Blood–brain barrier

Your brain has a special wall.

This wall protects your brain. It sits around your tiny blood tubes.

The wall is very picky. It lets good food in. It helps the brain get what it needs.

But the wall also keeps bad things out. It stops germs from entering. It stops many things in your blood from getting inside.

This wall is ready from the day you are born. It works hard to keep your brain safe. It is a very smart wall.

Your brain has a special border. We call this the blood–brain barrier.

It is a set of steps that controls what enters your brain. This border is made of many parts. It uses cells in the walls of tiny blood tubes. It also uses cells called astrocytes. These cells wrap around the tubes like feet.

The barrier is very picky. It lets small things like oxygen and carbon dioxide pass through easily. It also uses special proteins to move nutrients like glucose. Glucose is a type of sugar that the brain needs for power. However, the barrier blocks many other things. It stops germs and large molecules from getting inside. This helps keep the brain safe from harm.

This barrier is working even before you are born. It is very strong. It even blocks most medicines. This makes it hard for doctors to treat brain sickness. Some parts of the brain have different rules. These areas have tubes that are more open. They help the brain talk to the rest of the body.

Your brain is a very delicate organ. It needs a special way to stay safe from the blood. This is the job of the blood–brain barrier, or BBB. This border is a highly selective wall of cells. It regulates what moves between your blood and your central nervous system. This helps protect your brain from unwanted or harmful substances. Without this barrier, the brain might be damaged by many things in the blood.

How does this barrier work? It is made of several different parts working together. First, endothelial cells form the walls of the tiny blood tubes, called capillaries. These cells are joined by tight junctions. These junctions are made of proteins like occludin and claudins. These proteins act like a seal to stop things from leaking through. Next, pericytes are embedded in the capillary wall to help. Finally, astrocyte cells wrap their "feet" around the capillaries. These feet provide biochemical support to the cells.

The barrier is very picky about what it lets in. It allows small, non-polar molecules like oxygen and carbon dioxide to pass through. It also uses special transport proteins to move nutrients. These include things like glucose and amino acids. These nutrients are crucial for your brain to work. However, the barrier blocks many other things. It stops large molecules and most pathogens, which are germs. It even stops antibodies from the immune system. This keeps the brain insulated from many immune events.

Scientists have studied this barrier for a long time. In 1898, a study showed that certain salts did not affect animal behavior when injected. This suggested the salts did not enter the brain. In 1900, Max Lewandowsky may have first used the term "blood–brain barrier." Some people think the Russian scientist Lina Stern actually created the term. In 1913, Edwin Goldmann showed that dyes could enter the brain if injected directly into the cerebrospinal fluid. This proved there was a clear divide between the blood and the brain. This barrier is already working by the time a baby is born.

Because the barrier is so strong, it can be a hard job for doctors. The BBB blocks 100% of large-molecule medicines. It also blocks more than 98% of small-molecule drugs. This makes it very difficult to treat many brain disorders. Some parts of the brain do not have this strict barrier. These are called circumventricular organs, or CVOs. These areas have more open capillaries. They allow the brain to communicate with the rest of the body. This helps the brain sense signals in the blood.

The blood–brain barrier, or BBB, is a highly selective semipermeable border. It regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system. This system is vital because it protects the brain from harmful or unwanted substances found in the blood. By acting as a filter, the BBB ensures the brain maintains a stable environment. Without this protection, the brain could be damaged by many things circulating in the body.

This barrier is formed by several specialized components working in coordination. The primary structure consists of endothelial cells that make up the capillary walls. These cells are joined together by tight junctions. These junctions are composed of transmembrane proteins such as occludin, claudins, and junctional adhesion molecules. To stabilize these junctions, the cells use scaffolding proteins like tight junction protein 1 (ZO1). Additionally, pericytes are embedded within the capillary basement membrane. Finally, astrocyte cell projections, known as astrocytic feet, wrap around the endothelial cells to provide biochemical support.

The BBB uses different methods to move substances into the brain. It allows small, non-polar molecules to pass through via passive diffusion. This includes essential gases like oxygen and carbon dioxide, as well as certain hormones. For more complex needs, the barrier uses active transport. Specific transport proteins move metabolic products, such as glucose and amino acids, across the border. These nutrients are crucial for neural function. However, the barrier also restricts the passage of many things. It blocks large or hydrophilic molecules and most pathogens. It even prevents peripheral immune factors, such as antibodies and immune cells, from entering. This insulation protects the brain from damage caused by peripheral immune events.

While most of the brain is highly protected, some areas are different. These are called circumventricular organs, or CVOs. These structures, such as the area postrema and the pineal gland, have highly permeable capillaries. Unlike the rest of the brain, these capillaries allow for rapid detection of signals in the blood. CVOs act as points of bidirectional communication. Sensory CVOs allow the brain to detect circulating signals. Secretory CVOs help transport brain-derived signals into the blood. There are also specialized hybrid capillary zones. These zones act as rapid transit regions between the protected brain tissue and the open CVOs.

Scientists have been uncovering the secrets of this barrier for over a century. In 1898, researchers noticed that certain bile salts did not change animal behavior after injection. This suggested the salts could not reach the brain. By 1900, Max Lewandowsky may have first used the term "blood–brain barrier." However, some believe the Russian scientist Lina Stern actually created the term. In 1913, Edwin Goldmann provided crucial evidence for the barrier's existence. He injected dye directly into the cerebrospinal fluid of animal brains. He found the brain became dyed, while the rest of the body did not. This proved there was a clear compartmentalization between the blood and the brain.

The strength of the BBB creates significant challenges for modern medicine. The barrier excludes 100% of large-molecule neurotherapeutics from the brain. It also blocks more than 98% of all small-molecule drugs. This makes delivering medicine to specific brain regions a major hurdle for treating disorders. Researchers are looking for ways to bypass this. Some methods involve disrupting the barrier using ultrasound or chemical substances. Others try to use the brain's own transport systems, such as insulin carriers. Scientists are even studying nanotechnology to help move drugs across. Another non-invasive method is intranasal administration, which uses the olfactory or trigeminal nerves to reach the brain.

Maintaining the integrity of the BBB is essential for health. The barrier is functional by the time a person is born. However, it can be damaged by various neurological and systemic diseases. Conditions like Alzheimer's disease, epilepsy, and ischemic stroke are linked to BBB damage. Even liver failure can affect its function. When the barrier is compromised, it may allow proinflammatory factors to enter the brain. This can lead to metabolic dysfunction. Emerging evidence also suggests that a healthy gut microbiome is necessary to maintain the barrier during development and aging. Understanding these connections is key to treating brain diseases.