Which of the following is a true statement about the blood-brain barrier?

• A. It is absent in all regions of the central nervous system.
• B. It allows all substances, including toxins and pathogens, to freely enter the brain tissue.
• C. It is composed primarily of loosely connected endothelial cells with large gaps.
• D. It restricts the passage of most large and hydrophilic molecules from the bloodstream into the brain.

Answer: (D)

Verified step by step guidance

1. Understand the concept of the blood-brain barrier (BBB): The blood-brain barrier is a selective permeability barrier that protects the brain from harmful substances in the bloodstream while allowing essential nutrients to pass through. It is primarily formed by tightly connected endothelial cells lining the brain's capillaries.
2. Analyze the first option: 'It is absent in all regions of the central nervous system.' This is incorrect because the blood-brain barrier is present in most regions of the central nervous system, although there are exceptions like the hypothalamus and the area postrema where the barrier is less restrictive.
3. Analyze the second option: 'It allows all substances, including toxins and pathogens, to freely enter the brain tissue.' This is incorrect because the blood-brain barrier is specifically designed to restrict the entry of harmful substances, including toxins and pathogens, into the brain.
4. Analyze the third option: 'It is composed primarily of loosely connected endothelial cells with large gaps.' This is incorrect because the endothelial cells in the blood-brain barrier are tightly connected by tight junctions, which prevent large molecules and most hydrophilic substances from passing through.
5. Analyze the fourth option: 'It restricts the passage of most large and hydrophilic molecules from the bloodstream into the brain.' This is correct because the blood-brain barrier selectively allows small, lipophilic (fat-soluble) molecules and certain essential nutrients to pass through, while restricting large and hydrophilic (water-soluble) molecules.