Cerebral Blood Flow Flashcards
Describe the normal blood flow through the brain?
50-55 ml/100g of brain per minute
> Approx 750 ml/min for the entire brain or 15% of total resting cardiac output
Note: total cessation of blood flow to the brain causes unconsciousness within 5 to 10 sec
Describe the dual circulation to the brain?
anterior > carotid arteries
posterior > vertebral arteries
- they meet up to form the circle of Willis
Describe the anterior blood supply of the brain?
- common carotid
- external carotid artery
- internal carotid artery
- middle cerebral artery
> temporal + parietal lobes - anterior cerebral artery
> frontal lobes + superior medial parietal lobes - anterior communicating artery
> connect the anterior cerebral arteries of each side
Describe the posterior blood supply of the brain?
What it supplies?
- vertebral arteries
- posterior inferior cerebellar artery
> medulla + part of cerebellum - basilar artery
> mid-pons - anterior inferior cerebellar artery
> lateral pons + part of the cerebellum - superior cerebellar artery
- posterior cerebral artery
- posterior communicating artery
> connect posterior cerebral arteries to the internal carotids
What is the circle of willis?
a ring of connected arteries at the brain’s base, formed by the posterior cerebral and communicating arteries, the internal carotids, and the anterior cerebral and communicating arteries
What are the arteries of the circle of willis?
a ring of connected arteries at the brain’s base, formed by the
> posterior cerebral and communicating arteries
> the internal carotids, and the anterior cerebral and communicating arteries
Describe the venous drainage system of the brain?
- superficial surfaces
sagittal sinuses and cortical veins > jugular vein - deep structures
lateral sinus, straight sinus and sigmoid sinus > jugular vein
What are the 4 mechanisms that control cerebral blood flow?
- myogenic
- neurogenic
- endothelial
- metabolic
What is cerebral autoregulation?
the ability of the cerebral vasculature to maintain stable blood flow despite changes in blood pressure (cerebral perfusion pressure)
> Cerebral flow is autoregulated extremely well between the pressure limits of 60 & 140 mm Hg
Describe the myogenic control of arterial pressure?
- The myogenic tone gets produced when arteriole and small artery smooth muscle cells contract in response to increased pressure.
- myogenic tone relaxes in response to decreased pressure
What is neurogenic control of flow?
Neurogenic mediation of cerebral vasoreactivity involves the control of small- and medium-sized vessel diameters.
How is neurogenic control of flow achieved?
Neurons, astrocytes and microglia secrete a variety of neurotransmitters with vasoactive properties.
1. acetylcholine and nitric oxide
> are relatively potent vasodilators
2. serotonin and neuropeptide Y
> stimulate vasoconstriction
How are changes in blood flow in response to neuronal activation observed?
as the blood oxygen level-dependent (BOLD) signal employed in functional magnetic resonance imaging (fMRI)
Describe the endothelial control of flow?
The endothelium secretes
1. vasodilators > nitric oxide (NO)
2. vasoconstrictors > thromboxane A2 and endothelin-1 in a paracrine manner
What are the 3 metabolic factors which have effects in controlling cerebral blood flow and how do they do so?
- Carbon dioxide concentration
- Hydrogen ion concentration
- Oxygen concentration
> An increase in either carbon dioxide or hydrogen ion concentration increases cerebral blood flow
> Decrease in oxygen concentration increases the flow
Describe the regulation of blood flow in response to excess CO2 and H+ concentration?
CO2 and H+ are potent vasodilators
> An increase in CO2 concentration in arterial blood perfusing the brain greatly increases cerebral blood flow
How does CO2 increase cerebral blood flow?
combining first with water in the body fluids to form carbonic acid, with subsequent dissociation to form hydrogen ions
Name other metabolic substances that increase acidity?
lactic acid and pyruvic acid
What is the importance of CO2 and H+ ion control of cerebral blood flow?
> Increased hydrogen ion concentration greatly depresses neuronal activity
fortunately hydrogen ions increase cause increase in blood flow, which in turn carries both carbon dioxide and other acidic substances away from the brain tissues
Loss of carbon dioxide removes carbonic acid from tissues and this along with removal of other acids which reduces hydrogen ions back towards normal
Describe how oxygen deficiency can regulate cerebral blood flow?
