Blood-Brain Barrier: Definition & Function

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  • 0:00 Definition of…
  • 1:15 Structure and Function
  • 2:38 Blood-Brain Barrier Breakdown
  • 3:46 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we will learn about the blood-brain barrier, which is an important layer of cells that helps keep our brains safe. We will discuss the structure and function of this barrier and look at what happens when it breaks down.

Definition of Blood-Brain Barrier

When you get sick, why don't bacteria or viruses invade your brain? The answer is because of the blood-brain barrier. This layer of cells keeps your brain safe from pathogens and other toxins. It is composed of brain cells and blood vessel cells. It plays a crucial role in protecting the most important organ in your body: the brain. The blood-brain barrier is like the moat outside a castle, preventing harmful invaders from entering. The image below shows the blood-brain barrier (BBB) separating the brain from the blood.

The blood-brain barrier separates the brain from the blood vessels
blood-brain barrier

To figure out how the blood-brain barrier does this, we first have to talk about the blood vessels that surround the brain. Blood vessels bring oxygen and glucose to your whole body, including the brain. However, when you eat or breathe in a pathogen or a toxin, sometimes it can also get into your blood. Your brain is such an important organ, that nature has evolved a way to let oxygen and glucose in, but keep other potentially harmful substances out. Other parts of the body do not have this capability.

Structure and Function

Since the blood-brain barrier is made of brain cells and blood vessel cells, we first need to explain how blood vessels work. Capillaries are tiny blood vessels that allow for distribution of carbon dioxide and nutrients in your body. Cells that make up the capillaries in your body are called endothelial cells. Normally, these cells are loosely linked together, letting nutrients, immune cells, and fluid in and out of the tissue. These types of blood vessels are like a loosely connected net, able to let things through.

In the brain however, capillaries are closely linked together using tight junctions. Think of the endothelial cells as a gate; they only let certain things in and out. In the image below, you can see different types of capillaries. Capillaries with connected endothelial cells are the type in the blood-brain barrier, as shown in the far left. Other types would be harmful to the brain.

The blood-brain barrier has tightly linked endothelial cells compared to other types of capillaries, as shown on the left
types of capillaries

To enhance this barrier, brain cells called astrocytes surround the outside of the capillaries. These cells latch onto the endothelial cells, creating an even stronger seal, allowing only healthy substances, like glucose and oxygen, to the brain. This combination of tightly linked endothelial cells and astrocytes is what makes up the blood-brain barrier.

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