Cerebral blood flow bloodbrain barrier Dr Mohammed Jeelani

Cerebral blood flow & blood-brain barrier Dr Mohammed Jeelani

Specific learning outcomes Explain the blood- brain barrier. Mention the normal volume of cerebral blood flow Describe the autoregulation of cerebral blood flow Describe the metabolic regulation of cerebral blood flow Explain the role of intracranial pressure in the regulation of cerebral blood flow

Blood- Brain Barrier • Restricts the movement of large molecules and highly charged ions • Formed : CNS capillary endothelial cells, their intercellular junctions and lack of vesicular transport • Substances crossed are not lipid soluble (specific carrier-mediated transport system)

Some areas of the brain do not have a BBB • Posterior pituitary and circumventricular organs. • Isolated from the rest of the brain by specialized ependymal cells called tanycytes.

Disruption of blood–brain barrier • Brain tumours and Bacterial meningitis. • Detected : radiologically

Normal cerebral blood flow • In an adult averages 50– 65 m. L/100 g, or about 750– 900 m. L/min. • Brain receives approximately 15% of the total resting cardiac output

REGULATION OF CEREBRAL BLOOD FLOW • Factors which affect the cerebral blood flow are: v. Arterial blood pressure v. Intracranial pressure v. Resistance, i. e. viscosity of the blood v. Diameter of the cerebral blood vessels

Metabolic regulation The important metabolic factors which play important role are: (i) Carbon dioxide. (ii) p. O 2. (iii) K+ ions.

Autoregulation of cerebral blood flow • 60 and 140 mm Hg blood pressure • below 60 mm Hg • extremely compromised and syncope • above 140 mm Hg • disruption of blood–brain barrier and cerebral oedema or cerebral haemorrhage

Role of intracranial pressure in regulation of cerebral blood flow (i) Monro-Kellie doctrine: the brain, CSF and blood in the cerebral vessels are three elements enclosed in a rigid cranial cavity and when any of them increases, it is at the expense of other ØHelps to maintain the cerebral blood flow when changes in the arterial blood pressure occur at the level of head.

(ii) Cushing reflex: When intracranial pressure is increased and becomes equal to the arterial pressure, it compresses the arteries in the brain and blood supply to vasomotor area is compromised. ØThe hypoxia and hypercapnia produced increases the discharge from VMC ØThe resultant rise in a systemic pressure tends to restore the cerebral blood flow

Nervous regulation of cerebral blood flow Innervated by the noradrenergic vasoconstrictor fibres and cholinergic vasodilator fibres. § Normal Conditions : do not regulate the cerebral blood flow. In severe hypertension: vasoconstriction In severe hypotension: vasodilation

Reference • Indu khurana. Medical physiology for UG, 1 st edition, Elsevier publication chapter 4. 6 page 272273, chapter 10. 6 page 775 -776