Examples of Substances Transported Across the Blood-Brain Barrier

📚 Quick Study Guide

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• The blood-brain barrier (BBB) is a highly selective semipermeable membrane that separates the circulating blood from the brain extracellular fluid in the central nervous system (CNS).
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• Its primary function is to protect the brain from harmful substances while allowing essential nutrients to reach it.
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• Substances can cross the BBB through various mechanisms:
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• Transcellular pathway: Substances pass directly through the endothelial cells.
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• Paracellular pathway: Substances pass between endothelial cells (highly restricted due to tight junctions).
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• Transporter-mediated transport: Specific transporters facilitate the movement of molecules.
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• Receptor-mediated transcytosis: Molecules bind to receptors on the cell surface, are internalized, and then released on the other side.
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• Examples of substances that cross the BBB:
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• Water: Via aquaporins.
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• Gases: Oxygen ($O_2$) and Carbon Dioxide ($CO_2$) by diffusion.
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• Glucose: Via GLUT1 transporter.
amino acids: Via specific amino acid transporters. 💊
• Lipid-soluble molecules: Such as some drugs (e.g., anesthetics).
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• Substances that generally do NOT cross the BBB easily:
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• Large molecules: Such as proteins.
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• Most antibodies.
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• Highly charged molecules.

Practice Quiz

1. Which of the following substances crosses the blood-brain barrier via the GLUT1 transporter?
   1. Proteins
   2. Glucose
   3. Antibodies
   4. Large Lipids
2. What is the primary function of the blood-brain barrier?
   1. To produce red blood cells
   2. To protect the brain from harmful substances
   3. To regulate body temperature
   4. To synthesize neurotransmitters
3. Which pathway involves substances passing directly through endothelial cells of the BBB?
   1. Paracellular pathway
   2. Receptor-mediated transcytosis
   3. Transcellular pathway
   4. Advective pathway
4. Which of the following gases crosses the BBB by diffusion?
   1. Nitrogen
   2. Helium
   3. Oxygen
   4. Ammonia
5. Large molecules like proteins typically cross the BBB:
   1. Easily via diffusion
   2. Via GLUT1 transporters
   3. With difficulty
   4. Through paracellular pathways
6. Which of the following is NOT a typical characteristic of substances that cross the BBB easily?
   1. Small size
   2. High lipid solubility
   3. High charge
   4. Presence of specific transporters
7. Receptor-mediated transcytosis involves:
   1. Direct diffusion through the cell membrane
   2. Binding to receptors on the cell surface, internalization, and release
   3. Passing between endothelial cells
   4. Active transport against a concentration gradient