Blood Supply Flashcards
What are the metabolic demands of the brain like?
Very high:
- Only 2% of body weight
- 20% O2 consumption
How sensitive is the brain to O2 deprivation?
Very sensitive - with ischemic cell death resulting within minutes.
The skull is very rigid and cannot be expanded. What is the pathological effect of this?
If there is anything in the brain that there shouldn’t be, this will act as a mass and cause compressions or herniations (e.g. tumour)
What is a lesion?
an area of tissue that has been damaged through injury or disease
What can cause focal cerebral lesions?
- Vascular (most common)
- Tumours (1ary or 2ary)
- Abscess
- Hydrocephalus
What are examples of vascular cerebral lesions?
- Transient Ischaemic Attack (TIA)
- Stroke
What is a stroke?
A neurological deficit of vascular origin, lasting more than 24 hours (may be due to infarction or haemorrhage)
Which 2 main arteries supply the brain?
There are two paired arteries which are responsible for the blood supply to the brain:
- Vertebral arteries
- Internal carotid arteries
Where are the vertebral arteries and the internal carotid arteries connected?
The terminal branches of these arteries form an anastomotic circle at the base of the brain –> the Circle of Willis
The vertebral artery is a major artery in the neck. Where does it arise from?
Branch of the subclavian artery
What is the transverse foramen? What is it specific to?
Specific to the cervical vertebra is the transverse foramen (also known as foramen transversarium). This is an opening on each of the transverse processes which gives passage to the vertebral artery and vein and a sympathetic nerve plexus.
How does the vertebral artery ascend in the neck?
Ascends through the transverse foramina of the cervical vertebrae until they reach the foramen magnum
how do the vertebral arteries enter the skull?
Via the foramen magnum
After passing through the foramen magnum and entering the skull, what happens to the 2 vertebral arteries (one each side)?
They fuse at the base of the brainstem to form the basilar artery
Where do the internal carotid arteries (one each side) arise from?
At the bifurcation of the left and right common carotid arteries
What vertebral level is the bifurcation of the left and right common carotid arteries?
C4
As the interal carotid arteries move superiorly, what are the enclosed in?
The carotid sheath
How do the internal carotid arteries enter the skull?
Via the carotid canal of the temporal bone
What does the external carotid artery supply?
Gives off many branches that supply blood to the face and neck
Does the internal carotid artery give off any branches in the neck?
No - straight to brain
Once in the cranial cavity, the internal carotid artery has a series of characteristic bends inside which region?
The cavernous sinus
After the cavernous sinus, where does the ICA emerge?
Either side of the optic chiasm
Inferior view of skull and foramen
Before converging to form the basilar artery, what 3 branches does each vertebral artery give off?
- Meningeal branc
- Anterior and posterior spinal arteries
- Posterior inferior cerebellar artery (PICA)
Labelled diagram of PICA
What does the PICA supply?
The posterior and inferior portion of the cerebellum
Labelled diagram of anterior spinal artery
How many PICAs are there?
2 (paired)
How many anterior spinal arteries are there?
Only 1 (unpaired)
How is the anterior spinal artery formed?
Contribution from each vertebral artery which fuse to form the single anterior spinal artery
Where does the anterior spinal artery travel? What does it supply?
All the way down the length of the spinal cord to supply its anterior aspect
Labelled diagram of posterior spinal artery
How many posterior spinal arteries are there?
2 (paired)
How is the posterior spinal artery formed?
Origin is variable:
- In some people they come straight off the vertebral arteries
- In others they come from the PICA (seen in this image)
Where does the posterior spinal artery travel? What does it supply?
All the way down the length of the spinal cord to supply its posterior aspect
There is also an anterior inferior cerebellar artery (AICA). Where does this arise from?
Most commonly arises from the basilar artery (after the 2 vertebral arteries have fused)
How many AICAs are there?
2 (paired)
What does the AICA supply?
The anterior inferior portion of the cerebellum
Several branches arise from the basilar artery. What are these called? What do they supply?
(Transverse) pontine branches - supply the pons
Distal to these pontine branches arising, which branch arises from the distal basilar artery near the termination of the basilar artery?
The superior cerebellar artery (paired)
What does the superior cerebellar artery supply?
The superior aspect of the cerebellum
How does the basilar artery terminate?
by bifurcating into the posterior cerebral arteries (PCA)
What do the posterior cerebral arteries supply?
The posterior aspect of the cerebrum
How many posterior cerebral arteries are there?
2 (paired)
How many pairs of cerebellar arteries are there?
3: PICA, AICA, SCA
Cerebrum vs cerebellum?
Cerebrum: is the largest part of the brain and is composed of right and left hemispheres.
Cerebellum: is located under the cerebrum.
Diagram of ICA emerging
Either side of optic chiasmus
Distal to the cavernous sinus, what branches does the ICA give rise to?
- Middle cerebral artery (N.B. this can be viewed as the continuation of the ICA)
- Ophthalmic artery
- Posterior communicating artery
- Anterior choroidal artery
- Anterior cerebral artery
Diagram of middle cerebral artery
What does the middle cerebral artery supply?
