You have six carotid arteries in your neck, arranged as a matched set of three on each side. Each side starts with a single common carotid artery that climbs upward and then splits into two branches: an internal carotid artery, which feeds the brain, and an external carotid artery, which supplies the face, scalp, and other structures outside the skull. The real story of these arteries goes well beyond the count, though, because the way they branch, where they sit, and how they change over a lifetime all have surprisingly practical consequences.
The Two Common Carotid Arteries and Where They Come From
The journey starts just above your heart. On the right side, the common carotid artery branches off the brachiocephalic trunk, a short, stout vessel that also gives rise to the right subclavian artery heading toward the arm. On the left side, the common carotid artery comes directly off the aortic arch, the large curved vessel that channels blood out of the heart. This difference in origin means the left common carotid is slightly longer than the right, since it has to travel a bit farther before entering the neck.1PMC Central (Anatomy & Cell Biology). Right and left common carotid arteries arising from the branchiocephalic, a rare variation of the aortic arch – Section: Discussion
Once they enter the neck, both common carotids run upward on either side of the trachea, tucked within a sheath of connective tissue alongside the internal jugular vein and the vagus nerve. If you gently press two fingers along the front edge of the large neck muscle (the sternocleidomastoid), you can usually feel the common carotid pulse. Paramedics and first responders check this spot because it is one of the most reliable places to confirm a heartbeat.
The Bifurcation and Its Two Branches
Roughly at the level of the upper border of the thyroid cartilage (the Adam’s apple), each common carotid artery divides into its two terminal branches. This splitting point is called the carotid bifurcation, and its exact height varies from person to person. Anatomical studies note that the thyroid cartilage is the most commonly cited landmark for locating the bifurcation during clinical exams, but the position can sit higher or lower depending on individual anatomy.2PubMed. Topography of carotid bifurcation: considerations for neck examination
The internal carotid artery (ICA) continues upward without giving off any branches in the neck. It enters the skull through an opening in the temporal bone and becomes the main supplier of blood to the brain, contributing to the large arterial circle at the base of the skull known as the circle of Willis. The external carotid artery (ECA), by contrast, is a prolific brancher. Almost immediately after the split, it begins sending off smaller arteries that supply the thyroid gland, tongue, face, jaw, scalp, and the membranes surrounding the brain. If you have ever watched a scalp wound bleed profusely, you have seen the external carotid system at work.
Built-In Sensors at the Fork
The bifurcation is not just a plumbing junction. It houses two specialized sensory structures that quietly monitor your blood around the clock. The carotid sinus is a slight widening at the very beginning of the internal carotid artery. Its walls are packed with stretch-sensitive nerve endings that detect changes in blood pressure. When pressure rises, those sensors fire signals through the glossopharyngeal nerve to the brainstem, which responds by slowing the heart rate and relaxing blood vessels to bring pressure back down. When pressure drops, the signaling decreases and the body ramps things up. This feedback loop, the arterial baroreflex, is your primary defense against sudden swings in blood pressure.3PubMed Central. Denervation of carotid baro- and chemoreceptors in humans
Nestled right beside the sinus is the carotid body, a tiny cluster of specialized cells that acts as a chemical sensor. It continuously samples the blood for oxygen levels, carbon dioxide levels, and acidity. If oxygen drops or carbon dioxide climbs, the carotid body triggers faster and deeper breathing. These two structures together explain why pressure on the side of the neck can cause dizziness or even fainting: you are mechanically activating the baroreflex, fooling the brainstem into thinking blood pressure is dangerously high.
