Central Nervous System Physiology

Question 1

What is the normal cerebrospinal fluid pressure?

Answer 1

When lying in a horizontal position, the normal cerebrospinal
fluid pressure is about 130 mm of water or 10 mmHg.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 747.

Question 2

What are some common causes of increased

cerebrospinal fluid pressure?

Answer 2

A brain tumor can result in the decreased reabsorption of the
CSF, which can result in an increased CSF pressure.
Hemorrhage or infection are other potential causes as the
appearance of red blood cells or white cells can obstruct the
absorption channels in the arachnoid villi. Hydrocephalus is an
inborn cause of increased CSF pressure.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 748.

Question 3

Where is the cerebrospinal fluid secreted and where

is it absorbed?

Answer 3

Cerebrospinal fluid is secreted by the choroid plexus and
absorbed by the arachnoid villi.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 747.

Question 4

How does secretion of cerebrospinal fluid by the

choroid plexus occur?

Answer 4

It is dependent upon the active transport of sodium through the
epithelium of the choroid plexus. As the sodium is transported
out, chloride is pulled outward as well because of its
electrochemical attraction to sodium. The combination of
sodium and chloride increases the osmotic pressure on the
outside of the plexus which results in the osmosis of water
through the plexus membrane. It is the fluid that forms the
principal constituent of cerebrospinal fluid.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 747.

Question 5

About how much cerebrospinal fluid is in the

subarachnoid space at any given time?

Answer 5

About 125 mL
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 746.

Question 6

What is the rate of formation of cerebrospinal fluid?

Answer 6

About 500 mL of CSF is formed each day. This translates into
about 21 mL/hour.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 746.

Question 7

Where in the brain are the choroid plexuses
located? Which locations are associated with the
greatest quantity of cerebrospinal fluid produced?

Answer 7

The choroid plexuses are located in the four ventricles. The
ones located in the two lateral ventricles produce the greatest
quantity of cerebrospinal fluid.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 746.

Question 8

What is the brain’s preferred energy source?

Answer 8

The brain is able to metabolize some lactate as a source of
energy although glucose is the preferred energy source.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 576.

Question 9

How much glucose does the brain utilize each

minute?

Answer 9

Brain glucose utilization is about 5 mg/100g/min. This is about
75 mg/min in the average adult.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 576.

Question 10

About how long will the glycogen stored inside
neurons last if it must be converted to glucose during
a period of hypoglycemia?

Answer 10

The neurons only contain about a 2 minute supply of glycogen.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 749.

Question 11

What percentage of cardiac output goes to the brain?

Answer 11

The brain only comprises about 2% of body mass but receives
about 15% of the cardiac output.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 743.

Question 12

What are the four arteries that supply blood to the

brain?

Answer 12

The left and right carotid arteries and left and right vertebral
arteries.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 743.

Question 13

What is the most effective method for protecting the

brain during ischemic episodes?

Answer 13

Profound hypothermia is the most effective method for
protecting the brain during episodes of ischemia. It is often
used for up to 60 minutes of total circulatory arrest for
cardiopulmonary or neurosurgical procedures with little
evidence of neurologic sequelae. Even mild hypothermia at 33-
35 degrees Celsius provides some protective benefits.
Although propofol, thiopental, and isoflurane offer many
protective advantages, no anesthetic has been shown to
consistently provide these effects.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 587

Question 14

What is the normal cerebral blood flow?

Answer 14

The normal blood flow to the brain is about 50-65 milliliters per
100 grams of brain tissue per minute. This amounts to about
750-900 milliliters/minute in the average adult.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 743.

Question 15

What is the normal cerebral metabolic rate (CMRO2)

in adults?

Answer 15

The average CMRO2 in adults is 3-3.8 mL/100g/min or about
50 mL/min
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 576.

Question 16

How do changes in temperature affect the cerebral

metabolic rate?

Answer 16

For every 1 degree Celsius that the core temperature drops, the
cerebral metabolic rate decreases 5-7% and vice versa. The
EEG becomes isoelectric at a body temperature of 20 degrees
Celsius.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 578.

