Cerebral circulation is the movement of

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...

through a network of cerebral arteries and veins supplying the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

. The rate of cerebral

blood flow Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuousl ...

in an adult

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

is typically 750 milliliters per

minute The minute is a unit of time usually equal to (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a n ...

, or about 15% of

cardiac output In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: th ...

. Arteries deliver oxygenated blood,

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, usi ...

and other nutrients to the brain.

Vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenate ...

s carry "used or spent" blood back to the

heart The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon diox ...

, to remove

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natur ...

, and other

metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...

products. Because the

would quickly suffer damage from any stoppage in blood supply, the cerebral circulatory system has safeguards including autoregulation of the

blood vessel Blood vessels are the structures of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from ...

s. The failure of these safeguards may result in a stroke. The

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...

of blood in circulation is called the cerebral blood flow. Sudden intense accelerations change the

gravitational forces ''Gravitational Forces'' is an album by Texas-based country/ folk singer-songwriter Robert Earl Keen. It was first released in the United States on August 7, 2001, on Lost Highway Records. One reviewer described this album, Keen's ninth, a ...

perceived by bodies and can severely impair cerebral circulation and normal functions to the point of becoming serious life-threatening conditions. The following description is based on idealized human cerebral circulation. The pattern of circulation and its

nomenclature Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal conventions of everyday speech to the internationally agre ...

vary between organisms.

Anatomy

Blood supply

Blood supply to the brain is normally divided into anterior and posterior segments, relating to the different arteries that supply the brain. The two main pairs of arteries are the Internal carotid arteries (supply the anterior brain) and vertebral arteries (supplying the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

and posterior brain).. The anterior and posterior cerebral circulations are interconnected via bilateral posterior communicating arteries. They are part of the Circle of Willis, which provides backup circulation to the brain. In case one of the supply arteries is occluded, the Circle of Willis provides interconnections between the anterior and the posterior cerebral circulation along the floor of the cerebral vault, providing blood to tissues that would otherwise become

ischemic Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

Anterior cerebral circulation

The anterior cerebral circulation is the blood supply to the anterior portion of the brain including

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

s. It is supplied by the following arteries: * Internal carotid arteries: These large arteries are the medial branches of the common carotid arteries which enter the skull, as opposed to the external carotid branches which supply the facial tissues; the internal carotid artery branches into the anterior cerebral artery and continues to form the middle cerebral artery. * Anterior cerebral artery (ACA) ** Anterior communicating artery: Connects both anterior cerebral arteries, within and along the floor of the cerebral vault. * Middle cerebral artery (MCA)

Posterior cerebral circulation

The posterior cerebral circulation is the blood supply to the posterior portion of the brain, including the occipital lobes,

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cere ...

and

. It is supplied by the following arteries: * Vertebral arteries: These smaller arteries branch from the subclavian arteries which primarily supply the shoulders, lateral chest, and arms. Within the

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, ...

the two vertebral arteries fuse into the

basilar artery The basilar artery () is one of the arteries that supplies the brain with oxygen-rich blood. The two vertebral arteries and the basilar artery are known as the vertebral basilar system, which supplies blood to the posterior part of the circle o ...

. **

Posterior inferior cerebellar artery The posterior inferior cerebellar artery (PICA) is the largest branch of the vertebral artery. It is one of the three main arteries that supply blood to the cerebellum, a part of the brain. Blockage of the posterior inferior cerebellar artery can ...

(PICA) *

Basilar artery The basilar artery () is one of the arteries that supplies the brain with oxygen-rich blood. The two vertebral arteries and the basilar artery are known as the vertebral basilar system, which supplies blood to the posterior part of the circle o ...

: Supplies the

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

, and usually branches into the posterior cerebral artery **

Anterior inferior cerebellar artery The anterior inferior cerebellar artery (AICA) is one of three pairs of arteries that supplies blood to the cerebellum. It arises from the basilar artery on each side at the level of the junction between the medulla oblongata and the pons in th ...

(AICA) **

Pontine branches The pontine arteries are a number of small arteries which come off at right angles from either side of the basilar artery and supply the pons and adjacent parts of the brain. The pontine arteries include the paramedian arteries, the short circumfer ...

** Superior cerebellar artery (SCA) * Posterior cerebral artery (PCA) * Posterior communicating artery

Venous drainage

The venous drainage of the cerebrum can be separated into two subdivisions: superficial and deep. The superficial system is composed of dural venous sinuses, which have walls composed of dura mater as opposed to a traditional vein. The dural sinuses are therefore located on the surface of the cerebrum. The most prominent of these sinuses is the superior sagittal sinus which flows in the sagittal plane under the midline of the cerebral vault, posteriorly and inferiorly to the confluence of sinuses, where the superficial drainage joins with the sinus that primarily drains the deep venous system. From here, two transverse sinuses bifurcate and travel laterally and inferiorly in an S-shaped curve that forms the sigmoid sinuses which go on to form the two

jugular vein The jugular veins are veins that take deoxygenated blood from the head back to the heart via the superior vena cava. The internal jugular vein descends next to the internal carotid artery and continues posteriorly to the sternocleidomast ...

s. In the neck, the

s parallel the upward course of the carotid arteries and drain blood into the

superior vena cava The superior vena cava (SVC) is the anatomical terms of location#Superior and inferior, superior of the two venae cavae, the great vein, venous trunks that return deoxygenated blood from the circulatory system, systemic circulation to the atrium ...

. The deep venous drainage is primarily composed of traditional veins inside the deep structures of the brain, which join behind the midbrain to form the vein of Galen. This vein merges with the inferior sagittal sinus to form the straight sinus which then joins the superficial venous system mentioned above at the confluence of sinuses.