Why the Bifurcation Is a Trouble Spot
The geometry of the carotid bifurcation creates flow patterns that make it one of the most common sites for atherosclerotic plaque to develop. Blood traveling through a straight tube flows in smooth, parallel layers. At a branch point, though, the flow becomes turbulent. Areas where the vessel wall faces low or oscillating shear stress (the drag force of flowing blood) are more prone to plaque formation, and the outer wall of the carotid sinus is particularly vulnerable. Research has confirmed that the shape of a person’s bifurcation, including the angle between the two branches, influences how much disturbed flow occurs and how likely plaque is to form there.4PubMed. Association Between Carotid Bifurcation Geometry and Atherosclerotic Plaque Vulnerability: A Chinese Atherosclerosis Risk Evaluation Study
This matters enormously for stroke risk. At least a fifth of ischemic strokes are caused by a blood clot or piece of plaque breaking off from the carotid bifurcation or the internal carotid artery and traveling to the brain.5PubMed Central. Unstable carotid artery plaque: new insights and controversies in diagnostics and treatment Screening for carotid artery disease typically starts with ultrasound, which can visualize the bifurcation and measure how much the channel has narrowed. When more detail is needed, CT angiography and MR angiography can map the arteries and the plaque in finer resolution.6PubMed. Diagnostic accuracy of colour Doppler ultrasonography, CT angiography and blood-pool-enhanced MR angiography in assessing carotid stenosis: a comparative study with DSA in 170 patients
If a carotid artery is severely narrowed and the patient is at high stroke risk, treatment options include carotid endarterectomy, a surgical procedure that opens the artery and physically removes the plaque, and carotid artery stenting, where a mesh tube is threaded into the narrowed section to hold it open. Both are established treatments, and large trials have compared them head to head.7PubMed Central. Stenting versus Endarterectomy for Treatment of Carotid-Artery Stenosis A Cochrane review has assessed their relative outcomes, confirming that stenting is an accepted alternative to surgery.8PubMed Central. Carotid artery stenting versus endarterectomy for treatment of carotid artery stenosis The choice between them depends on factors like the patient’s age, the anatomy of the blockage, and comorbid conditions.
How the Brain Protects Itself When a Carotid Is Blocked
One of the more reassuring features of the brain’s blood supply is its built-in redundancy. The circle of Willis at the base of the brain connects the internal carotid arteries to each other and to the vertebral arteries, which supply blood from the back of the neck. If one carotid artery becomes blocked, blood can often reroute through these connections. Studies using flow-sensitive MRI in patients with a completely occluded carotid have found that rerouting through the anterior communicating artery (connecting the two sides) and the posterior communicating artery (connecting front and back circulation) is common.9PubMed. The role of the circle of Willis in carotid occlusion: assessment with phase contrast MR angiography and transcranial duplex
This backup system is not perfect. The circle of Willis varies quite a bit from person to person; some people are missing one or more of the connecting arteries entirely, which leaves them more vulnerable if a carotid is compromised. Still, the principle holds: the brain does not rely on a single pipeline for its blood supply, and the six carotid arteries are only part of a larger network that includes the vertebral arteries entering through the back of the skull.
Anatomical Variations and Missing Arteries
Not everyone has the textbook arrangement of six carotid arteries. Congenital absence of the internal carotid artery is extremely rare but well documented. When it occurs, the brain develops alternative blood supply routes during fetal development, and many people with this condition are never aware of it until an imaging scan reveals the absent vessel.10PubMed Central. Internal Carotid Artery Agenesis: A Rare Entity Even rarer is congenital absence of the common carotid artery itself; a comprehensive review found only about 87 recorded cases in the medical literature spanning centuries. In those cases, the internal and external carotid arteries still exist, but they arise separately rather than sharing a common trunk.11PubMed Central. Congenital Aplasia of the Common Carotid Artery: A Comprehensive Review
There are also variations at the aortic arch level. The “normal” pattern, where three separate branches come off the arch, occurs in most people, but a significant minority have a different arrangement. Sometimes the left common carotid artery shares its origin with the brachiocephalic trunk (a so-called “bovine arch”), or additional branches arise directly from the arch. These variants are usually harmless, but surgeons and radiologists need to know about them before performing procedures on the chest or neck.
Another category of variation involves embryonic blood vessels that normally disappear during development but occasionally persist into adult life. During early fetal development, temporary connections exist between the carotid system and the vertebrobasilar system at the back of the brain. These are supposed to regress, but sometimes one survives, creating an extra connection that can alter blood flow and slightly increase the risk of aneurysm formation.12PubMed Central. The transclival artery: a variant persistent carotid-basilar arterial anastomosis not previously reported The most common of these persistent connections is the trigeminal artery, followed by the hypoglossal, proatlantal, and otic arteries.13PubMed Central. Carotid-vertebrobasilar Anastomoses with Reference to Their Segmental Property
How Your Carotid Arteries Change with Age
The carotid arteries you have at age twenty are not the same arteries you have at age seventy, even if nothing pathological has happened. One of the most visible changes is increasing tortuosity: the arteries become more twisty and kinked over time. MR angiography studies comparing younger and older adults have found that internal carotid arteries in older subjects are significantly more tortuous, with measurably higher kinking and bending.14PubMed Central. Age-Related Tortuosity of Carotid and Vertebral Arteries: Quantitative Evaluation With MR Angiography The bifurcation angle tends to widen, and the diameter of the carotid bulb (the slight expansion at the base of the internal carotid) increases.15PubMed. Age and disease-related geometric and structural remodeling of the carotid artery
The underlying cause is the gradual breakdown of elastin, the protein that gives artery walls their springiness. As elastin fibers fragment and degrade, the artery wall becomes stiffer and less able to maintain its straight course under the force of pulsating blood. Hypertension accelerates this process. Research has shown that both advancing age and the duration of high blood pressure contribute independently to carotid tortuosity. The elastic layer degenerates, blood vessel resistance increases, and smooth muscle cells in the artery wall are remodeled in ways that promote further twisting.16PubMed Central. Age and duration of hypertension are associated with carotid artery tortuosity Severe tortuosity can sometimes kink the artery enough to reduce blood flow, though this is uncommon.
