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I understand that Circle of Willis is a circulatory anastomotic system that provided blood to the brain. I want to understand if this system is unique to humans or does it exist in other species or mammals to be more specific?
I found two references that talk about the circle of Willis in different kind of mammals so i think it's right to say that this particular anastomotic system of arteries is not exclusive to humans.
Here are the two references:
Is Circle of Willis unique to humans? - Biology
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Pinoy Nurses Galore
The brain represents about 2% of the total body weight in humans, but it receives one fifth of the resting cardiac output and accounts for 15-20% of the body's blood supply. Brain cells die if the supply of blood carrying oxygen is blocked therefore the brain has the highest priority for the blood. The human body endeavours to supply the brain with an uninterrupted flow of blood.
Blood supply to the brain is carried by two pairs of arteries: the internal carotid arteries and vertebral arteries, which anastomose at the base of the brain to form what is known as the circle of Willis. The right and left vertebral arteries form a single basilar artery at the base of the brain. This then provides the brain blood supply that feeds the circle of Willis.
The carotid arteries and their branches supply the front portion of the brain and the vertebrobasilar system supplies the rear portion of the brain. These are known as anterior and posterior circulation respectively.
What Relevance is the Brain Blood Supply?
If the supply of blood to the brain is stopped, brain cells die. Ischemic strokes are caused by a blood clot. These can occur with in the brain itself, or in the arteries that make up the brain blood supply. Both situations result in a stroke as the brain is deprived of critical blood.The area of brain blood supply is unique because as a foetus grows, the arteries grow from both theheart and the brain, meeting up in the circle of Willis. Sometimes, the anastomosis (how these arterise join into a network) is not perfect and this can make a person susceptible to problems where plaque can build up at the join. In such cases, the risk of stroke is increased.It's not essential to understand all the parts of the brain blood supply. However, it's worthwhile to understand that this is a complex part of human anatomy.
The Circle of Willis is a vital formation of arteries at the base of the brain which supplies all thought processes with the necessary fuel.
There is a grouping of arteries near the base of the brain which is called the Arterial Circle of Willis. It is named after a very influential English physician named Thomas Willis, who discovered it and then published his findings in his 1664 work, a seminal peace on the inner workings of the brain entitled, Cerebri anatomi (from the Latin for “Anatomy of the Brain”).
The Circle of Willis or the Circulus Arteriosus is an arterial polygon where the blood carried by the two internal carotid arteries and
the basilar system comes together and then is redistributed by the anterior, middle, and posteriorcerebral arteries. The posterior cerebral artery is connected to the internal carotid artery by the posterior communicating artery.
The anterior communicating artery joins the anterior cerebral arteries of the two hemispheres together. The middle cerebral arteries are connected to the posterior cerebral arteries by the posterior communicating arteries. This anastomosis between arteries is responsible for developing collateral circulation. It provides a safety mechanism. If one of the major vessels becomes occluded within the
Circle or proximal to it, the circle will still provides the brain with continued supply of blood. Thus the circle of Willis is of great use in preventing neurological damage. As long as this circle is successful at maintaining blood pressure at fifty
Percent of normal, no infarction or death of brain tissue will occur in the blocked area and no permanent effects are produced.Smaller arteries arise from the circle of Willis and from the major cerebral arteries. They form four groups which include the anteromedial, the anterolateral, the posteromedial and the posterolateral.
The internal carotid artery divides into two main branches called the middle cerebral artery and the anterior cerebral artery. The middle cerebral artery supplies blood to the frontoparietal somatosensory cortex.
The anterior cerebral artery supplies blood to the frontal lobes and medial aspects of the parietal and occipital lobes. Before this divide, the internal carotid artery gives rise to the anterior communicating artery and the posterior communicating artery.
The two vertebral arteries run along the medulla and fuse at the pontomedullary junction to form the midline basilar artery, also called the vertebro-basilar artery.
Before forming the basilar artery, each vertebral artery gives rise to the posterior spinal artery, the anterior spinal artery, the posterior inferior cerebellar artery (PICA) and branches to the medulla.
At the ponto-midbrain junction, the basilar artery divides into the two posterior cerebral arteries. Before this divide, it gives rise to numerous paramedian, short and long circumferential penetrators and two other branches known as the anterior inferior cerebellar artery and the superior cerebellar artery.
