**Core Concept**
The blood-brain barrier (BBB) is a highly selective permeable barrier that separates the circulating blood from the brain's extracellular fluid in the central nervous system (CNS). It is composed of endothelial cells that line the brain's capillaries, astrocytes, and pericytes, which work together to regulate the passage of substances between the blood and the brain.

**Why the Correct Answer is Right**
The BBB is formed by the tight junctions between the endothelial cells, which create a continuous and impermeable layer. The astrocytes extend their foot processes to the endothelial cells, further reinforcing the barrier. The pericytes, which are specialized cells that surround the capillaries, also contribute to the maintenance of the BBB. This complex structure prevents the free diffusion of substances, including ions, nutrients, and waste products, between the blood and the brain.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because the BBB is not formed by the blood vessels themselves, but rather by the specialized cells that line the capillaries.

**Option B:**
This option is incorrect because the BBB is not a physical barrier, but rather a selective permeable barrier that allows certain substances to pass through while restricting others.

**Option C:**
This option is incorrect because the BBB is not a single layer of cells, but rather a complex structure composed of multiple cell types, including endothelial cells, astrocytes, and pericytes.

**Clinical Pearl / High-Yield Fact**
The BBB is essential for maintaining the homeostasis of the CNS, and disruptions to the BBB can lead to a variety of neurological disorders, including multiple sclerosis, Alzheimer's disease, and stroke.

**Correct Answer: C. The endothelial cells, astrocytes, and pericytes.**