Question 17

What is the relationship between the cerebral
metabolic rate and glucose consumption by the
brain?

Answer 17

CMRO2 maintains a direct relationship with glucose
consumption.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 576.

Question 18

How is cerebral blood flow calculated?

Answer 18

Cerebral blood flow is calculated as: CBF = Cerebral Perfusion
Pressure/Cerebral Vascular Resistance
Mccance KL, Huether SE, Brashers VL, & Rote NS.
Pathophysiology: The Biologic Basis of Disease. 6th ed.
Philadelphia, PA: Mosby, Inc.; 2010: 557.

Question 19

How does hypertension affect the cerebral

autoregulation curve?

Answer 19

Normally, the body maintains a constant cerebral blood flow
between mean arterial pressures of 60 and 160 mmHg.
Chronic hypertension will shift both the upper and lower limits of
the cerebral autoregulation curve to the right.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 577.

Question 20

How does carbon dioxide affect cerebral blood flow?

Answer 20

CBF is directly proportional to PaCO2 between tensions of 20
and 80 mmHg.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 578.

Question 21

Most body tissues can survive for several minutes
(and some as long as 30 minutes) without oxygen.
Why is the brain not capable of tolerating a similar
loss of oxygen?

Answer 21

Non-neural tissues can obtain energy from anaerobic
mechanisms, enabling them to acquire energy from glucose
and glycogen without the use of oxygen. The brain has a
metabolic rate that is about 7.5 times that of non-neural tissues
and cannot utilize anaerobic mechanisms effectively. As a
result, the brain depends upon second-by-second delivery of
oxygen for it to function correctly. Whereas peripheral tissues
can tolerate a loss of arterial flow for a considerable period of
time before severe damage occurs, the interruption of blood
flow to the brain for even 5 seconds can produce
unconsciousness.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 749.

Question 22

What is the normal tissue PO2 in the brain? How do

changes in tissue PO2 affect cerebral blood flow?

Answer 22

The normal tissue PO2 in the brain is between 35-40 mmHg.
Derangements in cerebral function occur at a tissue PO2 of 20
mmHg. If the tissue PO2 drops below 30 mmHg, the body
increases cerebral blood flow to protect the tissue from
ischemia.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 744

Question 23

How does hydrogen ion concentration affect

neuronal activity and cerebral blood flow?

Answer 23

An increase in hydrogen ion concentration depresses neuronal
activity. It also causes an increase in blood flow to the brain.
By doing so, it will help ‘wash away’ the hydrogen ions, carbondioxide,
and other acid precursors away from the brain which
returns the hydrogen ion concentration back to normal.
Guyton AC, & Hall, JE. Textbook of Medical Physiology. 12th
ed. Philadelphia: Saunders; 2011: 744.

Question 24

Between what blood pressures does the brain
maintain cerebral blood flow at a normal level? How
does it do this?

Answer 24

Normally, the cerebral blood flow remains constant between
mean arterial pressures of 60 to 160 mmHg. As the MAP
decreases within this range, the cerebral vessels dilate to keep
CBF constant. As the MAP increases within this range, the
cerebral vessels constrict to maintain a constant rate of flow.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 577.

Question 25

How is PaO2 related to cerebral blood flow?

Answer 25

PaO2 has little effect on CBF until it reaches a tension of 50
mmHg at which point it dramatically increases CBF.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 578.

Question 26

What level can cerebral blood flow fall to before

brain damage occurs?

Answer 26

Cerebral blood flow between 15 and 20 mL/100g/min is
associated with a flat EEG and values < 10 mL/100g/min are
associated with irreversible brain damage.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 576.

Question 27

How does hyperglycemia affect the brain during

periods of hypoxia?

Answer 27

Hyperglycemia can exacerbate global hypoxic brain injury,
therefore glucose containing IV solutions are contraindicated in
hypoxic injuries unless a concomitant diagnosis of
hypoglycemia has been established.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 576.