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== Circle of Willis  ==

== Circle of Willis  ==

[[Image:376px-Circle of Willis en.svg.png|thumb|right|150px|Generalised Structure of the Circle of Willis, Rhcastilhos, 2007, WikiMedia Commons]]

[[Image:376px-Circle of Willis en.svg.png|thumb|right|150px|Generalised Structure of the Circle of Willis, Rhcastilhos, 2007, WikiMedia Commons]]

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The Rete Mirable is a complex network of arteries and veins lying very close to each other and depends on a countercurrent blood flow between the arterioles and venuoles (blood flowing in opposite directions). It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient. 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.

The Rete Mirable is a complex network of arteries and veins lying very close to each other and depends on a countercurrent blood flow between the arterioles and venules (blood flowing in opposite directions). It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient. 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.

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==Species Differences: Blood Supply==

==Species Differences: Blood Supply==

The [[#Vasculature|blood supply to the brain]] has major differences depending on species. Across all species, there are variations in four major potential channels to facilitate blood supply to the Circle of Willis. As mentioned above, these are;  

The blood supply to the brain has major differences depending on species. Across all species, there are variations in four major potential channels to facilitate blood supply to the Circle of Willis. As mentioned above, these are;  

*Internal carotid artery  

*Internal carotid artery  

*Basiliar artery

*Basiliar artery

===Dog and Man (and many other species)===

===Dog and 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 comparison. RVC 2008]]  

The circle of Willis in the dog is supplied from three sources; paired internal carotid arteries laterally and the basilar artery caudally. The internal carotid artery is a terminal branch of the common carotid artery. The internal carotid artery 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 arteries are responsible for almost all supply to the occipital lobes of the cerebral hemisphere in the dog.

The circle of Willis in the dog is supplied from three sources; paired internal carotid arteries laterally and the basilar artery caudally. The internal carotid artery is a terminal branch of the common carotid artery. The internal carotid artery blood reaches all of the cerebral hemisphere except for its most caudal part. Vertebral blood supplies the remainder of the cerebral hemisphere and the rest of the brain. Vertebral arteries are responsible for almost all supply to the occipital lobes of the cerebral hemisphere in the dog.

On the right side of the brain in canines and humans, the internal carotid also receives a blood supply via two other sources. Firstly from the ramus of the vertebral artery (bypassing the occipital artery) and secondly from the maxillary ramus (bypassing the external carotid artery). Both of these rami appear to have a rete mirabile function prior to connecting to the internal carotid artery. On the left side of the brain in canines and humans there are no rami interconnecting between the main supply channels.

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===Sheep and Cat===

===Sheep and Cat===

Again the main supply to the brain in both species is via three main sources; paired internal carotid arteries and the basilar artery. On the right side of an ovine and feline brain there is a similar physiological setup to that found in canines and man (above). The right internal carotid artery receives blood supply from the normal source (describes in the section above) but also has a maxillary ramus and a vertebral artery ramus, both of which have rete mirabile functions.

Again the main supply to the brain in both species is via three main sources; paired internal carotid arteries and the basilar artery.  

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On the left side of the brain in these species there is a distinct missing connection. Where the vertebral artery supplies the occipital artery and this then supplies the internal carotid, this connection is missing so the occipital is only able to join the external carotid and the common carotid arteries. Similarly the vertebral artery ramus is also missing so there is no direct blood supply to the internal carotid from the vertebral artery and also no associated rete mirabile. The maxillary ramus supplying the internal carotid is still patent on the left side. Maxillary blood is distributed to all of the brain except the caudal part of medulla oblongata, which is supplied by vertibral blood. Therefore blood supply to the circle of Willis in ovines and felines is not bi-laterally symmetrical.

Where the vertebral artery supplies the occipital artery and this then supplies the internal carotid, this connection is missing so the occipital is only able to join the external carotid and the common carotid arteries. Similarly the vertebral artery ramus is also missing so there is no direct blood supply to the internal carotid from the vertebral artery and also no associated rete mirabile. The maxillary ramus supplying the internal carotid is still patent on the left side. Maxillary blood is distributed to all of the brain except the caudal part of medulla oblongata, which is supplied by vertebral blood.

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===Ox===

===Ox===

As with the previous species, the blood supply to the circle of Willis is via three main routes; paired internal carotid arteries and a basilar artery. The right side of the brain is similar to that seen in the dog/man or ovine/feline with patent vertebral and maxillary rami with associated rete mirabile. On the left side of the brain the ox is also missing a connection from the occipital artery/external carotid/common carotid to the internal cartoid in the same manner as the ovine/feline physiology. However, where the ovine/feline physiology included only a maxillary ramus, the ox has maxillary and vertebral rami on the left side also, with associated rete mirabile. Therefore the ox has four functional rete mirabile involved in blood supply to the circle of Willis. A mixture of maxillary and vertebral blood reaches all parts of the brain.

As with the previous species, the blood supply to the circle of Willis is via three main routes; paired internal carotid arteries and a basilar artery. The ox is also missing a connection from the occipital artery/external carotid/common carotid to the internal cartoid in the same manner as the ovine/feline physiology. However, where the ovine/feline physiology included only a maxillary ramus, the ox has maxillary and vertebral rami, with associated rete mirabile. Therefore the ox has four functional rete mirabile involved in blood supply to the circle of Willis. A mixture of maxillary and vertebral blood reaches all parts of the brain.

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In the Ox the maxillary rami containing the rete mirabile are associated with the cavity of the frontal sinus which helps to facilitate cooling of the blood.

In the Ox the maxillary rami containing the rete mirabile are associated with the cavity of the frontal sinus which helps to facilitate cooling of the blood.

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==Species Differences: Regions of Blood Supply==

==Species Differences: Regions of Blood Supply==

===Dog and Man===

===Dog and Man===

In dog and man the internal carotid artery supplies approximately 3/4 of the cerebrum whilst the vertebtral artery supplies the very caudal aspect of the cerebrum, the cerebellum, medulla oblongata and brain stem.

In dog and man the internal carotid artery supplies approximately 3/4 of the cerebrum whilst the vertebral artery supplies the very caudal aspect of the cerebrum, the cerebellum, medulla oblongata and brain stem.

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===Sheep and Cat===

===Sheep and Cat===

Maxillary blood (via the maxillary rami) supplies the whole of the cerebrum, the cerebellum and the medulla oblongata. vertebral blood supplies structures caudal to the cerebellum and the brain stem.

Maxillary blood (via the maxillary rami) supplies the whole of the cerebrum, the cerebellum and the medulla oblongata. Vertebral blood supplies structures caudal to the cerebellum and the brain stem.

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===Ox===

===Ox===

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

 

[[Category:Nervous_System_-_Anatomy_&_Physiology]] [[Category:A&P Done]]