Cerebral blood flow
Encyclopedia
Cerebral blood flow, or CBF, is the blood supply to the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 in a given time. In an adult, CBF is typically 750 millitres per minute or 15% of the cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...

. This equates to 50 to 54 millilitres of blood per 100 grams of brain tissue per minute. CBF is tightly regulated to meet the brain's metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 demands. Too much blood (a condition known as hyperemia) can raise intracranial pressure
Intracranial pressure
Intracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...

 (ICP), which can compress and damage delicate brain tissue. Too little blood flow (ischemia
Ischemia
In medicine, ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia...

) 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 brain tissue, a biochemical cascade
Biochemical cascade
A biochemical cascade is a series of chemical reactions in which the products of one reaction are consumed in the next reaction. There are several important biochemical cascade reactions in biochemistry, including the enzymatic cascades, such as the coagulation cascade and the complement system,...

 known as the ischemic cascade
Ischemic cascade
The ischemic cascade is a series of biochemical reactions that are initiated in the brain and other aerobic tissues after seconds to minutes of ischemia . This is typically secondary to stroke, injury, or cardiac arrest due to heart attack. Most ischemic neurons that die do so due to the...

 is triggered when the tissue becomes ischemic, potentially resulting in damage to and death of brain cells
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

. Medical professionals must take steps to maintain proper CBF in patients who have conditions like shock, stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, and traumatic brain injury
Traumatic brain injury
Traumatic brain injury , also known as intracranial injury, occurs when an external force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...

.

Cerebral blood flow is determined by a number of factors, such as viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 of blood, how dilated blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s are, and the net pressure of the flow of blood into the brain, known as cerebral perfusion pressure
Cerebral perfusion pressure
Cerebral perfusion pressure, or CPP, is the net pressure gradient causing blood flow to the brain . It must be maintained within narrow limits because too little pressure could cause brain tissue to become ischemic , and too much could raise intracranial pressure .-From resistance:CPP can be...

, which is determined by the body's blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

. Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called autoregulation
Autoregulation
Autoregulation is a process within many biological systems, resulting from some internal adaptive mechanism that works to adjust the systems response to stimuli. While most systems of the body show some degree of autoregulation, it is most clearly observed in the kidney, the heart, and the brain...

; they constrict when systemic blood pressure is raised and dilate when it is lowered Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 in the blood.

CBF is equal to the cerebral perfusion pressure
Cerebral perfusion pressure
Cerebral perfusion pressure, or CPP, is the net pressure gradient causing blood flow to the brain . It must be maintained within narrow limits because too little pressure could cause brain tissue to become ischemic , and too much could raise intracranial pressure .-From resistance:CPP can be...

 (CPP) divided by the cerebrovascular resistance (CVR):
CBF = CPP / CVR


Control of CBF is considered in terms of the factors affecting CPP and the factors affecting CVR. CVR is controlled by four major mechanisms:
  1. Metabolic control (or 'metabolic autoregulation')
  2. Pressure autoregulation
    Autoregulation
    Autoregulation is a process within many biological systems, resulting from some internal adaptive mechanism that works to adjust the systems response to stimuli. While most systems of the body show some degree of autoregulation, it is most clearly observed in the kidney, the heart, and the brain...

  3. Chemical control (by arterial pCO2 and pO2)
  4. Neural control


Functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...

 and positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...

 are neuroimaging
Neuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...

 techniques that can both be used to measure CBF. These techniques are also used to measure regional CBF (rCBF) within a specific brain region.

Role of intracranial pressure

Increased intracranial pressure
Intracranial pressure
Intracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...

 (ICP) causes decreased blood perfusion of brain cell
Brain Cell
Brain Cell is a mail art project begun by Ryosuke Cohen in June 1985. The project is basically a networked art project where individual artists create their own 30x42cm work of art with stamps, drawings, stickers and so forth. This is sent to Cohen, who prints each cell - 150 copies each - with a...

s by mainly two mechanisms:
  • Increased ICP constitutes an increased interstitial hydrostatic pressure that, in turn, causes a decreased driving force for capillary filtration
    Starling equation
    The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces in the movement of fluid across capillary membranes.Capillary fluid movement may occur as a result of three processes:...

    from intracerebral blood vessels.
  • Increased ICP compresses cerebral arteries, causing increased cerebrovascular resistance (CVR).
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