The lateral portions of the cerebrum
Diagram of anterior cerebral artery (ACA)
How many anterior cerebral arteries are there?
2 (paired)
What does the anterior cerebral artery supply?
Tucks under the midline portion of each cerebral hemisphere to supply its anterior and medial aspect
What is the Circle of Willis?
A closed loop, forming the vertebral/basilar system and the interal carotid system
What are the components of the Circle of Willis?
3 main paired constituents:
- Anterior cerebral arteries - branches of the internal carotid arteries.
- Internal carotid arteries - located immediately proximal to the origin of the middle cerebral arteries
- Posterior cerebral arteries - terminal branches of the basilar artery
To complete the circle, two ‘connecting vessels’ are also present:
- Anterior communicating artery
- Posterior communicating artery
What does the anterior communicating artery connect?
is a short artery connecting the left and right anterior cerebral arteries
What does the posterior communicating artery connect?
Connects the posterior cerebral artery and the middle cerebral artery
How many anterior communicating arteries are there?
1 (unparied?
How many posterior communicating arteries are there?
2 (paired)
Diagram of ant and post communicating artery
What does the Circle of Willis surround?
Surrounds optic chiasma and hypothalamus
Purpose of the Circle of Willis?
Ensures compensation of decreased blood flow if one vessel is occluded
Typical arrangement of Circle of Willis. In most cases, the circle of Willis is formed by which arteries?
the anterior, middle, and posterior cerebral arteries; the anterior and posterior communicating arteries; the internal carotid arteries; and the basilar artery.
There are often variations in the Circle of Willis. What are the 6 most common?
- the anterior communicating artery is absent
- Both anterior cerebral arteries may arise from one internal carotid artery
- The posterior communicating artery may be absent or hypoplastic (underdeveloped)
- Both posterior communicating arteries may be absent or hypoplastic
- The posterior cerebral artery may be absent or hypoplastic on one side.
- Both posterior cerebral arteries may be absent or hypoplastic (in addition, the anterior cerebral arteries may arise from a common trunk)
What is a berry aneurysm? What typically causes them?
Abnormal dilations around the circle of Willis or at at the points of arterial branching within the brain (more common here).
Caused by a weakening in the vessel wall.
Why are aneurysms more common at areas of arterial branching?
Aneurysms develop at branch points of high intravascular turbulence and abnormal vessel wall shear stress
What can a rupture of a berry aneurysm lead to?
A vascular stroke (~ 10% of vascular strokes are caused by rupture of a berry aneurysm)
What happens when a berry aneurysm ruptures?
Bleeding into the subarachnoid space (subarachnoid haemorrhage)
Labelled angiogram of a berry aneurysm. Where is this aneurysm located?
Aneurysm located at the terminal bifurcation of the basilar artery
Angiogram from ICA system (one side). What does the green line denote?
ICA ascending through neck and entering skull through the carotid canal (forming the characteristic bend as it passes through the cavernous sinous)
Labelled angiogram of ICA
There is a very close relationship between the cranial nerves and blood supply. What can rupture of a vessel lead to?
Functional impairment of one of nerves
What area is supplied by the ACA?
- Superior and medial areas of frontal and parietal lobes
- Corpus callosum
(area shown in blue)
What area is supplied by the MCA?
•Lateral areas of frontal, temporal and parietal lobes
(shown here in pink)
What area is supplied by the PCA?
- Occipital lobe
- Inferior and medial surface of temporal lobe
(shown here in orange)
Lateral view of areas supplied by MCA (pink), ACA (blue) and PCA (orange)
Medial view of areas supplied by the ACA (blue) , PCA (orange) and MCA (pink)
The occitpital lobe is responsible for vision. A rupture of which artery can affect vision?
PCA
Deep, perforating branches also arise in the brain. Some of these are called striate arteries (delicate). What structures do these supply?
These supply structures inside hemisphere:
- Deep nuclei
- Deep grey matter (e.g. thalamus)
- Internal capsule (major fibre bundle to and from cerebral cortex - deep white matter)
- Supply includes major descending motor pathway
Where do the these striate branches arise from?
MCA
What is the internal capsule of the brain?
- A white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain
- Descending motor pathway travels in this capsule
Why can damage to the striate arteries lead to motor deficits?
As major descending motor pathway travels in the internal capsule supplied by these arteries
What are watershed areas in the brain?
Regions that receive blood supply from distal areas of adjacent cerebral arteries
Why are watershed areas susceptible to ischemia (Watershed stroke) when decreased systemic blood pressure?
Tissue is furthest from arterial supply and thus most vulnerable to reductions in perfusion
Describe the ACA-MCA watershed area
- Supplied by distal branches of the ACA
- Supplied by distal branches of MCA
- (overlapping territory)
What can the veins of the cerebrum be divided into?
Superficial and deep groups, which are flamboyantly arranged around the gyri and sulci of the brain
Where do the superficial veins drain into?
Straight into the dural sinuses
Where do the deep veins drain into?
