CEREBRAL CIRCULATION
CEREBRAL CIRCULATION:
Introduction:
- Cerebral blood flow (CBF) – 750 ml/min (15 % of total cardiac output) or 54ml/l00g brain tissue/min.
- Fairly constant blood flow – Characteristic feature of cerebral blood supply.
Brain metabolism & oxygen consumption:
- Brain does not contain any fuel reserves & blood need to be maintained continuously.
- Glucose – Only major & ultimate source of energy.
Glucose entry:
- Enters brain via GLUT-1 in cerebral capillaries.
- During starvation – Brain adapts to ketone bodies usage.
O2 consumption by human brain (cerebral metabolic O2 rate/CMRO2):
- 20% of total body resting O2 consumption.
- Brain is very sensitive to hypoxia – unconsciousness within 10 seconds.
Regulation of cerebral blood flow:
1. Autoregulation:
- Total cerebral blood flow to be held constant, despite systemic changes in blood pressure (60-150mm Hg).
Cerebral perfusion pressure:
- Difference between mean arterial pressure (MAP) & intracranial pressure (ICp)/central venous pressure (CVP) whichever is greater.
- Autoregulation maintains a steady CBF, despite pressure variation between 60 & 160 mm/Hg. Decreases in CPP result in cerebral vasodilation; Increased CPP induce vasoconstriction.
2. Metabolic regulation:
- Cerebral blood flow varies directly with metabolic activity.
Metabolic factors:
Blood Co2:
- Increased blood Co2 (Main) → Vasodilatation.
- CO2 rapidly diffused into BBB.
- CO2 produced by activated neurons.
- Vasodilatory effect is indirect & mediated by plasma H+ formation. H+ – Has direct vasodilatory effect.
- CBP doubled at an arterial PCO2 of 58 mm Hg.
Blood O2:
- Decreased blood PO2 → Vasodilatation.
- Increased blood Po2 → Vasoconstriction.
Other factors:
- Potassium ions & adenosine.
3. Intracranial pressure (ICP):
- Increased ICP→ Impaired blood supply to VMC (RVLM) → Increased local hypercapnia & hypoxia → Increased discharge to systemic resistance vessels.
- Causes raised BP → Restores CBF.
- Referred as “Cushing reflex”.
Cushing reflex:
- Consists of hypertension, bradycardia & tachypnea.
Note:
- Increased BP → Results reflex bradycardia
Exam Important
- Glucose is the only major & ultimate source of energy for brain, which enters via GLUT-1 in cerebral capillaries.
- During starvation, brain adapts to ketone bodies usage.
- Cerebral blood flow (CBF) – 750 ml/min (15 % of total cardiac output) or 54ml/l00g brain tissue/min.
- Brain is very sensitive to hypoxia – unconsciousness within 10 seconds.
- Total cerebral blood flow to be held constant, despite systemic changes in blood pressure (60-150mm Hg).
- Autoregulation maintains a steady CBF, despite pressure variation between 60 & 160 mm/Hg. Decreases in CPP result in cerebral vasodilation; Increased CPP induce vasoconstriction.
- CBP doubled at an arterial PCO2 of 58 mm Hg.
- Decreased blood PO2 → Vasodilatation & increased blood Po2 –> Vasoconstriction.
- Cushing reflex consists of hypertension, bradycardia & tachypnea.
- Increased ICP causes impaired blood supply to VMC (RVLM), in turn increasing local hypercapnia & hypoxia further resulting in increased discharge to systemic resistance vessels. This is referred as “Cushing Reflex”.
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