Ganong's Review of Medical Physiology, 23rd Edition

572
SECTION VI
Cardiovascular Physiology

PROTECTIVE FUNCTION

The most critical role for CSF (and the meninges) is to protect
the brain. The dura is attached firmly to bone. Normally, there
is no “subdural space,” with the arachnoid being held to the
dura by the surface tension of the thin layer of fluid between the
two membranes. As shown in Figure 34–4, the brain itself is
supported within the arachnoid by the blood vessels and nerve
roots and by the multiple fine fibrous
arachnoid trabeculae.
The brain weighs about 1400 g in air, but in its “water bath” of
CSF it has a net weight of only 50 g. The buoyancy of the brain
in the CSF permits its relatively flimsy attachments to suspend
it very effectively. When the head receives a blow, the arachnoid
slides on the dura and the brain moves, but its motion is gently
checked by the CSF cushion and by the arachnoid trabeculae.
The pain produced by spinal fluid deficiency illustrates the
importance of CSF in supporting the brain. Removal of CSF dur-
ing lumbar puncture can cause a severe headache after the fluid
is removed, because the brain hangs on the vessels and nerve
roots, and traction on them stimulates pain fibers. The pain can
be relieved by intrathecal injection of sterile isotonic saline.

HEAD INJURIES

Without the protection of the spinal fluid and the meninges,
the brain would probably be unable to withstand even the mi-
nor traumas of everyday living; but with the protection afford-
ed, it takes a fairly severe blow to produce cerebral damage.
The brain is damaged most commonly when the skull is frac-
tured and bone is driven into neural tissue (depressed skull
fracture), when the brain moves far enough to tear the delicate
bridging veins from the cortex to the bone, or when the brain
is accelerated by a blow on the head and is driven against the
skull or the tentorium at a point opposite where the blow was
struck
(contrecoup injury).

THE BLOOD–BRAIN BARRIER

The tight junctions between capillary endothelial cells in the

brain and between the epithelial cells in the choroid plexus ef-

fectively prevent proteins from entering the brain in adults

and slow the penetration of some smaller molecules as well.

An example is the slow penetration of urea (Figure 34–5). This

uniquely limited exchange of substances into the brain is re-

ferred to as the

blood–brain barrier,

a term most commonly

used to encompass this barrier overall and more specifically

the barrier in the choroid epithelium between blood and CSF

.

Passive diffusion across the tight cerebral capillaries is very

limited, and little vesicular transport takes place. However,

there are numerous carrier-mediated and active transport sys-

tems in the cerebral capillaries. These move substances out of

as well as into the brain, though movement out of the brain is

generally more free than movement into it.

PENETRATION OF SUBSTANCES

INTO THE BRAIN

Water, CO

2

, and O

2

penetrate the brain with ease, as do the

lipid-soluble free forms of steroid hormones, whereas their

FIGURE 34–3
CSF formation and absorption in humans at
various CSF pressures.
Note that at 112 mm CSF, formation and ab-
sorption are equal, and at 68 mm CSF, absorption is zero.
(Modified and
reproduced with permission from Cutler RWP, et al: Formation and absorption of
cerebrospinal fluid in man. Brain 1968;91:707.)

1.6

1.2

0.8

0.4

0

0 68 100 112 200

Outflow pressure (mm CSF)

Flow (mL/min) Formation

Absorption

FIGURE 34–4

Investing membranes of the brain, showing

their relation to the skull and to brain tissue.

(Reproduced with

permission from Wheater PR et al:

Functional Histology.

Churchill Livingstone, 1979.)

Subarachnoid

space

Arachnoid

trabeculae

Arachnoid

Artery

Pia mater

Brain

Perivascular

spaces

Dura mater

Inner table

of skull

Subdural

(potential)

space

Trabecular

bone

Outer table

of skull