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L1- Regional Circulation 2

Regional Circulation 2

4) Cerebral Circulation

Cerebral blood flow

-In normal adult the brain weights 1400 gm and receives 750 ml blood /min (14% of cardiac output).

-In children cerebral blood flow is nearly double  it’s value in adult and it falls to the adult level at puberty. The sex hormones are responsible for this drop.

-Adequate blood supply to the brain is essential.

-Arrest of cerebral circulation more than 5 seconds is followed by loss of consciousness .

-And more than 3minutes>>>>irreversible damage of the grey matter of the  cortex.

-Blood flow of the grey matter is about six times that of the white matter.

-It is enclosed within a solid structure (skull)

-This render the volume of the brain ,blood and cerebrospinal fluid to be constant at any time.

-Is regulated to maintain constant cerebral blood flow sufficient for the metabolic need of the brain.

-Blood flow increases in active  area and decreases In  the inactive one. i.e change in regional blood flow.

-Glucose is the main source of energy in the brain.

-Brain tissues is very sensitive to hypoglycemia and hypoxia.

-The brain is the least tolerant organ to ischaemia

-Interruption of cerebral blood flow for even 5 sec may lead to fainting (syncope)

-The brain represents ~ 2.5% of TBW

-Receiving ~ 15% of Total CO

-The AV O2 difference is~13Vol%

-The cerebral BF is tightly coupled to O2 consumption

-The brain is enclosed within the rigid cranium

-Thus, the volume within the cranium must be kept constant

-So, an increase in arterial vascular volume with arterial dilatation at one site must be associated with a decrease in another site

-Maintenance of Cerebral BF is ensured by a balance between mean arterial pressure (MAP) & intracranial pressure (ICP)

-This balance or difference is cerebral perfusion pressure (CPP) = 80 mm Hg

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Cerebral blood vessels

-The cerebral arterial blood is provided from:

  1. 2 internal carotid arteries.
  2. 2 vertebral arteries unit together>>basilar artery.
  3. Both the internal carotid arteries and the basilar artery forms the circle  of Willis at the base of the brain .
  4. from which 6 cerebral arteries arise to supply the brain 3 on each side.

-Cerebral arteries are considered functionally as end arteries ,there is no anastomoses.

-There is normally no crossing of circulation from one side of the circle to the other because the pressure is equal in both sides.

-Cerebral veins drain venous >>>internal jugular veins.

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Regulation of cerebral blood flow

-Auto regulation. (intrinsic mechanism)

-Extrinsic mechanisms:

  1. Nervous factors.
  2. Chemical factors.
  3. Mechanical factors.

Auto regulation (Intrinsic Mecha)

-It is the ability of the brain to maintain relative constant level of blood flow over a wide range of ABP (70-150 mmHg)

-Autoregulation means the intrinsic ability of an organ to maintain a constant blood flow in spite of changes in perfusion pressure •

-This ensures adequate supply of O2 to critically important organs

-This autoregulation occurs in absence of nervous or hormonal factors

-It is attributed to myogenic, metabolic & endothelial mechanisms

-Increased ABP>>increase CBF>>increase O2 & decrease CO2 tension >> V.C

-Decrease ABP>>>V.D

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-Cerebral BF remains more or less constant with a change in ABP between 60-150 mm Hg (autoregulatory range)

-Theories for autoregulation:

1) myogenic (↑ABP→ ↑ BF → stretch of vascular smooth muscle → stretch-induced ms contraction → ↓ BF back to normal)

•↓ ABP → ↓BF→ inhibition of smooth ms→ vasodilatation→ ↑BF back to normal

2metabolic theory:

↓BF→ accumulation of metabolites (adenosine, K+, lactate, ATP) →VD→↑BF

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Extrinsic mechanism

-Nervous factors:

  • Sympathetic stimulation

•The cerebral vessels have rich sympathetic innervations

•However, under normal conditions they have no role in regulation of cerebral BF (overridden by autoregulatory mechanism)

•Their importance have been outlined as protective in cases of marked increased of ABP where it constrict the large & medium sized vessels thus protecting the smaller vessels from rupture

-Chemical factors:

  • O2 ,CO2 , H

•Changes in CO2, O2 & H+ concentrations have important effect on Cerebral BF

•70%↑in PCO2 will double CBF

•The effect is not direct but through H+ which causes VD of cerebral Bl.V

•This is beneficial to get rid off excess H+ that causes depression of nervous system

•A decrease in cerebral tissues PO2 below 30 mm Hg stimulates VD of cerebral Bl.V

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-Mechanical factors:

  • Blood viscosity
  • Intracranial tension

•ICP is produced by presence of CSF & normally=0-10 mm Hg

•Cerebral perfusion pressure (CPP) which is the pressure pushing blood inside the brain

= MAP-ICP

•Thus, any factor that increases ICP will decreases CPP & consequently can reduce cerebral BF

•Marked rise in ICP >33 mm Hg will compress cerebral Bl.V → marked reduction in cerebral BF

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Blood Brain Barrier

-It means very low and selective permeability of the cerebral capillaries. -It is due to:

  1. Tight junction between the capillary endothelial cells
  2. The end cells of the astrocyte glial cells covers most of capillary wall.

