Blood brain barrier is not present in –
**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 capillaries, which are tightly joined together by tight junctions, and are surrounded by astrocytic end-feet that regulate the exchange of ions, nutrients, and waste products.
**Why the Correct Answer is Right**
The BBB is crucial for maintaining the brain's internal environment and preventing the entry of harmful substances, such as toxins, pathogens, and excess ions, into the brain. It achieves this by regulating the passage of molecules through the endothelial cells, using specific transport mechanisms, including carrier-mediated transport and receptor-mediated endocytosis. The BBB is particularly effective in preventing the entry of large molecules, such as proteins and peptides, into the brain.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the BBB is present in the brain capillaries, which are a critical component of the BBB.
* **Option B:** This option is incorrect because the BBB is not present in the spinal cord, which is also part of the CNS.
* **Option C:** This option is incorrect because the BBB is present in the choroid plexus, which is responsible for producing cerebrospinal fluid, but it is not absent in this region.
**Clinical Pearl / High-Yield Fact**
The BBB is disrupted in various neurological conditions, such as stroke, traumatic brain injury, and multiple sclerosis, leading to increased permeability and edema in the brain.
**Correct Answer: B. The spinal cord.**