Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 34

Circulation Through Special Regions 571

neuropeptide Y is a vasoconstrictor. Touching or pulling on
the cerebral vessels causes pain.

CEREBROSPINAL FLUID

FORMATION & ABSORPTION

CSF fills the ventricles and subarachnoid space. In humans, the
volume of CSF is about 150 mL and the rate of CSF production
is about 550 mL/d. Thus the CSF turns over about 3.7 times a
day. In experiments on animals, it has been estimated that 50–
70% of the CSF is formed in the choroid plexuses and the re-
mainder is formed around blood vessels and along ventricular
walls. Presumably, the situation in humans is similar. The CSF
in the ventricles flows through the foramens of Magendie and
Luschka to the subarachnoid space and is absorbed through
the
arachnoid villi
into veins, primarily the cerebral venous si-
nuses. The villi consist of projections of the fused arachnoid
membrane and endothelium of the sinuses into the venous si-
nuses. Similar, smaller villi project into veins around spinal
nerve routes. These projections may contribute to the outflow
of CSF into venous blood by a process known as
bulk flow,
which is unidirectional.
However, recent studies suggest that,
at least in animals, a more important route for CSF reabsorp-
tion into the bloodstream in health is via the cribriform plate
above the nose and thence into the cervical lymphatics. How-
ever, reabsorption via one-way valves (of uncertain structural
basis) in the arachnoid villi may assume a greater role if CSF
pressure is elevated. Likewise, when CSF builds up abnormally,
aquaporin water channels may be expressed in the choroid
plexus and brain microvessels as a compensatory adaptation.
CSF is formed continuously by the choroid plexus in two
stages. First, plasma is passively filtered across the choroidal
capillary endothelium. Next, secretion of water and ions
across the choroidal epithelium provides for active control of
CSF composition and quantity. Bicarbonate, chloride, and
potassium ions enter the CSF via channels in the epithelial cell

apical membranes. Aquaporins provide for water movement

to balance osmotic gradients. The composition of CSF (Table

34–2) is essentially the same as that of brain extracellular fluid

(ECF), which in living humans makes up 15% of the brain

volume. In adults, free communication appears to take place

between the brain interstitial fluid and CSF, although the dif-

fusion distances from some parts of the brain to the CSF are

appreciable. Consequently, equilibration may take some time

to occur, and local areas of the brain may have extracellular

microenvironments that are transiently different from CSF.

Lumbar CSF pressure is normally 70 to 180 mm H

2

O. Up to

pressures well above this range, the rate of CSF formation is

independent of intraventricular pressure. However, absorp-

tion is proportional to the pressure (Figure 34–3). At a pres-

sure of 112 mm H

2

O, which is the average normal CSF

pressure, filtration and absorption are equal. Below a pressure

of approximately 68 mm H

2

O, absorption stops. Large

amounts of fluid accumulate when the capacity for CSF reab-

sorption is decreased

(external hydrocephalus, communi-

cating hydrocephalus).

Fluid also accumulates proximal to

the block and distends the ventricles when the foramens of

Luschka and Magendie are blocked or there is obstruction

within the ventricular system

(internal hydrocephalus, non-

communicating hydrocephalus).

FIGURE 34–2
Transport across cerebral capillaries.

Glial

endfoot

Glucose,

etc

Tight junction

Nucleus Mitochondrion

Lipid-soluble

diffusion,

carrier-mediated

transport

TABLE 34–2

Concentration of various

substances in human CSF and plasma.

Substance CSF Plasma

Ratio

CSF/Plasma

Na

+

(meq/kg H

2

O) 147.0 150.0 0.98

K

+

(meq/kg H

2

O) 2.9 4.6 0.62

Mg

2+

(meq/kg H

2

O) 2.2 1.6 1.39

Ca

2+

(meq/kg H

2

O) 2.3 4.7 0.49

Cl

• (meq/kg H
  2
  O) 113.0 99.0 1.14
  HCO
  3 –
  (meq/L) 25.1 24.8 1.01
  P
  CO 2
  (mm Hg) 50.2 39.5 1.28
  pH 7.33 7.40...
  Osmolality (mosm/kg H
  2
  O) 289.0 289.0 1.00
  Protein (mg/dL) 20.0 6000.0 0.003
  Glucose (mg/dL) 64.0 100.0 0.64
  Inorganic P (mg/dL) 3.4 4.7 0.73
  Urea (mg/dL) 12.0 15.0 0.80
  Creatinine (mg/dL) 1.5 1.2 1.25
  Uric acid (mg/dL) 1.5 5.0 0.30
  Cholesterol (mg/dL) 0.2 175.0 0.001