Cerebral blood flow, brain metabolism and intracranial pressure !
- Regulation
- Exam priority: high
The brain uses a fifth of the body's oxygen and stores no fuel, so its blood flow is tightly controlled; swelling inside the closed skull is dangerous.
Updated 2026-11-01
Also called: Cerebral perfusion pressure, Monro–Kellie doctrine
See it in 3D: Circle of Willis · Internal carotid artery · Vertebral artery · Basilar artery · Middle cerebral artery · Middle meningeal artery
What it is
The brain is about 2% of body weight but receives about 15% of the cardiac output and uses about 20% of resting O₂. Autoregulation holds its flow near 50–65 mL/100 g/min across a wide range of arterial pressure, and flow is very sensitive to PaCO₂. Because the skull is rigid, brain, blood and CSF share a fixed volume (Monro–Kellie doctrine): when one grows the others must shrink, or intracranial pressure rises and perfusion falls.
Equations
- MAP
- mean arterial pressure
- ICP
- intracranial pressure
When the jugular venous pressure exceeds ICP, it replaces ICP in the formula.
- CVR
- cerebrovascular resistance, set mainly by arteriolar radius
Autoregulation changes CVR to keep CBF constant as CPP changes.
Reference values (adult at rest)
| Parameter | Value | Note |
|---|---|---|
| Cerebral blood flow | ≈50–65 mL/100 g/min | ≈750–900 mL/min in total |
| Share of cardiac output | ≈15% | |
| Brain O₂ consumption | ≈3.5 mL O₂/100 g/min | ≈20% of the body at rest |
| Intracranial pressure | ≈5–15 mmHg | supine adult |
| Autoregulation range | MAP ≈60–150 mmHg | |
| Time to unconsciousness after flow stops | ≈5–10 s |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +↑ PaCO₂ (↑ perivascular H⁺): Arteriolar dilation: more flow and blood volume, ICP rises
- −↓ PaCO₂ (hyperventilation): Vasoconstriction: less flow and volume; lowers ICP briefly but risks ischemia
- +↓ PaO₂ (below ≈50 mmHg): Vasodilation that protects O₂ delivery
- +Local neuronal activity (K⁺, adenosine, NO): More flow to active regions (neurovascular coupling)
- −Myogenic response to ↑ arterial pressure: Arteriolar constriction keeps flow steady (autoregulation)
Clinical notes
- Epidural hematoma (middle meningeal artery) gives a lucid interval, then a rapid ICP rise and uncal herniation with a fixed dilated pupil on the side of the lesion (CN III).
- Raised ICP: headache worse lying down and on waking, vomiting, papilledema, falling level of consciousness.
- Lumbar puncture is avoided when a mass raises ICP, because the pressure gradient can drive herniation.
- At Bogotá's altitude (≈2600 m), hypoxic vasodilation partly offsets the lower PaO₂, while the hypocapnia of increased breathing opposes it.
- The brain runs almost only on glucose; in prolonged fasting ketone bodies (β-hydroxybutyrate) can cover up to about two-thirds of its energy.
Test yourself
Think of the answer, then tap the question to see it.
Why does brief hyperventilation lower ICP, and why is prolonged hyperventilation harmful?
Lower PaCO₂ raises perivascular pH, constricts arterioles and shrinks cerebral blood volume, so ICP falls. Flow falls too, so deep or long hypocapnia can cause ischemia, and the effect fades as pH re-adjusts.
A head-injured patient has MAP 80 mmHg and ICP 35 mmHg. What is the CPP and why is it a problem?
CPP = 80 − 35 = 45 mmHg, below the ≈50–60 mmHg usually needed. Autoregulation is often impaired after injury, so flow falls with pressure and ischemia follows.
What causes the Cushing triad?
High ICP compresses the brainstem; ischemia of the vasomotor centre drives sympathetic hypertension to restore perfusion, baroreceptors then slow the heart, and brainstem compression disrupts breathing.
Why may a patient with long-standing hypertension get cerebral symptoms when pressure is lowered quickly to normal?
Chronic hypertension shifts the autoregulation curve to the right, so its lower limit is higher. A fast drop can fall below that limit and reduce cerebral flow.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Circle of Willis
- Internal carotid artery
- Vertebral artery
- Basilar artery
- Middle cerebral artery
- Middle meningeal artery
Related topics
- CSF and the blood–brain barrier
- Local and humoral control of blood flow
- The baroreceptor reflex
- High altitude and acclimatization
- Control of breathing, chemoreceptors and lung reflexes
- Headache physiology
Related conditions
- Traumatic brain injury
- Ischaemic stroke
- Epidural haematoma
- Subdural haematoma
- Intracerebral haemorrhage
- Hydrocephalus
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 13.3 Circulation and the Central Nervous System (OpenStax (Rice University), 2022); Neuroanatomy, Circle of Willis (StatPearls (NCBI Bookshelf), 2023); Traumatic Brain Injury (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 62). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.