Two main sets of arteries feed blood to the human brain. At the front of the neck are the left and right common carotid arteries. The common carotids divide into the external and internal carotids. The carotid arteries are big, as far as arteries go, at about six millimetres in diameter for the average adult, and they account for about eighty percent of the total blood supply to our brains.
At the back of the neck, we find the left and right vertebral arteries—the verts. These major arteries run from a point behind the collarbones, up and through the vertebrae in our neck, and into the back of the brain. The verts originate from a junction with the subclavian arteries, another pair of large arteries that deliver blood into the arms. The verts are smaller than the subclavians, at about three millimetres in most adults.
As I’ve mentioned before, I look at medical subjects from a strictly layman’s perspective. One aspect of the arterial system feeding the brain that I find fascinating is the amount of redundancy that is built into it. In most people, the left and right vertebrals do the same job. The two separate arteries meet up in the back of the skull to form the larger basilar artery. It is the basilar artery that supplies the remainder of the blood to the brain, about twenty percent of the total.
Redundancy extends further than just between the left and right vertebrals. It also includes connectivity between the two sets of arteries feeding the brain, the carotids and the vertebrals. At the base of the brain, where the various arteries come together, there is a circle of connections between the carotid and vertebral arteries. This arrangement of communicating arteries is called the Circle of Willis, named for Thomas Willis, an English doctor who discovered it in the seventeenth century. Several other arteries meet up in the Circle of Willis and then take blood away to other parts of the brain.
The reason for the Circle of Willis is quite simple and elegant. If any of the main arteries is blocked, or occluded, then the parts of the brain that depend on the blocked artery can still get blood supply. In other words, blood can flow around the circle to get to its destination by another route.
There can be differences in each person’s arterial connections, and many people have an arrangement other than a textbook Circle of Willis. This is what a neurologist would refer to as an “incomplete Circle of Willis”. It doesn’t seem that having an incomplete Circle of Willis is necessarily a problem. Besides, there is no option to fix it, given the complexities involved. In other words, an incomplete Circle of Willis is likely something that would only ever be discovered in an autopsy.
So, why am I even writing about this?
I know from personal experience that having an incomplete Circle of Willis can lead to higher risks of stroke or other neurological problems.
Even if that weren’t the case, I look at this system from a chemical engineer’s perspective. And when I do, I can’t help but be fascinated by the perfection of the design for this most essential system.
Mine just doesn’t happen to be perfect.
My Second Running Life 