386
UNIT THREE
tration of sodium and lesser concentrations of glucose and
potassium than do other extracellular fl uids. Its function is
nutritive as well as protective. CSF helps maintain a stable
ionic concentration in the CNS and provides a pathway to
the blood for waste. The CSF may also supply information
about the internal environment to autonomic centers in the
hypothalamus and brainstem, because the fl
uid forms from
blood plasma and therefore its composition refl ects changes
in body fl uids. Clinical Application 11.1 discusses the pres-
sure that CSF generates.
CSF occupies the subarachnoid space of the meninges,
so it completely surrounds the brain and spinal cord. In
effect, these organs fl oat in the fl uid. The CSF protects them
by absorbing forces that might otherwise jar and damage
their delicate tissues.
PRACTICE
4
Where are the ventricles of the brain located?
5
How does CSF form?
6
Describe the pattern of CSF circulation.
the brain interstitial fl
uid (blood-brain barrier), ependymal
cells block passage of water-soluble substances between the
blood and the CSF (blood-CSF barrier). At the same time, the
cells selectively transfer certain substances from the blood
into the CSF by facilitated diffusion and transfer other sub-
stances by active transport (see chapter 3, pp. 93 and 95),
regulating CSF composition.
Most CSF forms in the lateral ventricles, from where it
slowly circulates into the third and fourth ventricles and into
the central canal of the spinal cord. It also enters the suba-
rachnoid space of the meninges by passing through the wall
of the fourth ventricle near the cerebellum.
Humans secrete nearly 500 milliliters of CSF daily.
However, only about 140 milliliters are in the nervous sys-
tem at any time, because CSF is continuously reabsorbed into
the blood through tiny, ±
ngerlike structures called
arachnoid
granulations
that project from the subarachnoid space into
the blood-±
lled dural sinuses (see ±
g. 11.4).
CSF is a clear, somewhat viscid liquid that differs in
composition from the fl
uid that leaves the capillaries in other
parts of the body. Speci±
cally, it contains a greater concen-
Interventricular
foramen
Lateral ventricle
Cerebral
aqueduct
Third ventricle
Fourth ventricle
To central canal
of spinal cord
Interventricular
foramen
Lateral
ventricle
Fourth
ventricle
To central canal
of spinal cord
Cerebral
aqueduct
Third ventricle
(a)
(b)
FIGURE 11.3
Ventricles in the brain. (
a
) Anterior view of the ventricles in the cerebral hemispheres and brainstem. (
b
) Lateral view.
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