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General Biology
“Maintenance of Life”
Circulation and Gas Exchange
Noppadon Kitana, Ph.D.
Department of Biology
Chulalongkorn University
August 29, 2012
Circulation and Gas Exchange
• Respiratory system
functions in gas
exchange (O2 v.s. CO2).
• Circulatory system
functions in carrying
oxygenated blood to
other parts of body.
• Blood capillaries carry
materials to interstitial
fluid surrounding cells.
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Circulation in Animals
• Every organism must exchange materials and
energy with its environment.
• This exchange ultimately occurs at the cellular
level.
– Cells live in aqueous environments.
– Nutrients and oxygen move across the plasma
membrane to the cytoplasm.
– Metabolic wastes (e.g. CO2) move out of the cell.
• Diffusion is the simplest and the most common
route of exchange.
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Circulation in Animals
• Diffusion alone is not adequate for transporting
substances over long distances in animals.
• Diffusion time is proportional to the square of the
distance, e.g. if diffusion through 0.1 mm
distance occurs in 1 second.
– 1 mm will occur in 100 seconds
– 10 mm will occur in 10,000 seconds (~ 3 hrs)
• Circulatory system solves this by ensuring that
no substance must diffuse very far to enter or
leave a cell.
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Circulation in Invertebrates:
Gastrovascular Cavity
• The body plan of cnidarians
(e.g. hydra) makes a circulatory
system unnecessary.
• A body wall of only two-cell
thick encloses a central
gastrovascular cavity that
serves for both digestion and
for diffusion of substances
throughout the body.
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• The fluid inside the cavity is continuous with the
water outside through a single opening (mouth).
• Thus, both the inner and outer tissue layers are
bathed in fluid.
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Circulatory System
• Open system
• Closed system
• Heart
• Vessels
• Circulating fluid
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Open Circulatory System
• Insects, other
arthropods, and most
mollusks
• No distinction
between blood and
interstitial fluid
(collectively called
hemolymph).
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• One or more hearts pump
the hemolymph into
interconnected sinuses
surrounding the organs.
• This process allows
exchange between
hemolymph and body cells.
• Heart is an elongated dorsal
tube.
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• Body movements that
squeeze the sinuses help
circulate the hemolymph.
Closed Circulatory System
• Earthworm, squid and
octopus.
• Blood is confined to
vessels and is distinct
from the interstitial fluid.
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• One or more hearts pump
blood into large vessels that
branch into smaller ones
surrounding organs.
• Materials are exchanged by
diffusion between the blood
and the interstitial fluid
bathing the cells.
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Circulation in Vertebrates:
Closed Circulatory System
• Collectively called cardiovascular system (CVS)
• Heart consists of
– atrium (atria) = chamber(s) that receive blood
returning to the heart
– ventricle (ventricles) = chamber(s) that pump
blood out of the heart
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• Blood vessels
– Arteries, arterioles = carry blood away from the heart to
organs
– Capillaries = very thin, porous wall vessels form
networks, called capillary beds, that infiltrate each
tissue
– Veins, venules = return blood to the heart
• Distinguished by the direction in which
they carry blood, not by the characters
of the blood they carry.
Heart
– Arteries carry blood from
the heart toward capillaries.
– Veins return blood to the heart
from capillaries.
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Capillaries
Circulation in Fish
• 2 chambers (atrium & ventricle)
• Blood is pumped from the
ventricle to gills (gill
circulation) where it picks up
O2 and disposes of CO2.
• Gill capillaries converge into a
vessel that carries oxygenated
blood to capillary beds at other
organs (systemic circulation)
and back to the heart.
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• Blood must pass through two
capillary beds, therefore,
oxygen-rich blood leaving the
gills flows to the systemic
circulation quite slowly.
• The heart pumps only oxygenpoor blood.
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Circulation in Amphibians
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• Three chambered heart
(2 atria & 1 ventricle)
• Ventricle pumps blood into a
forked artery that splits the
ventricle’s output into:
– pulmocutaneous
circulation
– systemic circulation
• Blood is pumped a second
time after it loses pressure in
the capillary beds of lung or
skin (= double circulation).
• Double circulation provides a
vigorous flow of blood to the
brain, muscles, and other
organs.
• In the ventricle, some oxygenrich blood from the lungs
mixes with oxygen-poor blood
that has returned from the rest
of the body
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Circulation in Reptiles
• Reptiles also have double circulation with
pulmonary (lung) and systemic circuits.
– However, there is even less mixing of oxygenrich and oxygen-poor blood than in
amphibians.