A decrease in cerebral tissue PO2 below 30 mm Hg increase cerebral blood flow
> At lower PO2 of 20 mm Hg brain function becomes deranged and coma can result
How do you calculate cerebral blood flow?
= cerebral perfusion pressure/cerebrovascular resistance
= (mean arterial pressure - intracranial pressure)/cerebrovascular resistance
Describe the role of sympathetic nervous system in regulating cerebral flow?
When autoregulatory mechanism fails to compensate enough, sympathetic control of cerebral blood flow is seen
e.g. when arterial pressure rises during exercise, the sympathetic system constricts the large and intermediate-sized arteries & prevent high pressures from reaching the smaller blood vessels
> This is important to prevent occurrence of a vascular haemorrhage into the brain – that is for preventing occurrence of cerebral stroke
Describe the cerebral microcirculation?
- The density of blood capillaries in the brain is greatest where the metabolic needs are greatest
- Metabolic rate of the gray matter is about 4 times as great as that of white matter so is the number of capillaries - Brain capillaries are much less leaky than capillaries in other tissues
- The capillaries are supported on all sides by glial feet
Note: The walls of small arterioles leading to capillaries greatly thickens in persons who develop high blood pressure to prevent transmission of high pressure to the capillaries
How much cerebral spinal fluid is formed a day?
500ml/day
Where is the CSF present?
This fluid is present in the:
> ventricles of the brain
> cisterns around the outside of the brain
> subarachnoid space around both the brain and the spinal cord
Where does the CSF originate?
⅔ originates from the choroid plexuses in the four ventricles, mainly in the two lateral ventricles
Ependymal surfaces of all the ventricles, arachnoidal membranes, perivascular spaces that surround the blood vessels passing through the brain
Describe the function of CSF?
CSF cushions the brain and the brain floats in the fluid
Describe the consequences of a mild blow to the head?
A mild blow to the head moves the entire brain simultaneously with the skull, causing no one portion of the brain to be momentarily contorted by the blow.
Describe the consequences of a severe blow to the head?
acoup injuryoccurs under the site of impact with an object, and acontrecoup injuryoccurs on the side opposite the area that was impacted.
> This phenomenon is known as coup-contrecoup injury
What is the blood brain barrier?
separates the lumen of the brain capillaries from the brain parenchyma
> Endothelial cells lining capillaries have tight junctions
Describe the blood brain barrier?
- separates the lumen of the brain capillaries from the brain parenchyma
> Endothelial cells lining capillaries have tight junctions - highly permeable to water, carbon dioxide, oxygen & most lipid-soluble substances such as alcohol & most anesthetics
- Slightly permeable to the electrolytes, such as sodium, chloride & potassium
Describe the blood-CSF barrier?
- found in the choroid plexus of each ventricle of the brain
> capillaries of the choroid plexus have no tight junctions and are fenestrated - are highly permeable to water, carbon dioxide, oxygen & most lipid-soluble substances such as alcohol & most anesthetics
- slightly permeable to the electrolytes, such as sodium, chloride & potassium
Name the regions with no blood brain barrier?
Some areas of the
1. hypothalamus
2. pituitary gland
3. median eminence
4. area postrema
5. preoptic recess
6. paraphysis
7. pineal gland
8. endothelium of choroid plexus
- This is important because these areas have sensory organs that respond to different changes in body fluids such as changes in osmolarity, glucose concentration etc
Notes: these organs are mainly involved in hormonal control
Describe brain metabolism during rest?
During resting, the brain metabolism accounts for 15% of the total metabolism in the body even though the mass of the brain is only 2% of the body
> Therefore, during resting brain metabolism is about seven & half times the average metabolism in the rest of the body
Most of the brain metabolism occurs in?
the neurons, not in the glial supportive tissues
- The major need for metabolism in the neurons is to pump ions through their membranes when conducting action potentials
- Almost all the energy used by the brain cells is supplied by glucose derived from the blood
The brain is not capable of much anaerobic metabolism because?
- Of high metabolic rate of neurons
- Glycogen stored in neurons is very little
- Stores of oxygen in brain tissues are little
> Therefore, neural activity depends on second-by-second delivery of oxygen from the blood
