhaled with each breath.
• Max. during forced breathing is 3-4.8L
• Residual volume is the amount
remaining in the lungs after a forced
exhale.
Breathing
• Human breathing is mostly under
autonomic control.
• 2 regions of the brain control this:
• The pons and the medulla.
• The pons controls the medulla which
sets a basic breathing rhythm.
Breathing
• Sensors in the aorta and carotid
arteries exert secondary control over
breathing.
• These sensors monitor O2, CO2 and
blood pH.
• The pH is largely controlled by CO2
levels.
Breathing
• When CO2 levels increase, carbonic
acid levels increase lowering the blood
pH.
• When pH drops, the depth and rate of
breathing increases helping to remove
excess CO2.
• O2 levels only have an effect on
breathing rate at high altitudes.
Breathing
• In addition to transporting O2,
hemoglobin helps transport CO2 and
assists in buffering.
• Respiring cells produce CO2. Carbonic
anhydrase catalyzes the reaction of
CO2 with H2O to form H2CO3.
• H2CO3 dissociates into H+ + HCO3• Most of the H+ attaches to hemoglobin
and other proteins minimizing the
change in blood pH.
Breathing
• HCO3- diffuses into the plasma.
• As blood flows through the lungs, the
process is reversed.
• Diffusion of CO2 out of the blood shifts
the chemical equilibrium in favor of
the conversion of HCO3- to CO2.