– Although the reptilian heart is threechambered, the ventricle is partially divided.
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Circulation in Other Vertebrates
• Crocodilians, birds, and
mammals
• Four-chambered heart
(2 atria & 2 ventricles)
• The left side of the heart
receives and pumps only
oxygen-rich blood.
• The right side handles only
oxygen-poor blood.
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• Also have double circulation
to restore pressure to the
systemic circuit.
• Evolution of a powerful fourchambered heart was an
essential adaptation in
support of the endothermic
way of life of birds and
mammals.
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Circulation Pattern in Human
1.
2.
3.
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Right ventricle
Pulmonary arteries
Capillary beds in left and
right lungs
4. Pulmonary veins, left
atrium
5. Left ventricle
6. Aorta
7. Capillary beds in head
and arms
8. Capillary beds in
abdominal organs and
legs
9. Superior (Anterior) vena
cava
10. Inferior (Posterior) vena
cava
11. Right atrium
Human Heart
•
•
•
•
•
•
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4 chambers
atria (thin)
ventricle (thick)
valve
artery
vein
Cardiac Cycle
• A complete sequence
of pumping
(contraction) and
filling (relaxation)
• Contraction phase =
Systole
• Relaxation phase =
Diastole
Cardiac cycle is regulated by electrical impulses that
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2012
radiate
the heart.
Control of Heartbeat
• Cells of vertebrate heart muscle are self-excitable
(contract without any signal from nervous system).
• Each cell has its own intrinsic contraction rhythm.
• However, these cells are synchronized by the
pacemaker, which sets the rate and timing of
contraction.
– In amphibians, pace maker is located in sinus venosus.
– In human, sinoatrial (SA) node in wall of the right atrium is
a pacemaker.
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Cardiac Electrical Impulse
• (1) The SA node generates electrical impulses that
spread rapidly (2) through the wall of the atria,
making them contract in unison.
• The impulse from the SA
node is delayed by about
0.1 sec at the
atrioventricular (AV) node,
the relay point to the
ventricle, allowing the atria
to empty completely before
the ventricles contract.
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• (3) Specialized muscle fibers called bundle
branches and Purkinje fibers conduct the signals
to the apex of the heart and (4) throughout the
ventricular walls.
• This stimulates the
ventricles to contract from
the apex toward the atria,
driving blood into the large
arteries.
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• The impulses generated during the heart cycle
produce electrical currents that are conducted
through body fluids to the skin.
• The currents can be detected by electrodes and
recorded as an electrocardiogram (ECG or
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EKG).
Heart of Animals
• Vertebrate heart has pace
maker within the heart
(=myogenic heart)
• Invertebrate heart has pace
maker from motor nerve
outside the heart
(=neurogenic heart)
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Blood Vessels
•
•
•
•
•
Artery
Arteriole
Capillary
Venule
Vein
Structural differences correlate with the different
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functions
of arteries, veins, and capillaries
• Arteries have thick middle and outer layers.
– The thicker walls provide strength to
accommodate blood pumped rapidly and at
high pressure by the heart.
– Their elasticity helps maintain blood pressure
even when the heart relaxes.
• Capillaries lack the two
outer layers and their
very thin walls consist of
only endothelium and its
basement membrane
(enhancing exchange).
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• Veins convey blood back to the heart at low
velocity and pressure.
– Blood flows as a result of skeletal muscle
contractions that squeeze blood in veins.
– Within larger veins, flaps of tissues act as oneway valves that allow blood to flow only toward
the heart.
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Blood Pressure
• Hydrostatic pressure that
blood exert against blood
vessel wall
• Much greater in arteries
than in veins
• Highest in arteries when
the heart contracts during
ventricular systole, creating
the systolic pressure.
• The narrow openings of
arterioles impeding the exit
of blood from the arteries,
the peripheral resistance.
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• When the heart contracts,
blood enters the arteries
faster than it can leave,
and the vessels stretch
from the pressure.
• The elastic walls of
arteries snap back during
diastole, but the heart
contracts again before
enough blood has flowed
into the arterioles to
completely relieve
pressure in the arteries,
the diastolic pressure.
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Normal value:
120/80 mm Hg
Blood Flow
• Blood travels 1,000 time
faster in the aorta than in
capillaries
• Law of continuity: fluid
will flow through
narrower segments
faster than wider
segments because the
volume of flow per sec
must be constant.
• The total cross-sectional
area of capillaries
determines flow rate.
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Capillary Blood Flow
• At any given time, only
about 5-10% of the
body’s capillaries have
blood flowing through
them.
