The blood-brain barrier is the network of blood vessels that supply the central nervous system. While they perform the same functions as other blood vessels — that is, delivering oxygen and nutrients — the vessels in the blood-brain barrier also regulate movement between the blood and the brain. This regulation controls the balance of the central nervous system (CNS) and protects the brain.
The blood-brain barrier has unique properties that allow it to regulate the movement of cells, ions, and molecules between the blood and the brain. These microvessels are heavily restrictive, maintaining balance to support proper brain function and protect the CNS from inflammation, injury, toxins, pathogens, and disease. The blood-brain barrier is complex and made of many types of cells.
Endothelial cells in the blood-brain barrier act as a highly selective nutrient transporter, carrying nutrients and waste products from the CNS into the blood. Endothelial cells exist throughout the body, but those in the CNS are unique. They have more mitochondria to produce more energy and a low level of leukocyte adhesion, limiting the number of immune cells that pass into the CNS.
Pericytes cover the walls of the system of arteries and veins. These cells contain proteins that cause them to contract, controlling the diameter of the small vessels. Pericytes regulate the formation of new blood vessels and immune cell infiltration and play a role in wound healing. These cells support the formation of the blood-brain barrier and maintain it as the body ages.
Many other cells make up the blood-brain barrier. These include the basement membrane that, among other things, provides an additional barrier for cells to cross before reaching the brain; astrocytes, which link the blood vessels and neurons and regulate blood flow in response to activity in the brain; and immune cells, which eliminate debris and impact vascular permeability.
One mechanism the blood-brain barrier uses is passive transport. Carrier cells are the primary way that many things cross the blood-brain barrier; the following can hitch a ride on these cells to get across: carbohydrates, amino acids, nucleotides, fatty acids, vitamins, sodium, glucose, thyroid hormones, insulin, and leptin. Other small molecules can diffuse through the membrane without a carrier, including oxygen, carbon dioxide, caffeine, ethanol, and barbiturates.
Some elements cross the blood-brain barrier using active transport. These transporters prevent unwanted substances from accumulating in the CNS by carrying them across the blood-brain barrier back into the plasma. Active transport also maintains the ion concentration in the brain by maintaining high concentrations of sodium (Na) and low concentrations of potassium (K). This mechanism also plays a critical role in various reactions that regulate cerebral blood flow and pH in the CNS.
The blood-brain barrier impacts how drugs reach the CNS and cerebral spinal fluid. Some lipid-soluble drugs cross the blood-brain barrier easily, while water-soluble drugs cross slowly. Choosing medications to treat the CNS is complicated as so many factors determine how effectively they can enter the brain. Some drugs can reliably cross the blood-brain barrier, but, in some extreme cases, direct injection of drugs into the CNS is required.
Researchers are looking for ways to bypass the blood-brain barrier. One approach is injecting hypertonic sugar solutions and disrupting the blood-brain barrier using ultrasound and microbubbles, allowing the drugs a temporary window to cross through. This method is experimental, and some researchers are concerned that it may cause inflammation and injury due to the restriction of blood. Another approach in testing is using so-called Trojan horse molecules to enclose treatment nanoparticles in a coating that allows them to cross the blood-brain barrier.
Blood-brain barrier disruption plays a role in some chronic neurological diseases, including ALS, Parkinson's disease, multiple sclerosis, and epilepsy. Researchers generally believe these disruptions to be secondary to the primary disease process, but they may be a cause in some cases. The cause of blood-brain barrier disruption itself is unknown. Studies show that these disruptions are multimodal — disrupting junctions and causing changes in metabolic processes and transport — though it is still unknown how these things interact.
The integrity of the blood-brain barrier is also affected by acute neurological disorders and injuries. Ischemic strokes cause fluid to leak into the brain, and white blood cells infiltrate the area, disrupting homeostasis and causing edema. Some studies show that a stroke combined with other conditions, like high blood sugar or hypertension, can worsen the degradation of the blood-brain barrier.
With traumatic brain injury, the vasculature of the blood-brain barrier is damaged. After the injury, inflammation further disrupts the blood-brain barrier, allowing things that should be blocked to pass freely into the brain.
This site offers information designed for educational purposes only. You should not rely on any information on this site as a substitute for professional medical advice, diagnosis, treatment, or as a substitute for, professional counseling care, advice, diagnosis, or treatment. If you have any concerns or questions about your health, you should always consult with a physician or other healthcare professional.