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Below the arachnoid lies the subarachnoid space.
The arteries and veins
that carry blood to and from the brain lie on the pial surface and are
surrounded by the cerebrospinal fluid (CSF) of the subarachnoid space.
The subarachnoid space is the space into which aneurysms of the cerebral
vessels hemorrhage. Blood in the subarachnoid space mixes with CSF which
circulates and covers the entire pial surface of the central nervous
system with a coat of red blood cells of greater or lesser density
according to the volume of blood released during the aneurysm rupture and
hemorrhage.
Over time the red blood cells in the subarachnoid space die and break open
releasing their contents in to the subarachnoid space. Red blood cells
contain many kinds of molecules but among the most important is hemoglobin
(the iron-carrying molecule of blood cells that makes them red).
Hemoglobin released into the subarachnoid space comes into contact with
the outer surface of cerebral arteries and stimulates contraction
of their smooth muscles (whose contraction and relaxation are the mechanism for
arterial constriction and expansion, respectively). Contraction
of the smooth muscles results in decreased diameter of the artery
which decreases blood supply to the brain and can result in stoke.
Subarachnoid hemorrhage
Vasospasm
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