66 73 Cerebral blood flow

Cerebral blood flow: about 13–15% of cardiac output, autoregulation, the role of CO₂ and stroke.

Cerebral blood flow supplies the brain, which makes up only about 2% of body weight but receives about 13–15% of cardiac output at rest, roughly 650–750 mL/min, or about 50 mL per 100 g of brain tissue per minute. The brain has almost no energy reserves and depends on a continuous supply of oxygen and glucose: consciousness is lost within seconds when flow stops. Several mechanisms keep cerebral flow matched to brain activity and protect it from changes in blood pressure.

BrainAutoregulationStroke

Share of cardiac output≈ 13–15%

At rest≈ 650–750 mL/min

Per tissue mass≈ 50 mL/100 g/min

Strong regulatorarterial CO₂

Figure 1. Resting distribution of a cardiac output of about 5 L/min (after Chapman and Mitchell, 1965). The highlighted bar shows cerebral blood flow.

This application estimates regional blood flow as a percentage of cardiac output and converts it into mL/min from the calculated cardiac output. In clinical practice, organ blood flow is measured invasively or with imaging (for example indicator dilution, Doppler ultrasound, PET or MRI perfusion).

Regulation

Autoregulation: cerebral vessels constrict when pressure rises and dilate when it falls, buffering flow against changes in blood pressure. This protection is limited and not perfect: flow is not kept constant at all pressures, and autoregulation is impaired in chronic hypertension, after stroke, in brain injury and in some older people.
Carbon dioxide: high CO₂ dilates cerebral vessels and increases flow; hyperventilation lowers CO₂ and reduces flow, which can cause dizziness
Neurovascular coupling: active brain areas receive more blood within seconds

Clinical significance

Reduced cerebral flow causes dizziness, fainting (syncope), confusion and, when prolonged, ischaemic stroke. Causes include narrowing or blockage of the carotid and cerebral arteries, embolism from the heart (for example in atrial fibrillation), low blood pressure, low cardiac output and arrhythmias. Raised intracranial pressure also reduces cerebral perfusion. In the model of this application, a share below 13% of cardiac output is flagged as a sign of possible cerebral circulatory disorders.

Seek emergency care

immediately for sudden weakness or numbness of the face, arm or leg, difficulty speaking, loss of vision, severe sudden headache or loss of balance. In stroke, every minute counts.

References

Claassen JAHR, Thijssen DHJ, Panerai RB, Faraci FM. Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation. Physiol Rev. 2021;101(4):1487–1559. doi:10.1152/physrev.00022.2020
Vander A, Sherman J, Luciano D. Human Physiology: The Mechanisms of Body Function. 8th ed. Boston: McGraw-Hill; 2001. Fig. 14-9 (after Chapman CB, Mitchell JH. Sci Am. 1965). Data: BioNumbers BNID 109465

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 30 April 2024