04 circulation web
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Chapter 42; pp. 867-879
Animal Circulatory Systems
� Types of circulatory systems:
gastrovascular, open, closed.
� Vascular system: arteries, veins, capillaries.
� Capillary - tissue fluid exchange.
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Absence of a Circulatory System
� Very small animals may not need a circulatory
system.
� Small size may permit nutrients and other
substances to reach all the body parts by simple
diffusion
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figure 49-01.jpg
Cnidarian Gastrovascular Systems
Fig. 33.5
Some larger animals such as
sea anemones, jellyfish, and
flatworms lack a true
circulatory system.
The gastrovascular cavity
extends to most areas of the
body in these animals andserves as a circulatory
system as well as a
digestive cavity.
Larger Animals Without a Separate Circulatory System
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Flatworm Gastrovascular System
Fig. 33.10
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Medusa Internal Transport
Fig. 42.2
� Radial canals branch from the centrally located stomach and
extend out to the circular canal at the margin of the bell.
� Nutrients and other material are carried around the body in
these canals and then diffuse out to nearby tissues.
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Circulatory Systems
� Two types of circulatory system are found:
Open Circulatory Systems
Closed Circulatory Systems
For larger or more active animals, some form of
more efficient circulatory system is necessary for
internal transport.
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Open Circulatory System
� Hemolymph leaves theheart in short, branchedarteries that open up intolarge spaces called sinuses.
� Hemolymph percolatesaround organs, directlybathing the cells.
� Hemolymph then returns tothe heart directly or throughshort veins.
Fig. 42.3a
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Open Circulatory System
� Advantage - Since cells are bathed by hemolymph,
the exchange of materials is direct between the
hemolymph and tissues. There is no diffusion
barrier.
� Disadvantage - There is little opportunity for fine
control over distribution of the hemolymph to body
regions. No mechanism for reducing flow to aspecific part of an organ.
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Open Circulatory System
� Open circulatory systems tend to be found in more
inactive animals.
� Most molluscs have an open system, but thehighly active cephalopods (squid and octopus)
have evolved a closed system.
� Insects have circumvented limitation of their opensystem by their tracheal system for oxygen supply.
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Origin of the Hemocoel
� The open sinuses where the hemolymph circulates
is called the hemocoel.
� This space is not a coelomic cavity.
� It is a persistent blastocoel that has never been
filled in by expanding mesodermal tissue during
development.
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Cross-sections
through
developingembryos.
See Fig. 47.9
for steps in
development.
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Closed Circulatory System
� The blood is containedwithin a completely closed
system of vessels.
� Vessels form a closed
loop, usually with some
sort of pumping organ like
a heart or contractile
vessels.
� Vessels branch into
smaller and smaller tubes
that penetrate among the
cells of tissues.
Fig. 42.3b
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Closed Circulatory System
� Fine-scale control over the distribution of blood
to different body regions is possible.
� Muscular walls of vessels can constrict and dilate
to vary the amount of flow through specific
vessels.
� Blood pressures are fairly high and the circulation
can be vigorous.
Advantages:
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Fig. 33.23
figure 49-03.jpg
Earthworm Anatomy
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Earthworm CirculationExtensive
capillary beds:
Body wall
Gut wall
Excretory tubules
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Coelomic Cavities - Circulatory Function
� Coelomic cavities are filled with fluid that can
transport materials around the body.
� Nematode worms have an extensive body cavity,
the pseudocoel, but lack a separate circulatory
system.
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Ascaris
Cross-Section
Pseudocoel
(fluid-filled space)
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The Vertebrate Vascular System: Arteries,
Veins, and CapillariesArteries and arterioleshave a layer of smooth
muscle tissue which
allows them to contract
(vasoconstrict) and
expand (vasodilate),
altering their diameter
and thus blood flow.
Walls of arteries andarterioles have many
elastic fibers enabling
them to withstand high
pressures.Fig. 42.9
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Artery and Vein
Artery
Vein
Note the much
thinner walls inveins.
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Blood Pressure and
Flow Velocity
� As arteries branch, the
cross-sectional area increases
causing blood pressure and
flow velocity to fall.
� In mammals there is a an
800-fold increase in cross-
sectional area from the aorta
to the capillaries.
� Velocity in the aorta is
around 40-50 cm/s but drops
to < 0.1 cm/s in capillaries.
Fig. 42.11
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Capillaries
� Capillaries are very small,
about the diameter of a red
blood cell (8µm or less).
� Capillary walls are a single
layer of very thin endothelial
cells, attached at their edges
and surrounded by a basementmembrane (extracellular
matrix).
Endothelial cells
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Blood cells,
most
proteins.
Vesicles; large,lipid-insoluble
(proteins)Filtration; fluid and
small, lipid-insoluble
molecules (water,
amino acids,NaCl, glucose,
urea)
Diffusion;
lipid-soluble
molecules
(O2, CO2,
lipids)
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Capillary Density in Tissues
� Penetration of tissues by capillaries is soextensive that in active tissues each capillary
serves a volume of tissue only about 10 times
its own volume.