The great cerebral vein
What is the superficial system of veins is largely responsible for?
Draining the cerebral cortex
What do the deep veins drain?
Internal structures of the brain (e.g. corpus callosum, thalamus, basal ganglia)
What does the great cerebral vein drain into?
The straight sinus and ultimately into the confluence of sinuses
Where are the dural venous sinuses located?
Located within the dural infoldings are various venous (dural) sinuses (ultimately where cerebral veins will empty blood)
Dural venous sinuses to be aware of: superior and inferior sagittal sinuses
- Superior sagittal sinus - travels along the attached border of the falx cerebri to enter into the confluence of sinuses at the back
- Inferior sagittal sinus - travels along the free edge of the falx cerebri to drain into the straight sinus (and eventually the confluence of sinuses)
What is the confluence of sinuses?
The connecting point of the superior sagittal sinus, straight sinus, and occipital sinuses.
Where is the confluence of sinuses found?
Found at the attached border of the falx and the tentorium cerebelli
What does the straight sinus empty its blood into?
The confluence of sinuses
Where do the transverse sinuses travel (one left and one right)? What do they become?
- Along the attached border of the tentorium cerebelli
- They then bend in a characteristic S shape to become the sigmoid sinus
What is the cavernous sinus? Where is it located?
- The cavernous sinus is one of the dural venous sinuses of the head
- Located laterally to the body of the sphenoid bone
Once blood enters the dural venous sinuses and circulates through them, where does that blood then drain?
Into the internal jugular veins
Describe the pathway of venous drainage from superficial veins
Superficial veins –> superior sagittal sinus –> confluence of sinuses –> transverse sinuses –> sigmoid sinus –> IJV
where does the confluence of sinus drain into?
Blood arriving at this point then proceeds to drain into the left and right transverse sinuses.
Where does blood from the superficial veins meet with blood from the deep veins?
At the confluence of sinuses
Describe the pathway of venous drainage from the deep veins?
Deep veins –> internal cerebral veins –> great vein (of Galen) –> straight sinus –> confluence of sinuses –> transverse sinuses –> sigmoid sinus –> IJV
Venous Drainage from Brain to IJV
Diagram of falx cerebri, tentorium cerebelli and falx cerebelli
Transverse sinuses drawn in black arrows
What is circled in black?
Confluence of sinuses
Where is the jugular foramen located?
At the meeting point between the occipital and the temporal bone
What is the cavernous sinus? Why is it clinically important?
- Is a paired dural venous sinus located within the cranial cavity. It is divided by septa into small ‘caves’
- Each cavernous sinus has a close anatomical relationship with several key structures in the head - important in infection
Within which cranial fossa are the cavernous sinuses?
In the middle cranial fossa
How does the pituitary gland relate to the paired cavernous sinuses?
The pituitary gland lies between the two paired cavernous sinuses.
Which paranasal sinus is located close to the cavernous sinuses? Why is this important during infection?
Sphenoid sinus - infection from here can spread to the cavernous sinus
What are the contents of the cavernous sinus? What is the clinical relevance of this?
- Internal carotid artery
- CN III (oculomotor)
- CN IV (trochlear)
- CN V1 (opthalmic division of the trigeminal)
- CN V2 (maxillary division of the trigeminal)
- CN VI (abducens)
Infections can affect all these structures
What is the ‘danger triangle’ of the face?
Consists of the area from the corners of the mouth to the bridge of the nose, including the nose and maxilla
Why are any cuts in the danger triangle important to treat?
Veins of the face connect via ophthalmic veins to cavernous sinus directly –> infections can spread to the cranial cavity
What can infections of the cranial cavity lead to?
- Meningitis
- Cavernous sinus thrombosis: headaches, oedema of the eyeball, numbness of the face, palsies of cranial nerves.
Where are all of the cerebral arteries and veins located?
Below the arachnoid –> subarachnoid space
Where is the middle meningeal artery located?
Within the outer layer of the dura (periosteal)
What can rupture of the middle meningeal artery lead to (e.g. blow to the pterion region)?
Haemorrhage between outer layer of dura and skull –> epidural haemorrhage
How is the superior sagittal sinus created?
- Separation of the layers of the dura mater
- Travels in midline
What are bridging veins? Where are they located?
Veins in the subarachnoid space that puncture the dura mater and empty into the dural venous sinuses (superior sagittal sinus)
What can a rupture of a bridging vein cause?
a subdural haematoma.
Diagram of relationship between vessels and meninges
What type of haemorrhage can a torn meningeal artery lead to?
An epidural haemorrhage
What type of haemorrhage can a torn bridging vein lead to?
A subdural haemorrhage (typical crescent shape, compresses ventricles)
What type of haemorrhage can a torn cerebral artery lead to?
A subarachnoid haemorrhage (blood fills spaces within brain tissue itself)
What are focal lesions?
Focal lesions are circumscribed areas of injury to brain tissue following brain injury. In closed head injury, such lesions are usually associated with vascular damage, such as contusions or hemorrhages.
