The Bloodstream

The bloodstream is the chief source of CNS infections, although it is very difficult for infectious agents to cross from it to the brain. This barrier to the passage of harmful agents is called the blood-brain barrier. It depends on special cells lining the central nervous system capillaries. These cells, only a single layer thick, are so close to each other that except for essential nutrients that are actively taken up, most molecules cannot pass through

26.1 Anatomy and Physiology 665

Cerebrospinal fluid formed

Cerebrospinal fluid formed

Spinal cord

Figure 26.2 Cerebrospinal Fluid (a) The fluid is formed in the ventricles, flows out over the surface of the brain and spinal cord, and is reabsorbed by structures at the top of the brain. (b) A sample of cerebrospinal fluid can be withdrawn from the subarachnoid space.

Spinal fluid

Spinal cord

Subarachnoid space

Lumbar

Spinal cord

Spinal cord

Subarachnoid space

Spinal fluid

Dura

Arachnoid

Dura

Arachnoid

Figure 26.2 Cerebrospinal Fluid (a) The fluid is formed in the ventricles, flows out over the surface of the brain and spinal cord, and is reabsorbed by structures at the top of the brain. (b) A sample of cerebrospinal fluid can be withdrawn from the subarachnoid space.

them. Consequently, many medications, including penicillin, cannot cross the barrier unless their concentrations in the blood are very high; antimicrobial medications must be able to cross the barrier in order to treat CNS infections. The blood-brain barrier generally prevents pathogens from entering nervous tissue except in rare cases when a high concentration of the infectious agent circulates for a long time in the bloodstream.

Infections of the face above the level of the mouth may involve veins that communicate through bone with veins on the brain surface. These communicating veins can provide a direct avenue for spread of infection to the central nervous system. A good rule, therefore, is to never squeeze pimples situated on the upper part of the face.

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