Venous Drainage Arteries provide the brain blood supply. The veins take the blood away,after the brain has taken nutrition from it.The prime course of venous drainage of the brain is through cerebral veins that empty into the dural venous sinuses and ultimately into the internal jugular vein.
Cerebral veins are divided into two groups, superficial and deep. The superficial veins usually lie on the surface of the cerebral hemispheres and empty themselves into the superior sagittal sinus. The deep veins drain internal structures and ultimately drain into the straight sinus.
Cerebral veins are thin-walled and valveless. They
are also interconnected by several functional anastomoses both within a group and between the superficial and deep groups. The numerous connections between cerebral veins and dural sinuses and venous systems of the meninges, skull, scalp and nasal sinuses assist the spread of thrombus or infection between these vessels.
The Veins of the Brain
The veins of the brain possess no valves, and their walls, owing to the absence of muscular tissue, are extremely thin. They pierce the arachnoid membrane and the inner or meningeal layer of the dura mater, and open into the cranial venous sinuses. They may be divided into two sets, cerebral and cerebellar.
The cerebral veins (vv. cerebri) are divisible into external and internal groups according as they drain the outer surfaces or the inner parts of the hemispheres.
The external veins are the superior, inferior, and middle cerebral.
The Superior Cerebral Veins (vv. cerebri superiores), eight to twelve in number, drain the superior, lateral, and medial surfaces of the hemispheres, and are mainly lodged in the sulci between the gyri, but some run across the gyri. They open into the superior sagittal sinus the anterior veins runs nearly at right angles to the sinus the posterior and larger veins are directed obliquely forward and open into the sinus in a direction more or less opposed to the current of the blood contained within it.
The Middle Cerebral Vein (v. cerebri media superficial Sylvian vein) begins on the lateral surface of the hemisphere, and, running along the lateral cerebral fissure, ends in the cavernous or the sphenoparietal sinus. It is connected (a) with the superior sagittal sinus by the great anastomotic vein of Trolard, which opens into one of the superior cerebral veins (b) with the transverse sinus by the posterior anastomotic vein of Labbé, which courses over the temporal lobe.
The Inferior Cerebral Veins (vv. cerebri inferiores), of small size, drain the under surfaces of the hemispheres. Those on the orbital surface of the frontal lobe join the superior cerebral veins, and through these open into the superior sagittal sinus those of the temporal lobe anastomose with the middle cerebral and basal veins, and join the cavernous, sphenoparietal, and superior petrosal sinuses.
The basal vein is formed at the anterior perforated substance by the union of (a) a small anterior cerebral vein which accompanies the anterior cerebral artery, (b) the deep middle cerebral vein (deep Sylvian vein), which receives tributaries from the insula and neighboring gyri, and runs in the lower part of the lateral cerebral fissure, and (c) the inferior striate veins, which leave the corpus striatum through the anterior perforated substance. The basal vein passes backward around the cerebral peduncle, and ends in the internal cerebral vein (vein of Galen) it receives tributaries from the interpeduncular fossa, the inferior horn of the lateral ventricle, the hippocampal gyrus, and the mid-brain.
The Internal Cerebral Veins (vv. cerebri internæ veins of Galen deep cerebral veins) drain the deep parts of the hemisphere and are two in number each is formed near the interventricular foramen by the union of the terminal and choroid veins. They run backward parallel with one another, between the layers of the tela chorioidea of the third ventricle, and beneath the splenium of the corpus callosum, where they unite to form a short trunk, the great cerebral vein just before their union each receives the corresponding basal vein.
The terminal vein (v. terminalis vena corporis striati) commences in the groove between the corpus striatum and thalamus, receives numerous veins from both of these parts, and unites behind the crus fornicis with the choroid vein, to form one of the internal cerebral veins. The choroid vein runs along the whole length of the choroid plexus, and receives veins from the hippocampus, the fornix, and the corpus callosum.
The Great Cerebral Vein (v. cerebri magna [Galeni] great vein of Galen) (Fig. 565), formed by the union of the two internal cerebral veins, is a short median trunk which curves backward and upward around the splenium of the corpus callosum and ends in the anterior extremity of the straight sinus.
The cerebellar veins are placed on the surface of the cerebellum, and are disposed in two sets, superior and inferior. The superior cerebellar veins (vv. cerebelli superiores) pass partly forward and medialward, across the superior vermis, to end in the straight sinus and the internal cerebral veins, partly lateralward to the transverse and superior petrosal sinuses. The inferior cerebellar veins (vv. cerebelli inferiores) of large size, end in the transverse, superior petrosal, and occipital sinuses.