Carotid Artery Dissection in Younger Adults
While atherosclerosis is typically a disease of middle and older age, a different threat to the carotid arteries disproportionately affects younger people: spontaneous dissection. This occurs when the inner lining of the artery wall tears, allowing blood to seep between the layers and form a pocket (a false lumen) that can narrow or block the artery. Though rare overall, spontaneous cervical artery dissection is one of the more common causes of stroke in younger adults.17PubMed Central. Spontaneous Bilateral Internal Carotid Artery Dissections in a Young Female with Headache
Dissection can happen after minor trauma, vigorous exercise, chiropractic neck manipulation, or sometimes with no identifiable trigger at all. Symptoms often start with sudden neck pain or a severe headache on one side, sometimes accompanied by a drooping eyelid on the same side (Horner syndrome, caused by disruption to sympathetic nerve fibers running alongside the carotid). If a clot forms at the dissection site and travels to the brain, a stroke can follow. Dissection can occur in one or both internal carotid arteries, and bilateral cases, while less common, are well documented.
The Name Itself Tells a Story
The word “carotid” traces back to the ancient Greek word “karos,” meaning deep sleep or stupor. The name stuck because the ancient Greeks observed that compressing these arteries in the neck caused a person to lose consciousness. The physician Rufus of Ephesus, active in the first century, is credited with connecting the word to the observation that pressure on these vessels induced a stupor-like state. Even earlier, the idea was embedded in art: a metope on the Parthenon in Athens depicts a centaur gripping a warrior’s neck during the legendary battle of the Centauromachy, compressing the carotid area. Galen, the influential Roman-era physician, refined the understanding by noting that blocking arterial flow, not just pressing on the neck, was the mechanism behind the loss of consciousness he called apoplexy.18PubMed Central. History of Carotid Artery Reconstruction around the World and in Japan – Section: Relationships between Carotid Artery Lesions and Cerebral Infarction
The ancient intuition was essentially correct: these arteries carry so much of the brain’s blood supply that blocking them, even briefly, has immediate neurological effects. What the ancients could not have known is just how elaborate the system is, with six distinct carotid vessels, specialized sensors, and a network of backup routes, all working continuously to keep the brain perfused.
Carotid Arteries in Other Animals
Humans are far from the only species with a paired carotid system, but other animals have made some creative modifications. Birds, for example, have right and left internal carotid arteries that can be connected by an intercarotid anastomosis, a bridging vessel between the two sides. The shape and thickness of this bridge varies by species. In some gallinaceous birds like roosters and pheasants, the connection forms an “H” shape, while in turkeys and geese it takes on more of an “X” configuration.19Academia.edu. Patterns of the internal and cerebral carotid arteries in various avian species: a comparative study
Even-toed ungulates like cattle, sheep, and antelope have evolved a remarkable structure called the carotid rete, a dense meshwork of small arteries derived from the carotid system that sits at the base of the brain. This network acts as a heat exchanger. Venous blood cooled by evaporation in the nasal passages flows past the warm arterial blood heading to the brain, chilling it before it arrives. The result is that these animals can regulate brain temperature independently of body temperature, a major advantage in hot climates. Odd-toed ungulates like horses lack this structure, which may have given artiodactyls a significant edge in colonizing warm, dry habitats.20PubMed Central. The carotid rete and artiodactyl success It is a reminder that the same basic arterial plan can be tuned by evolution to meet very different environmental demands.