Back To CourseBiology 105: Anatomy & Physiology
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I've got a great idea. We should take a trip to the mall and do some shopping together. If we can't afford to actually buy anything, that's okay. We can at least do some window shopping. As we walk into this very large mall, we see a directory and note that the store we want to go to is located on the second floor of the mall. Since this is a very busy mall with lots of people walking around, the architect of the mall did a very nice job designing several methods for us to get to the second floor. We can either walk up the stairs, take the elevator, or we can use the escalator to make it to the second floor. Why in the world would the architect create more than one way to get up to one floor? It takes quite a bit more money to construct all of this.
But, I think you already know the answer. What if the elevator breaks, the escalator stops working, or the stairs are shut down for repairs? If one or two of those occur, we will always have at least one other option to use in order to get to the second floor. The architect had a lot of foresight in making sure that congestion, in this case because of lots of people, can be relieved through more than one avenue.
Your brain is just as, if not more, special in this sense. Think of the mall as your brain. The people trafficking through the mall carrying money they want to spend in a store are like your red blood cells carrying oxygen to spend in brain cells. The red blood cells must have a way of making it into the mall, making their way around it, up and down, left and right. They do this via channels, like the halls in the mall, called blood vessels. And, like the stairs, elevators, and escalators in the mall, the blood vessels in the brain provide red blood cells more than one option of accessing the same exact point in the brain in case one option isn't working, is too congested, or is completely blocked off.
You'll find out exactly how this works in your brain as we explore the many arteries which supply your brain with blood, how they connect, and what important structures they form.
Your brain has quite a few arteries which supply it with oxygen. There's a reason for this plethora of choice. It's because your brain needs a large amount of oxygen constantly delivered to it in order to not only function on its own, but also to keep the rest of your body functioning as well. If the brain had only one artery supplying it, and it were to get blocked by a clot, then you would die almost instantaneously. This is why having so many ways of getting blood to your brain is critical: if one channel, or vessel, is blocked, there's always at least one other way to get around the blocked passageway.
With this in mind, let's take a look at some of the major arteries supplying your brain with oxygenated blood.
The most important artery running up the side of your neck is called the common carotid artery. It splits into two branches. One of the branches is called the external carotid artery, which supplies your neck and face with oxygenated blood. The other branch of the common carotid artery, which supplies the brain with oxygenated blood, is the internal carotid artery. The internal carotid artery, again, is a branch of the common carotid artery that supplies the brain with oxygenated blood. Each side of your body, more specifically, each side of your neck and head, has one internal carotid artery supplying blood to the brain. Therefore, you have a grand total of two internal carotid arteries.
As the internal carotid artery enters your brain, it gives off a branch called the anterior cerebral artery. The anterior cerebral artery is a branch of the internal carotid artery supplying the anterior, or front, portions of your brain with oxygenated blood. This artery is also important because it provides another avenue by which your brain can be supplied with blood, just in case another passageway is closed off.
In addition, because you have two internal carotid arteries and each one has a branch called the anterior cerebral artery, it should make sense that you have two anterior cerebral arteries as well.
Another artery entering your brain is called the the posterior cerebral artery, which supplies the posterior, or structures at the rear of your brain, with oxygenated blood. There is one posterior cerebral artery on each side of your body. Furthermore, each posterior cerebral artery connects to the internal carotid artery on the same side of the brain in order to complete a very special structure in the brain.
By now, you're hopefully dying to find out what this special structure is. The importance of blood supply to your brain explains why there are so many pathways for oxygenated blood. As the internal carotid artery enters your head, we know it gives off a branch to one side called the anterior cerebral artery. This anterior cerebral artery will then connect to the anterior cerebral artery on the other side of the head and, therefore, by extension, to the internal carotid artery on the other side of the head as well. We also know that the posterior cerebral arteries will also connect to the internal carotid artery through a small vessel. This, therefore, completes a circle of connected arteries, located at the base of your brain, which supply the brain with blood, and we call this structure the Circle of Willis.
Look at the circle above. You should be able to see the importance of it very clearly. Your oxygenated blood travels from your heart, into the internal carotid arteries, and into this circle. If we block off the anterior cerebral artery arising from the left carotid artery, the blood can take another direction, or shunt, to where the posterior cerebral arteries are and continue on into smaller vessels to reach important parts of the brain. It's just like our mall example. If the escalators aren't working, no problem! We'll just detour and use the elevator instead. If the anterior cerebral artery isn't working, no problem! We'll just detour and use another artery that connects to the circle.
The Circle of Willis is, therefore, basically a bunch of collateral connections between blood vessels we term anastomoses. An anastomosis, which is singular for anastomoses, is a connection between blood vessels. Anastomoses provide another avenue by which blood can travel from point A to point B; it's like a detour on a highway.
The blood supplying your brain isn't just full of red blood cells; it has many other particles dissolved in it, collectively called solutes. A lot of these solutes can be very dangerous to your brain's nerve cells and must be kept out of the brain at all costs. Therefore, your brain has a critical structure involving virtually every blood vessel which enters it. This structure restricts the passage of solutes into the brain and is called the blood brain barrier. The blood brain barrier is often abbreviated as the BBB.
The blood brain barrier works by having very tight adhesions between the cells which line the inner part of your blood vessels. These tight adhesions prevent the passage of many solutes out of the blood and into the brain. You can think of the adhesions between the cells which line the inside of your blood vessels, the endothelial cells, as super glue between two pieces of glass - not much is going to get through them.
In addition, the blood brain barrier has cells which actually surround blood vessels on the outside with tiny little feet. These little feet surround the blood vessels in order to provide an extra layer of support and protection to the endothelial cells, which also make up your blood brain barrier. These tiny little feet are provided by cells we call astrocytes because they look like stars under a microscope. This should make sense because the Greek root 'astro' refers to a star and the Latin root 'cyte' refers to a cell.
Remember, the blood brain barrier's main function is to prevent bad substances from getting into the brain from the arteries, which supply it with blood and other nutrients.
Let's recap the most important points we have learned in this lesson.
Your brain is supplied with a lot of oxygenated blood. This blood travels through vessels called arteries. The main arteries supplying your brain include the internal carotid artery, which is a branch of the common carotid artery that supplies your brain with oxygenated blood; the anterior cerebral artery, which is a branch of the internal carotid artery supplying the brain's anterior portions with oxygenated blood; and the posterior cerebral artery, which supplies the brain's posterior structures with oxygenated blood.
These three arteries have connections with one another, and the connection between one blood vessel and another is called an anastomosis. The internal carotid artery, anterior cerebral artery, and posterior cerebral artery anastomose with one another into a circle of connected arteries, located at the base of your brain, which supply the brain with blood, called the Circle of Willis. These anastomoses provide the blood traveling to your brain with plenty of options when it comes to making a choice of which way to go in case there is a blockage somewhere along its path.
Finally, your brain is protected from dangerous substances by tight adhesions between cells lining your blood vessels and tiny little feet that surround these blood vessels. All of these factors restrict the passage of solutes into the brain. Collectively, this combination of factors is known as the blood brain barrier. The blood brain barrier is what helps good things get into your brain and the bad things to stay out of it in the first place.
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Back To CourseBiology 105: Anatomy & Physiology
16 chapters | 179 lessons | 15 flashcard sets