The anterior circulation involves all the arteries that originate from the internal carotid arteries. It is responsible for the blood supply of the anterior and middle aspect of the brain. The arteries of this anterior circuit are:
- The internal carotid arteries
- The anterior cerebral arteries
- The anterior communicating artery
- The middle cerebral arteries
Internal carotid arteries
The internal carotid artery is one of two branches of the common carotid artery. It is responsible for supplying a large portion of the anterior and middle parts of the brain.
A new classification system divides the internal carotid artery into four parts cervical in the neck, petrous in the base of the skull, cavernous within the cavernous sinus and intracranial above the cavernous sinus.
Previously, the Cincinnati Classification (Bouthillier et. al., 1996) classified the internal carotid artery into seven segments cervical (C1), petrous (C2), lacerum (C3), cavernous (C4), clinoid (C5), ophthalmic or supraclinoid (C6), communicating or terminal (C7). It is undoubtedly easier to remember the new classification. However, here is a quick mnemonic to remember the C2-C7 intracranial segments of the internal carotid artery according to the Cincinnati classification - Please Let Children Consume Our Candy.
|New classification||Cervical part, petrous part, cavernous part, intracranial part|
|Cincinnati classification||C1 – Cervical Segment |
C2 – Petrous Segment
C3 – Lacerum Segment
C4 – Cavernous Segment
C5 – Clinoid Segment
C6 – Ophthalmic (Supraclinoid) Segment
C7 – Communicating (Terminal) Segment
Mnemonic (C2-C7): Please Let Children Consume Our Candy
When comparing the Cincinnati classification with the new system, the following differences can be observed:
- The part of the artery that was considered the lacerum segment is now referred to as a continuation of the petrous segment.
- The intracranial part involves the clinoid, ophthalmic and communicating portions (i.e. C5, C6, and C7)
The petrous part (C2) gives off the caroticotympanic and Vidian arteries. The cavernous segment (C4) gives numerous branches to the walls of the cavernous sinus and the surrounding nerves and dura mater. Of significance, the inferior hypophyseal artery also originates from this segment.
The ophthalmic segment (C6) gives of the ophthalmic artery and the superior hypophyseal artery. The communicating segment (C7) gives off the anterior cerebral (ACA), middle cerebral (MCA) and the anterior choroidal (AChA) arteries. The AChA supplies mesencephalic, diencephalic, and telencephalic derivatives.
Anterior cerebral artery
The anterior cerebral artery (ACA) is a much smaller branch of the internal carotid artery (when compared to the middle cerebral artery). It begins at the terminal portion of the internal carotid artery (after the ophthalmic branch is given off) on the medial part of the Sylvian fissure. It travels in an anteromedial course, superior to the optic nerve (CN II) towards the longitudinal cerebral fissure. Here it anastomoses with the contralateral counterpart via the short anterior communicating artery (AComm). The paired arteries then travel through the longitudinal cerebral fissure along the genu of the corpus callosum.
The anterior cerebral artery also gives off central and cortical branches. Central branches arise from the AComm to perfuse the optic chiasma, lamina terminalis, hypothalamus, para-olfactory areas, cingulate gyrus, and anterior columns of the fornix.
The cortical branches are named for the regions they supply. They are responsible for the somatosensory and motor cortices of the lower limbs.
- Frontal arteries supply the paracentral lobule, medial frontal and cingulate gyri, and the corpus callosum.
- Parietal branches perfuse the precuneus
- Orbital branches supply the frontal lobe (olfactory cortex, medial orbital gyrus, and gyrus rectus)
Anterior communicating artery
The anterior communicating artery (AComm) is a short, slender vessel that runs horizontally between the anterior cerebral arteries. The vessel crosses the ventral aspect of the median longitudinal fissure and is located anterior to the optic chiasm and posteromedial to the olfactory tracts. This vessel forms the anterior bridge between the left and right halves of the anterior circuit. It also completes the anterior part of the anastomotic ring known as the circle of Willis.
Middle cerebral artery
The middle cerebral artery (MCA) is the largest terminal branch of the internal carotid artery. It travels through the Sylvian (lateral) fissure before coursing in a posterosuperior direction on the island of Reil (insula). It subsequently divides to supply the lateral cortical surfaces along with the insula.
