Special Circulations Part 2 Flashcards
how does cerebral circulation meet the high demand for oxygen
High capillary density (large sa and reduced diffusion distance)
High basal flow rate
High oxygen extraction
Neurones continually use oxygen to maintain high conc Grad
Why is secure oxygen supply to brain vital
Neurones very sensitive to hypoxia
Loss of consciousness after a few seconds of cerebral ischeamia
Irreversible neurone damage after 4 mins
Stroke causes neuronal death
how is a secure cerebral blood supply ensured
Structurally- anastomoses between basilar and internal carotid arteries
Functionally- myogenic auto regulation maintains perfusion during hypotension, metabolic factors, brain stem
What is myogenic auto regulation
Cerebral resistance vessels have a myogenic response to changes in transmural pressure
Maintains cerebral blood flow when BP changes
What happens in myogenic auto regulation when there is raised blood pressure
Vasoconstriction
What is hypercapnia
Increased partial pressure of carbon dioxide
What happens in hypercapnia
Vasodilation to increase blood supply
Panic hyperventilation can cause
Hypocapnia and cerebral vasoconstriction leading to dizziness or fainting
What causes local cerebral increases in blood flow
Regional activity - areas with increased neuronal activity
What is a powerful vasodilator of cerebral arterioles
Adenosine
What leads to cerebral vasodilation
increased partial pressure of carbon dioxide, increased potassium, increase adenosine, decreased partial pressure of oxygen due to ATP breakdown
What is Cushing’s reflex
Rigid cranium protects the brain but does not allow for volume expansion
increases in inter cranial pressure impair cerebral blood flow
Impaired blood flow to vasomotor control regions of the brain stem increase sympathetic vasomotor activity
What happens when sympathetic vasomotor activity is increased
Increases arterial BP
Maintains cerebral blood flow
Increased bP causes reflex bradycardia
Bradycardia and acute hypertension - hallmark of space occupying legion
what must coronary circulation do
Deliver oxygen at high basal rate
Meet increased demand
Right and left coronary arteries arise from
Right and left aortic sinuses
Flow in the left coronary artery occurs mainly during
Diastole
Features of coronary circulation
high capillary density facilitates efficient oxygen delivery, diffusion distance short, continuous production of NO by coronary endothelium maintains a high basal flow
What happens to coronary blood blood with increased myocardial oxygen demand
Supplied by increased blood flow
Almost linear relationship
Small increase in oxygen extraction ]
Vasodilation due to metabolic hyperaemia
vasodilators = adenosine (up potassium and down pH)
How are coronary arteries functional end arteries
Few arterio-arterial anastomoses
Prone to atheromas
Narrowed leads to angina
Blood flow mostly during diastole
Stress and cold can cause
Sympathetic coronary vasoconstriction and angina
Sudden obstruction by thrombus caused
Myocardial infarction
Skeletal muscle circulation key features
Must increase oxygen and nutrient delivery and removal of metabolites during exercise
Helps regulate arterial blood pressure
resistance vessels have rich innervation by sympathetic vasoconstrictor fibres (baroreceptor reflex maintains bp)
What makes skeletal muscle circulation effective
Capillary density depends on muscle type
Very high vascular tone (permits lots of dilation)
At rest only 1/2 perfused at one time (allows for increased recruitment)
Opening of precapillary sphincters allows more capillaries to be perfused
What agents act as vasodilators during hyperaemia
Increased potassium Increased osmolarity Inorganic phosphates Adenosine Increased hydrogen
Adrenaline in arterioles in skeletal muscle through beta 2
Problems associated with skeletal muscle circulation
Atheroma formation Intermittent claudication (pain in calf or buttocks when you walk)
Cutaneous circulation key points
Special role in temperature regulation
Skin is main heat dissipating surface
Vasoconstriction maintains BP
Acral (apical) skin has specialised structures called
Arterovenous anastomoses (AVAs)
what do AVAs do
regulate heat loss from apical skin
High sa:v
Neural control (sympathetic vasoconstrictor fibres)
Not regulated by local metabolites
Increased core temperature
Opens AVAs
Reduced vasomotor drive allows AVAs to
Dilate- diverts blood to veins near surface
What happens in non-apical skin to increase blood flow to lose heat
Sympathetic cholinergic fibres activate sweat glands that may in turn release vasodilators such as bradykinin
Problems associated with cutaneous circulation
Decreased blood flow due to pressure e.g. pressure ulcers
Reynaud’s - XS vasoconstriction in response to cold, hands and feet
XS vasodilation in response to heat can cause postural hypotension
