Changes

==Circle of Willis==

== Circle of Willis ==

Blood is supplied to the brain by the ''internal carotid artery'' in dogs and horses whilst in other domestic species the main blood supply is from branches of the ''maxillary artery''. The Circle of Willis (also called the ''cerebral arterial circle'' or ''arterial circle of Willis'') is a circle of arteries that supply blood to the brain. The Circle of Willis is made up of the following arteries;

 

*Left and right inferior cerebral arteries

Blood is supplied to the brain by the ''internal carotid artery'' in dogs and horses whilst in other domestic species the main blood supply is from branches of the ''maxillary artery''. The Circle of Willis (also called the ''cerebral arterial circle'' or ''arterial circle of Willis'') is a circle of arteries that supply blood to the brain. The Circle of Willis is made up of the following arteries;  

*Anterior communicating artery

 

*Left and right internal carotid arteries

*Left and right inferior cerebral arteries  

*Left and right posterior cerebral arteries

*Anterior communicating artery  

*Left and right internal carotid arteries  

*Left and right posterior cerebral arteries  

*Left and right posterior communicating arteries

*Left and right posterior communicating arteries

The arrangement of the Circle of Willis means that if one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is stenosed, blood flow from the other blood vessels can continue to provide a continuous supply of blood to the brain.

 

The arrangement of the Circle of Willis means that if one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is stenosed, blood flow from the other blood vessels can continue to provide a continuous supply of blood to the brain.  

===Rete Mirable===

=== Rete Mirable ===

* A rete mirabile is a complex network of arteries and veins lying very close to each other.

[[Image:Carotidretesheep.jpg|thumb|right|150px|Carotid Rete of the Sheep. Sophie Stenner, RVC 2008]]

* The rete mirabile depends on a countercurrent blood flow between the arterioles and venuoles (blood flowing in opposite directions).

*A rete mirabile is a complex network of arteries and veins lying very close to each other.

* It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient.

*The rete mirabile depends on a countercurrent blood flow between the arterioles and venuoles (blood flowing in opposite directions).

The image shows the countercurrent exchange in the carotid rete mirable (carotid rete) of a sheep controlling the temperature of blood supplied to the brain.

<br> The image shows the countercurrent exchange in the carotid rete mirable (carotid rete) of a sheep controlling the temperature of blood supplied to the brain.  

===Species Differences===

=== Species Differences ===

[[#Vasculature|Blood supply to the brain]] has major differences within the species. The 4 potential channels supplying the cerebral arterial circle are:

[[#Vasculature|Blood supply to the brain]] has major differences within the species. The 4 potential channels supplying the cerebral arterial circle are:  

'''1. The internal carotid artery'''

'''1. The internal carotid artery'''  

'''2. The basiliar artery'''

'''2. The basiliar artery'''  

'''3. The anastomosing ramus from the maxillary artery to the internal carotid artery'''

'''3. The anastomosing ramus from the maxillary artery to the internal carotid artery'''  

'''4. The connection of the vertibral artery to the internal carotid artery'''

'''4. The connection of the vertibral artery to the internal carotid artery'''  

====Dog, man and many other species====

==== Dog, man and many other species ====

[[Image:calfvssheepblood.jpg|thumb|right|150px|Calf vs Sheep blood supply comparism. RVC 2008]]

[[Image:Calfvssheepblood.jpg|thumb|right|150px|Calf vs Sheep blood supply comparism. RVC 2008]]  

* '''Channels 1 + 2''' supply the arterial circle.

*'''Channels 1 + 2''' supply the arterial circle.  

* Neither channel has a '''[[#Rete Mirable|rete mirable]]'''.

*Neither channel has a '''[[#Rete_Mirable|rete mirable]]'''.  

* Internal carotid blood reaches all of the cerebral hemisphere '''except for it's most caudal part'''.

*Internal carotid blood reaches all of the cerebral hemisphere '''except for it's most caudal part'''.  

* '''Vertebral blood''' supplies the the remainder of the cerebral hemisphere and the rest of the brain.

*'''Vertebral blood''' supplies the the remainder of the cerebral hemisphere and the rest of the brain.

====Sheep and cat====

==== Sheep and cat ====

* '''Only channel 3''' supplies the arterial circle.

*'''Only channel 3''' supplies the arterial circle.  

* Channel 3 has a '''rete mirable'''.

*Channel 3 has a '''rete mirable'''.  

* '''Channel 2 carries blood away''' from the arterial circle.

*'''Channel 2 carries blood away''' from the arterial circle.  

* Maxillary blood is distributed to all of the brain except the caudal part of medulla oblongata, which is supplied by vertibral blood.

*Maxillary blood is distributed to all of the brain except the caudal part of medulla oblongata, which is supplied by vertibral blood.

====Ox====

==== Ox ====

* '''Channels 3 + 4''' both supply the arterial circle.

*'''Channels 3 + 4''' both supply the arterial circle.  

* Each has a '''rete mirable'''.

*Each has a '''rete mirable'''.  

* '''Channel 2 carries blood away''' from the arterial circle.

*'''Channel 2 carries blood away''' from the arterial circle.  

* A mixture of maxillary and vertibral blood reaches '''all parts of the brain'''.

*A mixture of maxillary and vertibral blood reaches '''all parts of the brain'''.

[[Category:Nervous System - Anatomy & Physiology]]

[[Category:Nervous_System_-_Anatomy_&_Physiology]] [[Category:To_Do_-_A&P]]

[[Category:To Do - A&P]]