� No cell is very far from the blood supply.
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Capillary - Tissue Fluid Exchange
� Capillary beds are the site of exchange of materials between bloodand the interstitial fluid that bathes the tissues.
� Fluid exchange between blood and interstitial fluid is determinedby the balance between the positive blood pressure (hydrostatic
pressure) and the net negative osmotic potential in the bloodplasma.
� The osmotic potential of the blood plasma in the capillary is more
negative than the osmotic potential of the surrounding interstitialfluid.
� Proteins in the blood plasma that cannot easily leave the capillaryare the source of this difference in osmotic potential. Otherwise,the interstitial fluid and blood plasma have similar concentrationsof ions and other small molecules.
Osmotic Gradient Between Interstitial Fluid and Blood
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Capillary - Tissue Fluid Exchange
Similar to
Fig. 42.14
Blood hydrostatic pressure exceeds the opposing negative
colloidal osmotic potential of the blood plasma.
Water, containing small dissolved molecules, is forced
out of the capillary through small pores in the capillary
wall by the excess hydrostatic pressure.
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Capillary Fluid Exchanges
Blood pressure(hydrostatic)
32 mm Hg
Plasma colloidalosmotic potential
-22 mm Hg
Net pressure
10 mm Hg
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Capillary - Tissue Fluid Exchange
Venous End
� At the venous end of the capillary, the balance of
forces reverses and the blood plasma¶s negative
colloidal osmotic potential exceeds the hydrostaticpressure of the blood.
� Water, containing small dissolved molecules,
moves back into the capillary by osmosis.
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Capillary Fluid Exchanges
Frictional
resistance Blood pressure(hydrostatic)
15 mm Hg
Plasma colloidalosmotic potential
-22 mm Hg
Net pressure
-7 mm Hg
Blood pressure(hydrostatic)
32 mm Hg
Plasma colloidalosmotic potential
-22 mm Hg
Net pressure
10 mm Hg
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Capillary - Tissue Fluid Exchange
� Note that the net force is less at the venous end(7 mm Hg inwards vs 10 mm Hg outwards at arterial end ).
� Less water re-enters the capillary than originally
left at the arterial end.
� The surplus fluid is taken up by the lymphatic
system
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The Lymphatic System
Fig. 43.5
� A separate system of
vessels, the lymphatic
system, returns excesstissue fluid to the blood.
� Lymphatic ducts drain
into the venous system
near the heart.
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Capillary - Tissue Fluid Exchange
� The bulk flow of fluid out of the capillary
exchanges material much faster than would be
possible by simple diffusion alone.
� Nutrients and O2 are released to the tissues
rapidly.
� Wastes from cell metabolism are more rapidlycleared away by the circulatory system.
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Control of Capillary Circulation
� Arteries, arterioles, and metarterioles that feedblood to the capillaries contain a circular layer of smooth muscle in their walls.
� Contraction of these smooth muscles(vasoconstriction) is important in controlling theblood flow through capillary beds.
� Relaxation of smooth muscles results invasodilation, an expansion of the vessel diameter that increases blood flow.
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figure 49-18.jpg
Precapillary sphincters are
rings of smooth muscle that
surround the junction of a
capillary with an arteriole or
metarteriole.
Contraction of precapillary
sphincters can completely
shut off blood flow to a
capillary bed.
Fig. 42.13
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Circulatory Patterns in
Vertebrates
The circulatory pattern has been modified
during evolution of the major groups of vertebrates.
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Based on Fig. 42.3
(and capillaries)
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Fish Heart
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Frog Circulation
� The frog has completely separated atria, but the
ventricle is a single chamber.
� There is surprisingly little mixing of oxygenated and
deoxygenated blood as it passes through the single
ventricle.
� Oxygenated blood from the pulmonary vein
probably soaks into the spongy walls of the ventricle
so it doesn¶t mix much with deoxygenated blood.
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Based on Fig. 42.3
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Frog Heart
� There is incomplete mixing
of the oxygenated anddeoxygenated blood.
� The most deoxygenatedblood passes into the
pulmocutaneous arteries.� The most oxygenated blood,
from the lung (pulmonaryvein) preferentially entersthe carotid arteries.
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Mammals and Birds
� Mammals and birds have completely divided atria
and ventricles so no mixing of oxygenated anddeoxygenated blood is possible.
� There is a complete double circulation pattern first
through the pulmonary circuit and then throughthe systemic circuit.
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Based on Fig. 42.3
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� Deoxygenated blood enters the right atrium and
goes on into the pulmonary circuit to the lungs.
� Oxygenated blood comes back to the left atrium in
the pulmonary vein and then goes on to the
systemic circuit
Mammals and Birds
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Cardiac cycle
Fig. 42.7Animation
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Structure of blood vessels
Fig. 42.9
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Fig. 42.10
Blood flow in veins
One-way flow of blood (toward heart) isdetermined by valves.
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Human
blood
components
Fig 42 15