• Capillaries in the brain,
heart, kidneys, and liver
are usually filled, but in
other sites, the blood
supply varies over times
as blood is diverted
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• 2 mechanisms regulate the distribution of blood in
capillary beds.
1) contraction of smooth muscle layer in the wall
of an arteriole constricts the vessel, decreasing
blood flow to a capillary bed.
2) rings of smooth muscles, precapillary
sphincters, control the flow of blood between
arterioles and venules.
• Some blood flows directly from arterioles to
venules through thoroughfare channels which are
always open.
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Exchange of Substances between
Blood & Interstitial Fluid
• Takes place across the thin endothelial walls of
the capillaries by diffusion and endocytosis.
• Bulk flow due to fluid pressure (blood pressure
v.s. osmotic pressure)
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Blood Component
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Cellular Elements
• Erythrocytes
– 5-6 million cells per microliter
– carry oxygen and carbondioxide
– mammals and birds has biconcave erythrocyte with
no nucleus (increase surface area and hemoglobin)
– no mitochondria, make ATP by anaerobic respiration
• Leukocytes
– 5,000-10,000 cells per microliter
– 5 types: monocytes, neutrophils, basophils,
eosinophils, lymphocytes
– play roles in immunity
• Platelets
– 250,000-400,000 platelets per microliter
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2012
–29,function
in blood clotting
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Gas Exchange in Animals
• Gas exchange is the uptake of molecular oxygen
(O2) from the environment and the discharge of
carbon dioxide (CO2) to the environment.
• Although it is often called respiration, this process
is distinct from the production of ATP in cellular
respiration.
• Other terms
– respiration, breathing
– cellular respiration
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Components of Gas Exchange
• Respiratory medium
(air or water)
• Respiratory surface
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Gas Exchange in Animals
• Sponges, cnidarians and flatworms: membrane of
every cell in the body is close enough to the
outside environment for gases to diffuse in and out.
• Other animal has a respiratory organ that is
extensively folded or branched, enlarging the
surface area for gas exchange:
– skin
– gill
– trachea
– lung
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Gas Exchange through Skin
• Earthworms & some
amphibians
• Below the moist skin
is a dense net of
capillaries.
• The respiratory
surface must be
moist, their possible
habitats are limited to
water or damp places.
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Gas Exchange by Gills
• Invertebrates & fishes
• Gills are outfoldings of
the body surface that
are suspended in
water.
• The total surface area
of gills is often much
greater than that of the
rest of the body.
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Water as a Respiratory Medium
• Pros: no problem in keeping the respiratory surface
moist since the gills are surrounded by the aqueous
environment.
• Cons: O2 concentrations in water are low, especially
in warmer and saltier environments.
• Thus, gills must be very effective to obtain enough
oxygen.
• Ventilation increases the flow of the respiratory
medium over the respiratory surface, ensures that
there is a strong diffusion gradient between the gill
surface and the environment.
– Crayfish and lobsters have appendages that drive
a29,current
of water over their gills.
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2012
• Fish gills are ventilated by a current of water that
enters the mouth, passes through the pharynx,
flows over the gills, and exits the body.
– Fishes must expend considerable energy in
ventilating their gills.
– Gas exchange at the gill surface is enhanced by
the opposing flows of water and blood at the gills.
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Countercurrent exchange
• As blood moves in a gill capillary, it becomes more
and more loaded with O2, but it simultaneously
encounters water with even higher concentration of
O2 because it just pass over the gills.
• All along the gill
capillary, there is a
diffusion gradient
favoring the transfer
of O2 from
water to blood.
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Air as a Respiratory Medium
• Pros: air is a good respiratory medium containing
high concentration of oxygen. Diffusion rate is
thus higher in air.
• Cons: high risk of losing moisture from the
respiratory surface due to evaporation.
• The terrestrial animal must kept its respiratory
surface within its body.
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Gas Exchange by Tracheal
System
• Insects
• Air tubes that branch through the body.
• The largest tubes open to the outside.
• The finest branches extend to the
surface of nearly every cell where gas
is exchanged by diffusion.
• The circulatory system does not
transport oxygen and carbon dioxide.
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Gas Exchange by Lungs
• Unlike branching tracheal systems, lungs are
restricted to one location.
• Respiratory surface of the lung is not in direct
contact with all other parts of the body.
• Circulatory system transports gases between the
lungs and the rest of the body.
• Lungs have a dense net of capillaries just under the
epithelium that forms the respiratory surface.
• Lungs have evolved in spiders, terrestrial snails,
and vertebrates.
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