The vessel gives numerous tributaries to both central and cortical regions of the brain. The central branches are relatively small and include the lenticulostriate arteries that pass through the anterior perforated substance to supply the lentiform nucleus and the posterior limb of the internal capsule.
The cortical branches include the frontal, orbital, parietal, and temporal branches:
- The frontal arteries perfuse the inferior frontal, middle, and precentral gyri.
- The lateral orbital parts of the frontal lobe, as well as the frontal gyrus, are supplied by the orbital branches.
- The inferior parietal lobe, the inferior part of the superior parietal lobe, and the postcentral gyrus receive blood from the parietal branch.
- Several temporal arteries then go on to perfuse the lateral aspect of the temporal lobe.
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Research output : Contribution to journal › Article (Academic Journal) › peer-review
T1 - The major cerebral arteries proximal to the Circle of Willis contribute to cerebrovascular resistance in humans
N2 - Cerebral autoregulation ensures constant cerebral blood flow during periods of increased blood pressure by increasing cerebrovascular resistance. However, whether this increase in resistance occurs at the level of major cerebral arteries as well as at the level of smaller pial arterioles is still unknown in humans. Here, we measure cerebral arterial compliance, a measure that is inversely related to cerebrovascular resistance, with our novel non-invasive magnetic resonance imaging-based measurement, which employs short inversion time pulsed arterial spin labelling to map arterial blood volume at different phases of the cardiac cycle. We investigate the differential response of the cerebrovasculature during post exercise ischemia (a stimulus which leads to increased cerebrovascular resistance because of increases in blood pressure and sympathetic outflow). During post exercise ischemia in eight normotensive men (30.4 ± 6.4 years), cerebral arterial compliance decreased in the major cerebral arteries at the level of and below the Circle of Willis, while no changes were measured in arteries above the Circle of Willis. The reduction in arterial compliance manifested as a reduction in the arterial blood volume during systole. This study provides the first evidence that in humans the major cerebral arteries may play an important role in increasing cerebrovascular resistance.
AB - Cerebral autoregulation ensures constant cerebral blood flow during periods of increased blood pressure by increasing cerebrovascular resistance. However, whether this increase in resistance occurs at the level of major cerebral arteries as well as at the level of smaller pial arterioles is still unknown in humans. Here, we measure cerebral arterial compliance, a measure that is inversely related to cerebrovascular resistance, with our novel non-invasive magnetic resonance imaging-based measurement, which employs short inversion time pulsed arterial spin labelling to map arterial blood volume at different phases of the cardiac cycle. We investigate the differential response of the cerebrovasculature during post exercise ischemia (a stimulus which leads to increased cerebrovascular resistance because of increases in blood pressure and sympathetic outflow). During post exercise ischemia in eight normotensive men (30.4 ± 6.4 years), cerebral arterial compliance decreased in the major cerebral arteries at the level of and below the Circle of Willis, while no changes were measured in arteries above the Circle of Willis. The reduction in arterial compliance manifested as a reduction in the arterial blood volume during systole. This study provides the first evidence that in humans the major cerebral arteries may play an important role in increasing cerebrovascular resistance.
Yawning: Fact vs. Fiction
Many people instantly associate yawning with being bored or tired, Therefore, it is a common assertion that yawning is a way to prevent us from nodding off. For many years, people believed that yawning delivered a burst of oxygen to the brain and bloodstream, fighting against the urge to sleep and waking us up. While that sounds like a logical explanation, it isn&rsquot true, as studies haven&rsquot shown an increase in bloodstream oxygen levels following a yawn.
That answer isn&rsquot too far off, however. The crucial purpose of yawning does lie in the air that your body inhales, just not the oxygen specifically. The prevailing theory relates to the cooling effect that air can have on the brain. Given that the brain is the most metabolically active organ, it also tends to generate the most heat. The human body likes to self-regulate and keep temperature at a standard level.
When our bodies are hot, standing in front of a fan can be a quick fix, and the brain is the same way. When we yawn, we pull in cool air through the nasal and oral cavities, and that air comes in contact with all of the blood vessels in those densely packed areas. Many of those blood vessels carry blood directly to the brain, and the surge of air cools the blood, and thus the brain.
Research has shown that our brains are at their hottest right before we fall asleep, and immediately after we wake up. Unsurprisingly, those are the two times when yawns are most common: before you drift off to sleep and in the groggy morning hours before your first cup of coffee. It makes sense that most people associate yawning with being sleepy, but that&rsquos clearly just not the case.
