You have four carotid arteries. Two common carotid arteries run up either side of your neck, and each one splits into two branches, giving you a total of four: two internal carotid arteries and two external carotid arteries. Together, they are the primary blood supply to your brain, face, and neck.
How the Carotid Arteries Are Arranged
The system starts with your two common carotid arteries. They originate differently: the right common carotid branches off from a short trunk near the top of your chest, while the left common carotid comes directly off the aorta, the body’s largest artery. Despite this difference in starting point, both arteries travel the same path up either side of your neck, running alongside your windpipe and just beneath the large muscle you can feel when you turn your head.
At roughly the level of your Adam’s apple, near the C3 or C4 vertebra, each common carotid splits into two. This fork is called the carotid bifurcation, and it’s the spot where a doctor or paramedic checks your neck pulse. From this split, the internal carotid artery heads upward into the skull, and the external carotid artery branches out toward the face and scalp.
What Each Pair Supplies
Your two internal carotid arteries enter the skull through openings at the base of the brain. Once inside, they branch into several smaller arteries that supply blood to your brain and eyes. These arteries are responsible for delivering the oxygen and glucose your brain constantly demands. Because the brain has no energy reserves, even a brief interruption in this blood flow can cause serious damage.
Your two external carotid arteries stay outside the skull. Each one gives rise to eight branches that fan out to supply your face, scalp, tongue, jaw, and the outer structures of your head and neck. These are the arteries responsible for blood flow to your facial muscles, your teeth, your ear, and the tissue around your throat. When your face flushes from heat or exercise, it’s the external carotid branches working overtime.
Built-In Sensors at the Fork
The carotid bifurcation isn’t just a fork in the road. It contains two specialized structures that act as real-time monitors for your circulatory system.
The carotid sinus sits in the wall of the artery right at the split point. It contains baroreceptors, which are pressure-sensitive nerve endings that constantly measure your blood pressure. When pressure rises too high, signals from the carotid sinus tell your heart to slow down. When it drops, the opposite happens. This is why pressing firmly on the side of your neck can sometimes make you feel lightheaded.
The carotid body is a tiny cluster of cells near the same location, and it monitors the chemical composition of your blood. Its primary job is detecting oxygen levels, but it also responds to carbon dioxide, blood acidity, and even low blood sugar. When oxygen drops, the carotid body triggers reflexes that increase your breathing rate and raise your blood pressure to compensate. You have one carotid body on each side, matching the two bifurcation points.
Why Carotid Health Matters
Because the carotid arteries supply roughly 80% of the blood reaching your brain, narrowing in these vessels is a major risk factor for stroke. The buildup of fatty plaque inside the artery wall, called carotid stenosis, reduces blood flow and can also send clots or debris into the brain’s smaller vessels.
The risk scales with severity. People with more than 50% narrowing in a carotid artery face an annual stroke risk of about 1%. That risk climbs to 2% or higher when narrowing exceeds 70%, particularly if the person has already experienced symptoms like temporary vision loss, sudden weakness on one side of the body, or difficulty speaking. These warning episodes, called transient ischemic attacks, signal that a full stroke may follow.
Carotid stenosis often develops silently over years. Risk factors are the same ones that drive heart disease: high blood pressure, smoking, high cholesterol, diabetes, and a sedentary lifestyle. Doctors can detect narrowing with an ultrasound of the neck, a painless test that takes about 30 minutes. For significant blockages, treatment ranges from medications that reduce clot risk and lower cholesterol to procedures that physically open or bypass the narrowed segment.
Left and Right Are Not Identical
Although the two sides mirror each other in structure, their origins create a subtle but meaningful difference. The left common carotid artery branches directly from the aortic arch, giving it a slightly longer path than its counterpart on the right. The right common carotid arises from a shared trunk called the brachiocephalic artery before heading upward. This asymmetry is a leftover from how blood vessels develop in the embryo, when paired arches form and then selectively disappear to create the adult pattern.
In practice, this means the left carotid can sometimes be more exposed to the turbulent blood flow coming straight from the heart, which may slightly influence where plaque tends to accumulate. Surgeons and radiologists also need to account for the different origins when planning imaging or procedures.