Compared to other tissues, the brain is extremely dependent on a stable and efficient blood supply. Despite making up only 2% of total body mass, the brain requires 15-20% of total cardiac output; this makes the brain extremely sensitive to hypoxia. Any hypoxic damage caused to the brain becomes irreversible after only a few minutes.
In normal tissue, there are three typical forms of metabolites utilisable for energy, these include; glucose, fatty acids and ketone bodies. However, in the brain only glucose can be utilised, except under extreme conditions, such as starvation. During these harsh conditions for the brain, ketone bodies may be used for energy. Because of the brains dependence on glucose, hypoglycaemia will result in dizziness and confusion as the brain is starved of energy.
An overview of Trophic Support:
- The heart pumps blood into the arterial system
- This moves blood to the skull
- The blood passes through a series of membranes (meninges)
- From here it moves into:
- Neuronal extracellular fluid
- Cerebrospinal fluid (CSF)
- Used blood then flows into venous sinuses (blood filled cavities between the skull & brain)
- Draining back into venous blood
- Moving back to the heart
The Circle of Willis
The Circle of Willis is a circular network of arteries that supply blood to the brain. It acts as a redistribution centre for blood which is supplied to the Circle of Willis; blood is brought together here and then moved to the brain. The Circle of Willis sits directly beneath the brain. The arrangement of the brain’s arteries into the circle of Willis creates redundancies in the cerebral circulation. If one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can often preserve the cerebral perfusion well enough to avoid the symptoms of ischemia (restriction of blood supply).
There are 4 routes which blood can take to reach the Circle of Willis, these are:
- Common carotid arteries -> internal carotid arteries (This is the most direct route)
- External carotid arteries -> maxillary arteries -> anatomising ramus ->internal carotid
- Vertebral arteries -> rete mirabile -> internal carotid
- Vertebral/Vertebral spinal arteries -> basilar artery
Fig. 1 – The arterial supply to the Circle of Willis, the different routes all begin by flowing in through the aorta.
The routes are colour coded to correspond with the above four routes:
In effect there are only two final routes into the Circle of Willis, either:
- Through either internal carotid artery
- Through the Basilar artery
At the Circle of Willis, the arteries anastamose (join together) to form a ring.
Fig. 2 – The Circle of Willis, upon reaching the Circle of Willis blood is then distributed further heading towards the brain. The blood supply from the Circle of Willis is shown in Fig. 3.
Fig. 3 (below) – Blood Supply from the Circle of Willis. The lines in red show which arteries the blood leaves the Circle of Willis from, whilst the lines in pink show the arteries which blood use to enter the Circle of Willis
Blood Supply From the Circle of Willis
There are essentially 5 pairs of arteries which supply the brain with blood. 4 of these are derived from the cerebral arterial circle, this is the red circle in Fig. 3, only the caudal cerebellar arteries are not derived from this ‘circle’. The five pairs of arteries which therefore supply the brain are:
- Rostral cerebral arteries
- Middle cerebral arteries
- Caudal cerebral arteries
- Rostal cerebellar arteries
- Caudal cerebellar arteries
The terms rostral (front) and caudal (back) are interchangable with the terms anterior and posterior respectively (these terms are used in human medicine).
The blood from different arteries emerging from the Circle of Willis tend to supply different parts of the brain:
- The medial surface of the brain is mainly supplied by:
- Rostral cerebral arteries
- Caudal cerebral arteries
- Whilst the lateral surface of the brain is mainly supplied by:
A rete mirabile is present in sheep, goats, swine, ox and dogs and is located in the venous cavernous sinus. The cat has a rete mirabile present also, however it is located extracranially. Retia Mirabilia are absent in the rat and rabbit.
Arterial Blood Supply to the Circle of Willis – Cats
During embryonic development in the cat, the internal carotid artery degenerates. In other species the internal carotid artery is typically a major artery in terms of direction of blood flow (see fig. 1). The maxillary artery compensates and becomes the major supply of blood to the Circle of Willis. Additional supply is also aided by a greater developed pharyngeal artery. This degeneration of the internal carotid also occurs in adult sheep, cows and pigs.
Variation of the Circle of Willis
The Circle of Willis’ structure can differ between species; this has even been observed between different breeds of dogs. The typical example is that some dogs completely lack a rostral communicating cerebral artery altogether.
Supply of Arterial Blood to the Cortex
The route of supply of blood to the brain may also differ between species:
- Human & Dog – The Circle of Willis receives blood by both the internal carotid and the basilar arteries. (Fig. 1) This means blood supply to the forebrain mainly originates from the internal carotids. Caudal areas of the brain are typically supplied with vertebral arterial blood.
- Cats, Sheep & Pigs – The internal carotid is much less important; the maxillary artery supplies the Circle of Willis via the anatomising ramus. In this situation the basilar artery carries blood away from the Circle of Willis (unlike in man). This means most blood supply to the forebrain and midbrain is derived from the maxillary artery.
- The Ox – Blood again flows away from the Circle of Willis via the basilar artery. Blood enters the Circle of Willis via maxillary and vertebral artery pathways, which are well mixed -meaning all areas of the brain, are supplied with blood from mixed maxillary and vertebral arterial origins.
Arterial Supply to the Spinal Cord
Like the brain, the spinal cord requires an equally rich supply of arterial blood, the general arrangement:
- Ventral spinal artery – a large, broad artery which runs the length of the ventral surface of the spinal cord
- Dorsal spinal arteries – a pair of smaller arteries which run parallel to the ventral spinal artery along the dorsolateral surface of the spine
- These arteries are joined by various anatomising arteries which form an arterial ring between the ventral spinal and dorsal spinal arteries.
Regulation of Arterial Blood Supply to the CNS
Should levels of CO2 or O2 alter in the blood, certain regulatory methods are in place to aid the return to normal levels. Hypoxic conditions occur when oxygen levels are low, whilst anoxic conditions occur when there is no oxygen. There are three main regulatory methods:
- Chemical autoregulation – In all systems, blood flow is locally regulated. When O2 tension falls or pCO2 increases local vasodilation occurs. This results in increased blood flow (Increased oxygen delivery and CO2 removal). The situation is similar for the brain within the restrictions of the inelastic skull.
- Sympathetic control – A sympathetic drive constricts blood vessels (vasoconstriction) which causes hypoxia. Vasodilation occurs as a result which increases the volume of the brain within the skull causing pressure and pain.
- Myogenic autoregulation – As pressure increases within cerebral arteries, muscle responds by constricting to prevent increased blood flow.
The Blood Brain Barrier
To protect the brain tissue, a blood-brain barrier is in place. This prevents unwanted substances crossing into brain tissue freely; certain transport systems are in place to selectively transport required substances.
Glucose and ketone bodies do not freely pass through the blood-brain barrier they are instead specifically transported across the barrier. It is believed than even molecules such as water require selective transport across the blood-brain barrier, water is moved across by aquaporin transport proteins.
In simple terms the brain capillaries are surrounded by ‘astrocyte feet’. These astrocytic feet serve to regulate the blood brain barrier in some way.
Differences between a regular capillary tissue boundary and the blood-brain barrier:
- The capillaries are not fenestrated, but bound together with tight junctions. This prevents simple diffusion of substances between the gaps as would happen outside the CNS.
- Pericytes site alongside the capillaries, these may be involved with capillary proliferation.
- Astrocytic feet surround the capillaries and their role appears to be in the formation and maintenance of the blood-brain barrier complex.