The cerebrum III

Veins of the brain

External cerebral veins	Internal cerebral veins
Superior cerebral vein : Drain to superior sagittal sinus Superficial middle cerebral vein : Drain to cavernous sinus Deep middle cerebral vein (It’s joined by the anterior cerebral & the striate veins to form the basal vein; basal vein drains to great cerebral vein & it drains to the straight sinus)	2 internal cerebral veins (Formed by the union of thalamostriate vein & the choroid vein) 2 veins unite to form the great cerebral vein

Veins of specific brain areas

Midbrain	Drain to basal or great cerebral veins
Pons	Drain to basal vein, cerebellar vein & neighboring venous sinuses
Medulla oblongata	Drain to spinal veins & neighboring venous sinuses
Cerebellum	Drain to great cerebral vein

Cerebral circulation

Cerebral blood flow is measured by the intra carotid injection or inhalation of radioactive Krypton or Xenon
Normal cerebral blood flow : 50 – 60ml per 100g of brain per minute
Right half of the brain is supplied by the vertebral & internal carotid arteries on the right side and vice versa
The streams of the vertebral & internal carotid arteries come together in the posterior communicating artery (At that point pressure is equal on both sides & don’t mix)
If one of the arteries is occluded, blood moves forward or backward to compensate the reduction
If the vertebral or internal carotid artery is occluded on one side, blood can flow across the midline (Via anterior communicating artery)
Streams of 2 vertebral arteries remain separate within the basilar artery & don’t mix
Cerebral blood flow remain constant despite changes in general blood pressure (Compensatory lowering of cerebral vascular resistance in low blood pressure & vice versa)

Brain capillaries

Greater in the grey matter than in white matter
Blood brain barrier isolates the brain from the rest of the body
BBB is formed by the tight junctions that exist between the endothelial cells in the capillary beds

Nerves of the cerebral arteries

Cerebral arteries are supplied by sympathetic post ganglionic fibers (derived from the superior cervical sympathetic ganglion) & cause vasoconstriction
Under normal circumstances, the local blood flow is mainly controlled by CO₂, H⁺ & O₂ levels
Local increase in CO₂ & H⁺ level (Viewing an object will increase O₂₂ tension will result in local increase in blood flow (Vasodilatation) & glucose consumption in the visual cortex of the occipital lobe) or decrease in O₂ tension will result in local increase in blood flow (Vasodilatation)