We found 122 families with at least two FDRs who had imaging of the circle of Willis. Twelve FDRs from 10 different families were excluded because of poor imaging quality. Of the 122 probands, 76 had one FDR with imaging of the circle of Willis, 25 had two, six had three, four had four, four had five, three had six, two had seven, one had 11, and one had 14 FDRs with imaging of the circle of Willis.
The prevalence of each circle of Willis variation among probands and FDRs is given in Table 2. Of the 122 probands, 78 (63.9%) were women 27 (22.1%) had a history of aSAH, 17 (13.9%) had unruptured aneurysms, and no aneurysms were identified upon screening in the remaining 78. Of the 258 FDRs, 112 (43.4%) were women 11 (4.3%) had a history of aSAH, and 27 (10.5%) had unruptured aneurysms.
Overall, the concordance in circle of Willis variations was higher for the index families than for the comparison families (Table 3). Of the four different circle of Willis variations, only incomplete PcomA occurred statistically significantly more often within the same family.
In group 2, 123 families with at least two FDRs with imaging of the circle of Willis were identified. One family was excluded because of poor imaging quality in both imaged FDRs. One FDR of an included family was excluded because of poor imaging quality. Of the 122 remaining probands, 87 had one FDR, 20 had two FDRs, four had three, seven had four, three had five, and one had six FDRs with imaging of the circle of Willis 76 (62.3%) were women one (0.8%) had a history of aSAH, and 20 (16.4%) had unruptured aneurysms. Of the 188 FDRs, 103 (54.8%) were women, one (0.5%) had a history of aSAH, and 13 (6.9%) had unruptured aneurysms. Table 2 also shows the prevalence of each circle of Willis variation in group 2. Similarly to group 1, the concordance in circle of Willis variations was higher in index families than in comparison families, and of the separate variations only incomplete PcomA had a statistically significant higher occurrence within families (Table 3).
Comparison and meta-analysis of groups 1 and 2
Heterogeneity between the two study groups was small with I 2 values ranging from 0% to 28%. On combining the results of both groups by means of a meta-analysis we found a higher occurrence within families of circle of Willis variations overall and of the incomplete PcomA variation (Table 3). The classical circle and fetal posterior circulation (fetal PC) variations had mildly elevated ORs, but with wide confidence intervals and no statistical significance.
Anastomosis: medical or Modern Latin, from Greek ἀναστόμωσις, anastomosis, "outlet, opening", Gr ana- "up, on, upon", stoma "mouth", "to furnish with a mouth".  Thus the -stom- syllable is cognate with that of stoma in botany or stoma in medicine.
An anastomosis is the connection of two normally divergent structures.  It refers to connections between blood vessels or between other tubular structures such as loops of intestine.
In circulatory anastomoses, many arteries naturally anastomose with each other for example, the inferior epigastric artery and superior epigastric artery, or the anterior and/or posterior communicating arteries in the Circle of Willis in the brain. The circulatory anastomosis is further divided into arterial and venous anastomosis. Arterial anastomosis includes actual arterial anastomosis (e.g., palmar arch, plantar arch) and potential arterial anastomosis (e.g. coronary arteries and cortical branch of cerebral arteries). Anastomoses also form alternative routes around capillary beds in areas that don't need a large blood supply, thus helping regulate systemic blood flow.
An example of surgical anastomosis occurs when a segment of intestine, blood vessel, or any other structure are connected together (anastomosed). Examples include intestinal anastomosis, Roux-en-Y anastomosis or ureteroureterostomy. Surgical anastamosis techniques include Linear Stapled Anastomosis,  Hand Sewn Anastomosis,  End-to-End Anastomosis (EEA).  Anastomosis can be performed by hand or with an anastomosis assist device.  Studies have been performed comparing various anastomosis approaches taking into account surgical "time and cost, postoperative anastomotic bleeding, leakage, and stricture". 
Pathological anastomosis results from trauma or disease and may involve veins, arteries, or intestines. These are usually referred to as fistulas. In the cases of veins or arteries, traumatic fistulas usually occur between artery and vein. Traumatic intestinal fistulas usually occur between two loops of intestine (entero-enteric fistula) or intestine and skin (enterocutaneous fistula). Portacaval anastomosis, by contrast, is an anastomosis between a vein of the portal circulation and a vein of the systemic circulation, which allows blood to bypass the liver in patients with portal hypertension, often resulting in hemorrhoids, esophageal varices, or caput medusae.
