Animal Circulatory Systems

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Animal Circulatory Systems
Chapter 44 pp. 944-1003

• 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
Larger Animals Without a Separate Circulatory
System
Cnidarian Gastrovascular Systems
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 and
serves as a circulatory system as well as a
digestive cavity.
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Flatworm Gastrovascular System
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Circulatory Systems
For larger or more active animals, some form of
more efficient circulatory system is necessary
for internal transport.

• Two types of circulatory system are found
• Open Circulatory Systems
• Closed Circulatory Systems

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Open Circulatory System

• Hemolymph leaves the heart in short, branched
  arteries that open up into large spaces called
  sinuses.
• Hemolymph percolates around organs, directly
  bathing the cells.
• Hemolymph then returns to the heart directly or
  through short veins.

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Open Circulatory System

• Advantage - Exchange of materials is direct
  between the hemolymph and tissues. There is no
  diffusion barrier.
• Disadvantage - Little fine control over
  distribution of the hemolymph to body regions. No
  mechanism for reducing flow to a specific 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 the highly
  active cephalopods (squid and octopus) have
  evolved a closed system.
• Insects have circumvented limitation of their
  open system by their tracheal system for oxygen
  supply.

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Closed Circulatory System

• The blood is contained within 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.

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Closed Circulatory System
Advantages

• 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.

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Earthworm Circulation
Extensive 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 Capillaries
Arteries and arterioles have 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 and arterioles have many
elastic fibers enabling them to withstand high
pressures.
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Artery and Vein
Artery
Vein
Note the much thinner walls in veins.
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Blood Pressure and Flow Velocity
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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 basement membrane (extracellular
  matrix).

Endothelial cells
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Filtration fluid and small, lipid-insoluble
molecules (water, amino acids, NaCl,
glucose, urea)
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Capillary - Tissue Fluid Exchange
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 colloidal osmotic potential -22 mm Hg
Net pressure 10 mm Hg
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Capillary Fluid Exchanges
Frictional
Blood pressure (hydrostatic) 32 mm Hg
resistance
Blood pressure (hydrostatic) 15 mm Hg
Plasma colloidal osmotic potential -22 mm Hg
Plasma colloidal osmotic potential -22 mm Hg
Net pressure 10 mm Hg
Net pressure -7 mm Hg
Less water re-enters the capillary than
originally left at the arterial end.
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The Lymphatic System
The lymphatic system, returns excess tissue fluid
to the blood.
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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 rapidly
  cleared away by the circulatory system.

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Control of Capillary Circulation

• Arteries and arterioles that feed blood to the
  capillaries contain a circular layer of smooth
  muscle in their walls.
• Contraction of these smooth muscles
  (vasoconstriction) is important in controlling
  the blood flow through capillary beds.
• Relaxation of smooth muscles results in
  vasodilation, an expansion of the vessel diameter
  that increases blood flow.

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figure 49-18.jpg
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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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(and capillaries)
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Cardiac cycle
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Blood flow in veins
One-way flow of blood (toward heart) is
determined by valves.
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Human blood components