Physiology

Cerebral blood flow (CBF) is the blood supply to the

in a given period of time.Tolias C and Sgouros S. 2006
"Initial Evaluation and Management of CNS Injury."
Emedicine.com. Accessed January 4, 2007. In an adult, CBF is typically 750 millilitres per minute or 15% of the

. This equates to an average perfusion of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute.Orlando Regional Healthcare, Education and Development. 2004
"Overview of Adult Traumatic Brain Injuries."
Accessed 2008-01-16.Shepherd S. 2004

Emedicine.com. Shepherd S. 2004

Emedicine.com. Accessed January 4, 2007.Walters, FJM. 1998.

Physiology. Issue 8, Article 4. Accessed January 4, 2007. CBF is tightly regulated to meet the brain's

demands.Singh J and Stock A. 2006
"Head Trauma."
Emedicine.com. Accessed January 4, 2007. 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. Too little blood flow (

ischemia Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

) 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 known as the

ischemic cascade The ischemic (ischaemic) cascade is a series of biochemical reactions that are initiated in the brain and other aerobic tissues after seconds to minutes of ischemia (inadequate blood supply). This is typically secondary to stroke, injury, or card ...

is triggered when the tissue becomes ischemic, potentially resulting in damage to and the death of brain cells. Medical professionals must take steps to maintain proper CBF in patients who have conditions like shock, stroke, cerebral edema, and

traumatic brain injury A traumatic brain injury (TBI), also known as an intracranial injury, is an injury to the brain caused by an external force. TBI can be classified based on severity (ranging from mild traumatic brain injury TBI/concussionto severe traumatic b ...

. Cerebral blood flow is determined by a number of factors, such as

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

of blood, how dilated

s 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 Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressur ...

. Cerebral perfusion pressure (CPP) is defined as the mean arterial pressure (MAP) minus the intracranial pressure (ICP). In normal individuals, it should be above 50 mm Hg. Intracranial pressure should not be above 15 mm Hg (ICP of 20 mm Hg is considered as intracranial hypertension). 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.Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York. p.1305 Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of

in the blood and constrict in response to lower levels of carbon dioxide. For example, assuming a person with an arterial partial pressure of carbon dioxide (

PaCO2 ''p''CO2, pCO2, or P_\ceis the partial pressure of carbon dioxide (CO2), often used in reference to blood but also used in meteorology, climate science, oceanography, and limnology to describe the fractional pressure of CO2 as a function of its c ...

) of 40 mmHg (normal range of 38–42 mmHg)Hadjiliadis D, Zieve D, Ogilvie I. Blood Gases. Medline Plus. 06/06/2015.
/ref> and a CBF of 50 ml per 100g per min. If the PaCO2 dips to 30 mmHg, this represents a 10 mmHg decrease from the initial value of PaCO2. Consequently, the CBF decreases by 1ml per 100g per min for each 1mmHg decrease in PaCO2, resulting in a new CBF of 40ml per 100g of brain tissue per minute. In fact, for each 1 mmHg increase or decrease in PaCO2, between the range of 20–60 mmHg, there is a corresponding CBF change in the same direction of approximately 1–2 ml/100g/min, or 2–5% of the CBF value.Giardino ND, Friedman SD, Dager SR. Anxiety, respiration, and cerebral blood flow: implications for functional brain imaging. Compr Psychiatry 2007;48:103–112. Accessed 6/6/2015. This is why small alterations in respiration pattern can cause significant changes in global CBF, specially through PaCO2 variations. CBF is equal to the cerebral perfusion pressure (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: #

Metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...

control (or 'metabolic autoregulation') # Pressure autoregulation # Chemical control (by arterial pCO₂ and pO₂) #

Neural In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

control

Role of intracranial pressure

Increased intracranial pressure (ICP) causes decreased blood perfusion of brain cells by mainly two mechanisms: * Increased ICP constitutes an increased interstitial hydrostatic pressure that, in turn, causes a decreased driving force for capillary filtration from intracerebral blood vessels. * Increased ICP compresses cerebral arteries, causing increased cerebrovascular resistance (CVR).

Cerebral perfusion pressure

Cerebral perfusion pressure, or CPP, is the net

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

gradient causing cerebral blood flow to the brain (brain perfusion). It must be maintained within narrow limits; too little pressure could cause brain tissue to become

(having inadequate blood flow), and too much could raise intracranial pressure (ICP).

Imaging

Arterial spin labeling (ASL), phase contrast magnetic resonance imaging (PC-MRI), and

positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, ...

(PET) are

neuroimaging Neuroimaging is the use of quantitative (computational) techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Incr ...

techniques that can be used to measure CBF. ASL and PET can also be used to measure regional CBF (rCBF) within a specific brain region. rCBF at one location can be measured over time by

thermal diffusion Thermal diffusion may refer to: * A thermal force on a gas due to a temperature gradient, also called ''thermal diffusion'' or Thermal transpiration. ** It is used to drive a gas pump with no moving parts called a Knudsen pump. ** It is the currentl ...

P. Vajkoczy, H. Roth, P. Horn, T. Lucke, C. Thome, U. Hubner, G. T. Martin, C. Zappletal, E. Klar, L. Schilling, and P. Schmiedek, “Continuous monitoring of regional cerebral blood flow: experimental and clinical validation of a novel thermal diffusion microprobe,” J. Neurosurg., vol. 93, no. 2, pp. 265–274, Aug. 2000

/ref>

References

External links

Computer Model of the Cerebral Circulation for Training and Education
{{VeinsHeadNeck Neurology Cardiology