In evolution, anastomosis is a recombination of evolutionary lineage. Conventional accounts of evolutionary lineage present themselves as the simple branching out of species into novel forms. Under anastomosis, species might recombine after initial branching out, such as in the case of recent research that shows that ancestral populations along human and chimpanzee lineages may have interbred after an initial branching event.  The concept of anastomosis also applies to the theory of symbiogenesis, in which new species emerge from the formation of novel symbiotic relationships.
In mycology, anastomosis is the fusion between branches of the same or different hyphae.  Hence the bifurcating fungal hyphae can form true reticulating networks. By sharing materials in the form of dissolved ions, hormones, and nucleotides, the fungus maintains bidirectional communication with itself. The fungal network might begin from several origins several spores (i.e. by means of conidial anastomosis tubes), several points of penetration, each a spreading circumference of absorption and assimilation. Once encountering the tip of another expanding, exploring self, the tips press against each other in pheromonal recognition or by an unknown recognition system, fusing to form a genetic singular clonal colony that can cover hectares called a genet or just microscopical areas. 
For fungi, anastomosis is also a component of reproduction. In some fungi, two different haploid mating types – if compatible – merge. Somatically, they form a morphologically similar mycelial wave front that continues to grow and explore. The significant difference is that each septated unit is binucleate, containing two unfused nuclei, i.e. one from each parent that eventually undergoes karyogamy and meiosis to complete the sexual cycle.
Also the term "anastomosing" is used for mushroom gills which interlink and separate to form a network. 
The growth of a strangler fig around a host tree, with tendrils fusing together to form a mesh, is called anastomosing. 
In geology, anastomosis refers to quartz (or other) veins displaying this property, which is often related to shearing in metamorphic regions. [ citation needed ]
Anastomosing streams consist of multiple channels that divide and reconnect and are separated by semi-permanent banks formed of cohesive material, such that they are unlikely to migrate from one channel position to another. They can be confused with braided rivers based on their planforms alone, but braided rivers are much shallower and more dynamic than anastomosing rivers. Some definitions require that an anastomosing river be made up of interconnected channels that enclose floodbasins,  again in contrast with braided rivers. Rivers with anastomosed reaches include the Magdalena River in Colombia,  the upper Columbia River in British Columbia, Canada,  the Drumheller Channels of the Channeled Scablands of the state of Washington, USA, and the upper Narew River in Poland.  The term anabranch has been used for segments of anastamosing rivers.
The Role Of Circle Of Willis Anatomy Variations In Cardio-embolic Stroke - A Patient-specific Simulation Based Study
We describe a patient-specific simulation based investigation on the role of Circle of Willis anatomy in cardioembolic stroke. Our simulation framework consists of medical image-driven modeling of patient anatomy including the Circle, 3D blood flow simulation through patient vasculature, embolus transport modeling using a discrete particle dynamics technique, and a sampling based approach to incorporate parametric variations. A total of 24 (four patients and six Circle anatomies including the complete Circle) models were considered, with cardiogenic emboli of varying sizes and compositions released virtually and tracked to compute distribution to the brain. The results establish that Circle anatomical variations significantly influence embolus distribution to the six major cerebral arteries. Embolus distribution to MCA territory is found to be least sensitive to the influence of anatomical variations. For varying Circle topologies, differences in flow through cervical vasculature are observed. This incoming flow is recruited differently across the communicating arteries of the Circle for varying anastomoses. Emboli interact with the routed flow, and can undergo significant traversal across the Circle arterial segments, depending upon their inertia and density ratio with respect to blood. This interaction drives the underlying biomechanics of embolus transport across the Circle, explaining how Circle anatomy influences embolism risk.
Our study is the first to investigate genetic versus environmental background of the CW variants with MRI in the frame of a twin study. Our results show that anatomical variants of the CW, which commonly occur and might pre-dispose to cerebrovascular events, might be shaped and influenced by environmental or stochastic factors however, our results do not allow us to draw conclusions on heritability. Anterior and posterior CW variants were not associated with current BMI, smoking, hypertension and hypercholesterolemia, and altered blood flow using TCD. Since CW variants are known risk factors of cerebrovascular ischemia, determining their origin is essential to develop preventive strategies. Our results should stimulate further research on the origin of CW variants in larger twin populations.