Properties Of Special Circulation Flashcards
What are 3 characteristics that special circulations have to consider?
- Special requirements met by the circulation
- Special structural or functional features of the circulation
- Specific problems relating to that circulation
What are the special requirements of coronary circulation?
Need a high basal supply of O2
Increase O2 supply in proportion to increased demand/cardiac work
What are special structural features of coronary circulation?
High capillary density
Large SA for O2 transfer
Together these reduce diffusion distance to myocytes
Time is proportional to diffusion squared so O2 transport is fast
What are special functional features during normal activity?
High blood flow
Relatively sparse sympathetic innervation
High nitric oxide released leading to vasodilatation
High O2 extraction (75%)
What are special functional features of coronary circulation during increased demand?
Coronary blood flow increases in proportion to demands
Production of vasodilators (adenosine, K+ and acidosis) outcompete low sympathetic vasoconstriction
Circulating adrenaline dilates coronary vessels due to abundance of B2 adrenoceptors
What are special problems with coronary circulation?
Ischaemic heart disease
- coronary arteries are functional end arteries
- heart is susceptible to sudden and slow obstruction
Sudden- acute thrombosis, produce MI
Slow- atheroma, chronic narrowing of lumen,produces angina
What are functional end arteries?
These are arteries that go alone, don’t cross link with other arteries
This means that if that arteriole is blocked, everything downstream is starved of blood
Thrombosis
E.g. total occlusion of left anterior descending artery
Occlusion leading to obstruction of blood flow to anterior left ventricle leading to MI
What are the mechanical factors for reducing coronary flow?
Shortening diastole e.g high heart rate
Increased ventricular end diastolic pressure e.g. heart failure
Reduced diastolic arterial pressure e.g. hypotension, aortic regurgitation (aortic valve is damaged)
What are special properties of cutaneous circulation?
Defence against the environment
Lewis triple response to trauma (increased blood flow)
Temp regulation
Skin temp depends on skin blood flow and ambient temperature
What are special structural features of cutaneous circulation?
Arterio-venous anastomoses (AVAs)
Sympathetic vasoconstrictor fibres
Sudomotor vasodilator fibres
What are special functional features of cutaneous circulation?
Responsive to ambient and core temperature
Help heat loss -> increase ambient temp causes vaso and venodilatation
Help to conserve heat -> decrease ambient temp causes vaso and venoconstriction
Severe cold causes paradoxical cold vasodilatation
Core temp receptors in hypothalamus control sympathetic activity to skin and hence skin blood flow
What is the effect of ambient temp on skin blood flow?
Cold induced vasoconstriction
- conserves heat -> want to maintain internal core temp at 37 degrees
-sympathetic nerves react to local cold by releasing NA which binds to a2 receptors in VSM in skin. A2 receptors bind to NA at lower temp than a1 receptors
Paradoxical cold vasodilation
-protects against skin damage
- caused by paralysis of sympathetic transmission leading to vasodilation
- long term exposure leads to oscillations of contract/relax
Cutaneous perfusion and core temp
Increased cutaneous perfusion with increased core temp (e.g. exercise)-> stimulate warmth receptors in anterior hypothalamus
Causing:
Sweating-> increased sympathetic activity to sweat glands mediated by Ach
Vasodilatation-> increase sympathetic sudomotor activity such that Ach act in endothelium to produce NO which dilate arterioles in extremities
What is Lewis triple response of skin to trauma?
Local redness: site of trauma
Local swelling: inflammatory oedema (wheal)
Spreading flare: vasodilatation spreading out from site of trauma
C fibre axon reflex mediates the flare to trauma