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5) Cutaneous circulation

-Skin comprise 4-5% of the total body weight.

-It receive about 6% of the COP.

-The A-V difference is very low because most of the blood flow is non nutritive.

-Types of cutaneous blood vessels:

  1. Nutritive vessels.
  2. Non nutritive

•The major function of skin blood flow is regulating body temperature

•Skin represents ~ 4-5% of TBW

•Receives ~2% of COP

•The Arterio-Venous O2 difference is only 3Vol%, indicating that the flow is mostly non-nutrient flow
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•The cutaneous circulation is under control of hypothalamic thermoregulatory centre that adjusts the sympathetic outflow to skin blood vessels

•If body temperature starts to rise, the hypothalamus decreases sympathetic flow to skin BV→ VD →↑cutaneous BF→ facilitates heat loss

•By contrast, ↓in body temperature →hypothalamus increases sympathetic outflow to skin BV→ VC of cutaneous BV→↓BF→ retention of heat

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Regulation of Cutaneous blood flow

-The blood flow is regulated by the heat regulation centre in the hypothalamus.

-Exposure to hot >>>V.D

-Exposure to cold >>>V.C

-The skin is supplied by sympathetic fibers.

-There is no parasympathetic supply to the skin.

-Regulated by:

1.Sympathetic innervation

2.Local Metabolites

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Factors regulating Skin Blood Flow:

Nervous

• skin arterioles (& AV anastomoses) are innervated by sympathetic nerves originating from sympathetic chain as in skeletal muscle

•The sympathetic supply to skin vessels is so powerful that cutaneous BF can range from 1% to 30% of COP

•A decreased ABP as in Haemorrhage → VC of skin arterioles shifting blood from skin to maintain blood volume

•Skin Blood flow increases (resistance arterioles) in response to stimulation of sympathetic cholinergic nerve fibres supplying sweat glands (paracrine effect)

•This response is mediated by Bradykinin; a potent vasodilator

•Cold pale clammy skin in cases of haemorrhagic shock is due to intense VC of skin blood vessels & sweat stimulation: both caused by sympathetic overactivity

Local Metabolites

•Skin blood flow is also regulated by local metabolites esp. during sweating & tissue injury (release of bradykinin 7 histamine increase BF →oedema)

•Occlusion of blood flow will be followed by reactive hyperaemia

Central Nervous system influences also affect skin blood flow:

  1. Fear (pale skin from VC)
  2. Blushing (red from VD)
  3. Turning white with rage (VC)

Is Skin Colour an accurate indicator of State of Cutaneous Blood Flow?

Not Always

6) Skeletal blood flow

  • In Resting muscle: Blood flow is under control of nervous regulation (sympathetic)
  • During exercise: the control is shifted to local factors overriding nervous regulation

•In an average weight adult, skeletal muscle represents ~ 50% of body w

•However, Skeletal muscle receives only 17% of COP

•Under resting conditions, O2 extraction by muscle is ~ 7volume% (arterial blood having 20volume%, venous blood leaving with 13vol%)

•Thus, resting muscle consuming ~ 25% of total O2 consumption/min

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-At rest skeletal muscle blood flow may be 1-4 ml/m/100gm.

-It account for 20% of the COP.

-During muscular exercise more than 80% of the COP is directed toward contracting muscles.

-Maximal blood flow may reach 50-100ml .

-Increase blood flow is mediated by:

  1. Increased COP
  2. VD of skeletal arterioles
  3. VC of other arterioles in the viscera.

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-Arterioles of skeletal muscle have :

  1. nalpha receptors>>>V.C
  2. beta receptors>>>>V.D
  3. muscarinic receptors.>>>V.D

-This permit muscle blood flow to adapt to different situation.

-Stress>>>sympathetic stimulation>>stimulate beta receptors>>VD.

-In hemorrhage>>>noradrenalin>>>stimulate alpha receptors>>>V.C>>>shift the blood to vital organs

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Nervous Control of Muscle BF

-In resting muscles, the BF is regulated by sympathetic innervation

-Stimulation of Sympathetic noradrenergic vasoconstrictor nerves supplying skeletal muscle (α adrenoreceptor-mediated)→↓ muscle BF by ½ or 1/3 of normal

-Blood-borne noradrenaline, secreted from suprarenal medulla→ VC of skeletal BV(α-receptor mediated effect)

-Whereas, blood-borne adrenaline→ VD of muscle BV (β-receptor mediated effect). This occurs in some conditions associated with generalised sympathetic activity (during exercise or mental stress)

Local Metabolic Factors

•Tremendous increase in BF occurs during muscle exercise

•This is mediated by local metabolic factors that override the nervous control

•The most important factor is reduction in O2 in muscle tissues (Hypoxia has a direct effect) &

•Other VD metabolites such as lactate, K+, ATP & CO2

Exercising muscles may use up to 80% of Cardiac Output How this is achieved?

•During exercise, the increased O2 demand by the muscles will be met by an increase in:

•O2 extraction; increases from 7Vol% to 15Vol%

•Blood flow to muscles which is mediated by local metabolites, increases temperature & acidosis

•This increase in BF occurs in spite of increased sympathetic activity that is associated with exercise