Week 10 Physiology

Cerebral circulation/circle of Willis

  • IC –> Opthalmic artery, MCA and ACA
  • Vertebral arteries(from subclavian) –> PICA, anterior spinal artery –> basilar artery –> AICA, SCA, pontine arteries –> PCA
  • Anterior and posterior communicating arteries

    Atlas of Human Anatomy, Sixth Edition- Frank H. Netter, M.D

    Atlas of Human Anatomy, Sixth Edition- Frank H. Netter, M.D

 

CSF

  • Made in all 4 ventricles but mostly in lateral ventricle
  • Made in choroid plexus by ependymal cells
  • 500ml made per day but only 100-160ml in circulation at one time – constantly reabsorbed by arachnoid granulations into venous circulation.
  • Lateral ventricle –>interventricular foramen –> 3rd ventricle –> cerebral aqueduct–> 4th ventricle –> foramen of luscka and magendie –> subarachnoid space.

 

Blood-brain barrier.

  • Highly selsective permeability that separates the blood and the brain’s ECF. Formed by endothelial cells with tight junctions with high electrical resistivity. Astrocytes feet and thick basement membrane also contribute.
  • Function is to provide a stable environment for the brain because many molecules that fluctuate in the body’s ECF with food, exercise etc can cause excessive neuronal stimulation.
  • P-glycoprotein in endothelial cells can actively pump out drugs and toxins.
  • Metabolic processes in the endothelium also limit the effect of neurotransmitters (enzymatic BBB)
  • Substances cross it easily: Lipid soluble, water, some gases; active transport of glucose and amino acids.¬†

 

  • What are the circumventricular organs ?
    Organs outside the blood brain barrier because the tight junctions between endothelial cells is discontinuous

    • Pituitary gland
    • Pineal gland
    • Area postrema
    • Median eminence
    • Preoptic recess
    • Paraphysis
    • Endothelium of choroid plexus

 

Measurement of cerebral blood flow.

  • Typically 750ml/min or 15% of CO
  • Perfusion of 50-55ml/100g brain tissue/min –> usually brain 1400 g –> generally flow = 756ml/min
  • Measured
  • – fMRI or PET scans
  • Kety method – flow = amount of substance removed/concentration in (artery-vein), done using N2O

 

Regulation of cerebral blood flow (Important):

  • Flow = perfusion pressure(usually BP) / resistance
  • Resistance depends on
    • Metabolic autoregulation – eg accumulation of CO2 vasodilates
    • Pressure autoregulation – high BP vasoconstricts and low BP vasodilates
    • Neural control – sympathetic tone increases the plateau ie you can increase pressure more without increasing flow.
    • ICP – raised will increase resistace

 

Monroe-Kellie doctrine.

  • Brain is a closed box and its volume is constant so any increase in one of the components(blood, csf or brain tissue) has to be balanced by a decrease in another otherwise ICP will go up.
  • Brain 1400g, CSF 75ml, blood 75mL
  • In this way it can compensate for 100-120mL.

 

What is the normal ICP ?

  • 7-15mmHg in supine adult

 

Auto regulation of cerebral blood flow: Maintains a steady flow in brain at pressures varying from 65-140mmHg

 

Brain metabolism and O2 requirements.

  • Doesn’t store excess energy
  • Derives almost all energy needs from aerobic oxidation of glucose
  • Minutes before permanent brain damage if blood flow stops
  • O2 requirements vary depending on the are of the brain
  • Imaging can locate using radioisotope tagged glucose – 2DG

 

Coronary circulation

  • Fills during diastole, affected by neural input, wall tension
  • R and left coroanry sinus –> RCA and LCA
  • Right coronary artery supple SA node in 60% time and AV node in 80% of time
  • RCA –> marginals –> Posterior interventricular artery
  • LCA/Left main–> circumflex(marginals) and L anterior descending (diagonals)

 

Skin circulation:

  • Big role in thermal regulation and heat loss
  • AV anastamoses in fingers, palms, earlobes, penis
  • White reaction
    • Sharp object drawn lightly on skin causes whitening due to precapillary sphinchters contracting. Appears in 15s
  • Triple response.
    • Firm stroke by sharp object
    • Red – 10s – reddening due to capillary dilation
    • Wheal – few mins – swelling, mottled reddening – due to increased cap permeability
    • Flare – redness spreading out from injury due to arteriolar dilation.
    • Thought to be an axonal reflex – not seen if anesthetised

 

Placental and foetal circulation:

  • Placenta is fetus’s lungs, 55% of fetal CO goes through placenta
  • 2 umbilical arteries – deoxygenated
  • 1 umbilical vein – oxygenated
  • Umbilical vein –> ductus venosum –> IVC, the rest mix with fetal portal blood.
  • High resistance in pulm circ and patent foramen ovale so blood shunts from RA to LA. And any blood that does go to pulmonary artery gets shunted to aorta via ductus arteriosus after the R common carotid
  • Always check saturation on R hand of neonate cos that reflects the better saturated blood reaching brain, there is more mixed venous blood post ductus arteriosus so lower limbs get lower saturation blood.

Changes at birth

  • Stimulus at birth causes the baby to take a breath –> pulmonary circulation opens –> low resistance so blood is diverted into it. Once umbilical cord is cut the total peripheral resistance increases for LV.
  • FO valve closes almost immediately and ductus arteriosus close(few hours – functional, permanent -24-48hrs)

 

Viva questions:

  1. Tell me about the control of cerebral blood flow
  2. What is cerebral perfusion pressure ?
  3. Tell me about the control of the coronary circulation.
  4. What is the triple response ?
  5. Describe the foetal circulation.
  6. What changes occur in the foetal circulation at birth ?