Brain Cooling and the Cranial Veins
Brain cooling is extremely important. The brain, just like the rest of the body produces heat as a byproduct of the work it does. The human brain is large and it does a great deal work producing a proportionate amount of heat, which is significant.
The brain is enclosed within the walls of the skull with few openings. This creates a problem when it comes to removing the heat produced by the brain. This page discusses the role of the veins in removing heat from within the skull and cooling blood before it enters the brain also helping to reduce heat.
With regards to brain cooling, an overview of the cranial veins will make the connection they have to cooling clear. The cranial veins include: the diploic veins, the dural sinuses, the veins of the face and scalp, and the emissary veins. Additionally, the cavernous and suboccipital cavernous sinus (atlantooccipital membrane) also serve as counter-current heat exchangers , which will be explained below.
The cranial veins of the face and scalp are on the outside of the skull where the cooling process begins with conduction, convection and sweat evaporation through the skin. The diploic veins, seen in the picture above, sit between the inner and outer plates of the membranous bones of the skull that cover the cranial vault. The emissary veins connect the veins of the face and scalp both to the diploic veins and the dural sinuses. If you look at the top picture, these veins are represented as little black semicircles. The dural sinuses seen in the picture below are the main drainage routes of the brain inside the cranial vault.
In contrast to the scalp and facial veins, the diploe and dural sinuses are on the inside of the skull covered from the outside by bones and fat that serve as insulation that help keep it cool inside by keeping heat out. Although they are part of the system of cranial veins, the dural sinuses are not veins. Instead they are tunnels formed by the outer coat of the brain itself, the dura mater. The inside walls of the dural sinuses are lined with the inner walls of veins. The dura mater, which means tough mother or material, makes the dural sinus drainage system much stronger than typical veins. As a result, they are better able to withstand stress and resist deformation from pressure and movement of the brain, which sits on top of and presses down against them. They are depicted in the top picture by the striped structures inside the skull.
The emissary veins play more than one important role in brain cooling. As was mentioned, one of their roles is the connection they make between the diploe and dural sinuses to the veins of the face and scalp.
The cooled venous blood from the face and scalp can thus flow through emissary veins and into the diploic veins between the inner and outer layers of bone that form the cap over the brain. It can also flow through the skull and into the dural sinuses inside the cranial vault. The emissary veins thus cool the dural sinuses directly (see picture above). They also keep a cooler layer of blood in diploe between the bones of the skull.
The other role some of the emissary veins play is in draining the head and brain during upright posture. In particular, (in the top picture) the ones located toward the back and bottom of the skull, as well as behind the outline of the ear, all drain into the vertebral veins of the spine. The emissary veins that are used to increase drainage capacity of the brain during upright posture empty into the internal and external vertebral veins inside and outside the spine and spinal canal. The vertebral veins thus help to transport heat away from the brain.
Lastly, the most important feature of the brain cooling system is the heat exchanger mechanism used by humans. In this case venous blood that has been cooled by cranial veins of the face and scalp flows through dural sinuses that serve as counter-current heat exchangers that cool incoming arterial blood before it enters the brain.
If you look at the picture above of the main dural sinus of the drainage system of the brain, you will see the internal carotid artery, depicted in red, passing through the venous cavernous sinus (blue) before it enters the brain. Several veins from the face drain cooled blood into the cavernous sinus thus allowing it to act as a heat exchanger for arterial blood entering the brain. As an aside, it is interesting to note that the largest vein that plays a role in cooling the cavernous sinus is the opthalmic vein, which drains the eye. The exposed surface of the eye thus helps cool the brain.
Cooled venous blood leaves the cavernous sinus by several routes. Two of the main routes are the superior and inferior petrosal sinuses seen in the picture above at the rear of the cavernous sinus. The petrosal sinuses pass through the jugular foramen located in the posterior fossa of the cranial vault and empty into the internal jugular veins.
The suboccipital cavernous sinus, which is also known as the atlantooccipital membrane as depicted in the picture below also acts as a heat exchanger. It is located just outside the skull between the first cervical vertebra and the occipital bone at the base of the skull. Although it is outside the skull, the suboccipital cavernous sinus is constructed of nearly identical materials, in the exact same way and serves similar functions to the cavernous sinus. For this reason, some scientists now consider the suboccipital cavernous sinus to be part of the intracranial dural sinuses of the brain. The vertebral arteries pass through the suboccipital cavernous sinus, which cools the blood before it enters the brain. The suboccipital cavernous sinus is cooled by scalp and emissary veins.
In brief, the net effect of heat production in the brain and brain cooling keeps the brain cooler than the rest of the body and is achieved by surrounding and bathing the brain with venous blood that has been cooled outside the cranial vault, by bone and fat acting as insulation, by the veins of the face and scalp through conduction, convection, sweat and evaporation and by cooled venous blood flowing through the cavernous and suboccipital cavernous sinuses cooling incoming blood in the internal carotid and vertebral arteries before it enters the brain. The combined effect of the brain cooling system keeps the temperature inside the vault and brain about two to three degrees cooler than the rest of the body. The effect is important enough that some physical anthropologists attribute the extra-large size of the human brain more to its exceptional cooling capacity than to the increase in arterial blood flow that comes with upright posture. Anthropologists refer to human encephalization due to enhanced brain cooling capacity as the "radiator theory".
It is interesting to note that recent research by Dr. Paulo Zamboni suggests that obstruction of the drainage system of the brain, which he calls chronic cerebrospinal venous insufficiency (CCSVI) can cause multiple sclerosis (MS). Dr. Zamboni attributes the cause of CCSVI to obstruction in the jugular veins (one of the main extracranial drainage routes). As I discuss in my book, it is similarly possible that, chronic craniocervical venous back pressure (CCVBP) due to misalignments and other problems of the upper cervical spine can lead to neurodegenerative processes and subsequent MS similar to CCSVI. In contrast to the internal jugular veins, CCVBP affects the vertebral veins. The cavernous and suboccipital cavernous sinues, as well as most of the brain, drain into two extracranial venous drainage routes. One is the internal jugular veins, the other system empties into the vertebral veins. Thus, in addition to venous outflow, CCSVI due to jugular stenosis and faulty valves, and CCVBP due to upper cervical misalignments can affect brain cooling capacity due their impact on venous outflow and arterial cooling in the cavernous and suboccipital cavernous sinuses. Interestingly, in addition to many other signs and symptoms a common complaint among MS patients is heat intolerance. It is possible that both CCSVI and CCVBP can decrease the cooling capacity of the brain and thus contribute to the heat intolerance seen in many MS patients.
In addition to it's impact on cooling the brain via arterial and venous blood flow, obstruction of the drainage system of the brain can also affect cerebrospinal fluid (CSF) flow. A decrease in CSF flow can further affect brain cooling causing heat intolerance by way of its impact on the midbrain and autonomic nervous system - but that's a topic for another page.
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