II' Aortic Arches A' Prototype condition consider 6 arches present in virtually all fishes plus amph

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EVOLUTION OF THE AORTIC ARCHES
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II. Evolution of the Aortic Arches

• A. Prototype condition - consider 6 arches -
  present in virtually all fishes plus amphibibians
  and amniotes
• 1. Dorsal aorta - paired throughout length of
  the pharynx in early verts.
• 2. Ventral aorta - paired early but fused singly
  later paired only anterior to IV arch.

Dorsal Aorta
Aortic arches
Ventral Aorta
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3. Extensions

• a. Internal carotid arteries - carry blood
  forward to head on dorsal
• b. External carotid arteriesVentral extensions
  of ventral aorta carrying blood to the head on
  ventral
• Remember - gills not functional for respiration

Internal carotid
Aortic arches
External carotid
Direction of Blood Flow
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B. Early fish state

• 1. Embryological stagesa. Loss of I afferent
  branchial artery leaving five afferent branchials
• b. Loss or reduction of dorsal aortic radices
  between arches II and III

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• c. Formation of afferent and efferent spiracular
  (pseudobranchial) arteries

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• d. Formation of efferent branchial loops
• e. Shift of external carotids from the ventral
  aorta to the II efferent loop

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f. Formation of efferent loops from efferent
branchial arteriesg. Connect loops across gill
slits
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Blood flow across gills
O2 taken up CO2 released Ion exchange
Remember - disadvantage of fish circulation -
loss of pressure across gill capillaries
resulting in reduced blood pressure in dorsal
aorta
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Bony fishes
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C. Amphibian state

• 1. Early statesa. Disappearance of spiracular
  arteries
• b. Loss of branchial function of the II
  visceral pouch
• c. Formation of pulmonary artery
• d. Formation of common carotid artery from
  base of III epibranchial

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2. Later state

• a. Loss of pouches so only arteries from III, IV,
  and VI remain
• b. Retention of complete aortic arch from
  embryonic development

Frogs
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D. Reptilian state

• 1. Dorsal part of III lost making a common
  carotid artery from ventral part of that arch
• 2. Division of ventral aorta into two or three
  vessels
• a. VI becomes pulmonary trunk and connected
  with right ventricle of heart
• b. Remainder of ventral aorta retained and
  attached to the left ventricle forming the
  systemic aorta

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E. Mammal state

• 1. Modifications a. Loss of most of right IV
  arch - Right subclavian was attached to this
  arch, only the dorsal part disappeared.
• b. Left half of IV remained along with base of
  right IV as trunk of right subclavian artery

Left IV remains as systemic aorta
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2. Mammalian derivatives of the hypothetical
prevertebrate system
ec
IC
I II III IV V VI

• a. Ventral aorta-external carotids and
  brachiocephalic arteries
• b. Cardiac region-sinauricular node, two
  ventricles, two atria

c. Aortic arches-I - lost II - lost III -
common carotids, part of internal carotid IV -
right base of right subclavian left
aortic arch retained as systemic aorta V -
lost VI - base of pulmonary arteries, left half
as ductus arteriosus in embryo and ligamentum
arteriosum in adult
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• d. Dorsal aorta - part of internal carotids,
  dorsal aorta
• e. External carotids - part of same

ec
IC
I II III IV V VI
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F. Avian stage

• 1. Modifications
• a. Loss of left IV arch
• b. Right half of IV remained as systemic aorta

Reptile
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Primitive Reptile
Reptile
Mammal
Bird
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Primitive and Developmental Condition
I
II
III
IV
V
VI
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Mammal
I
II
III
IV
V
VI
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Reptile
I
II
III
IV
V
VI
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Bird
I
II
III
IV
V
VI