Cerebral blood flow on 100g of tissue

Cerebral blood flow (CBF) is the blood supply to the brain in a given period of time. Perfusion of blood per 100 grams of brain tissue per minute is 50-54ml on average. CBF is tightly regulated to meet the brain's metabolic demands. It ensures proper delivery of oxygen and energy substrates and the removal of waste products of metabolism. Adequate brain perfusion is required to support normal brain function, achieve successful ageing, and navigate acute and chronic medical conditions.^1-3

Cerebral blood flow is determined by a number of external and internal factors. External factors include atmospheric pressure, the gas composition of air and partial gas pressure in the atmosphere. Internal factors are the viscosity of blood, how dilated blood vessels are, and the net pressure of the flow of blood into the brain, known as cerebral perfusion pressure, which is determined by the body's blood pressure. ⁵

Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called cerebral autoregulation; they constrict when systemic blood pressure is raised and dilate when it is lowered.⁶ Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of carbon dioxide in the blood and constrict in response to lower levels of carbon dioxide.

Abnormalities. Disorders or diseases caused or related.

Too much blood (a clinical condition of a normal homeostatic response of hyperemia) can raise intracranial pressure (ICP), which can compress and damage delicate brain tissue. As a result, the breakdown of the blood-brain barrier occurs. ^4,6

Too little blood flow, hypoperfusion, results if blood flow to the brain is below 18 to 20 ml per 100 g per minute, and tissue death occurs if flow dips below 8 to 10 ml per 100 g per minute. In this case, brain damage occurs through ischemic injury.

Both hypoperfusion (insufficient CBF) and hyperperfusion (excessive CBF) damage the brain which can cause seizures, headaches, encephalopathy, and both ischemic and hemorrhagic stroke. ⁶

 Autoregulation of Cerebral Blood Flow⁷

Autoregulation of cerebral blood flow is the ability of the brain to maintain relatively constant blood flow despite changes in perfusion pressure. Autoregulation is present in many vascular beds, but is particularly well-developed in the brain, likely due to the need for a constant blood supply and water homeostasis. In normotensive adults, cerebral blood flow is maintained at ~50 mL per 100 g of brain tissue per minute, provided CPP is in the range of ~60 to 160 mmHg. Above and below this limit, autoregulation is lost and cerebral blood flow becomes dependent on mean arterial pressure in a linear fashion. When CPP falls below the lower limit of autoregulation, cerebral ischemia ensues. The reduction in cerebral blood flow is compensated for by an increase in oxygen extraction from the blood. Clinical signs or symptoms of ischemia are not seen until the decrease in perfusion exceeds the ability of increased oxygen extraction to meet metabolic needs. At this point, clinical signs of hypoperfusion occur, including dizziness, altered mental status, and eventually irreversible tissue damage (infarction).

1.        Walters, FJM. 1998. "Intracranial Pressure and Cerebral Blood Flow." Archived May 14, 2011, at the Wayback Machine Physiology. Issue 8, Article 4. Accessed January 4, 2007

2.        Fantini S, et al. Cerebral blood flow and autoregulation: current measurement techniques and prospects for noninvasive optical methods. Neurophotonics. 2016 Jul; 3(3): 031411,doi: 10.1117/1.NPh.3.3.031411.

3.        Singh J and Stock A. 2006. "Head Trauma." Emedicine.com. Accessed January 4, 2007.

4.        Muoio, V; Persson, PB; Sendeski, MM (April 2014). "The neurovascular unit - concept review". Acta Physiologica. 210 (4): 790–8. doi:10.1111/apha.12250.

5.        Heinrich Mattle & Marco Mumenthaler with Ethan Taub (2016-12-14). Fundamentals of Neurology. Thieme. p. 129. ISBN 978-3-13-136452-4.

6.        Tzeng YC, Ainslie PN. Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges. Eur J Appl Physiol. 2014 Mar; 114(3):545-59.

7.        https://www.ncbi.nlm.nih.gov/books/NBK53082/#:~:text=In%20normotensive%20adults%2C%20cerebral%20blood,71%2C72%2C139%5D.