Special circulation Flashcards
Special circulations
There are many special circulations, including cerebral, pulmonary, skeletal muscle renal gastrointestinal and all have unique requirements
Special circulations have 3 characteristics to consider
1) Special requirements met by the circulation
2) Special structural or functional features of the circulation
3) Specific problems relating to that circulation
Cardiac veins function
- Carry blood with a poor level of oxygen, from the myocardium to the right atrium.
- Most of the blood of the coronary veins returns through the coronary sinus
Coronary circulation special requirements
- Needs a high basal supply of O2 – 20x resting skeletal muscle
- Increase O2 supply in proportion to increased demand/cardiac work
Coronary special structural features
- High capillary density
- Large surface area for O2 transfer
- Together these reduce diffusion distance to myocytes.
- Diffusion time is proportional to distance squared – so O2 transport is fast
- Cardiac muscle contains high numbers of fibres and capillaries giving rise to shorter diffusion distances
Special functional features of coronary circulation during normal activity
- High blood flow – 10x the flow per weight of rest of body
- Relatively sparse sympathetic innervation
- High nitric oxide released leading to vasodilation
- High O2 extraction (75%) – average in body is 25%
Special functional features of coronary circulation during increased demand
- Coronary blood flow increases in proportion to demands
- Production of vasodilators (adenosine, K+, acidosis) out-compete relatively low sympathetic vasoconstriction
- Circulating adrenaline dilates coronary vessels due to abundance of β2-adrenoceptors
Oxygen unloading to myocardium during normal activity
BOHR SHIFT
- Coronary sinus blood returning to right atrium from myocardial tissue has a greater carbon dioxide content due to high capillary density, surface area and small diffusion difference
- The high CO2 and low pH have shifted the curve to the right meaning that haemoglobin has less affinity for oxygen and more O2 is given up to the myocardial tissues
Sudden obstruction
Acute thrombosis - produce myocardial infarction
Slow obstruction
Atheroma (sub-endothelium lipid plaques) chronic narrowing of lumen - produces angina
Thrombosis can lead to
Ischaemic tissue, acidosis, pain (stimulation of C fibres) Impaired contractility Sympathetic activation Arryhthmias Cell death (necrosis)
Coronary blood flow during diastole
1) Shortening diastole, e.g. high heart rate
2) Increased ventricular end – diastolic pressure, e.g. heart failure (aortic stenosis, stiffening of ventricle)
3) Reduced disatolic arterial pressure, e.g. hypotension, aortic regurgitation
Special properties of cutaneous circulation
Defence against environment
Lewis triple response to trauma
Temperature regulation
Skin temperature depends on
Skin blood flow
Ambient temperature
How is skin temperature regulated
Blood flow delivers heat from body core
Radiation
Conduction to skin - convection from skin
Sweating
Special structural features of cutaneous circulation
Arterio-venous anastomoses (AVAs)
Convection, conduction, radiation, evaporation
Sympathetic vasoconstrictor fibres
Sudometer vasodilator fibres
Driven by temperature regulation nerves in hypothalamus
Arterio-venous anastomoses (AVAs)
Direct connections of arterioles and venules, expose blood to regions of high surface area
Sympathetic vasoconstrictor fibres
Release noradrenaline acting on α 1 receptors
Sudometer vasodilator fibres
ACh acting on endothelium to produce nitric oxide
Special functional features for cutaneous features
- Responsive to ambient core temperatures
- Help heat loss - increase ambient temperature causes vaso- and venodilation
- Help to conserve heat - decrease ambient temperature causes vaso and venoconstriction
- Severe cold causes ‘parodixcal cold vasodilation’
- core temperature receptors in hypothalamus control sympathetic activity to skin and hence skin blood flow
Cold induced vasoconstriction
Conserves heat
Sympathetic nerves react to local cold by releasing noradrenaline which binds to α2 receptors on vascular smooth muscle in skin
α2 receptors bind NA at lower temperatures than α1 receptors
Paradoxical cold vasodilation
Protects against skin damage
Caused by paralysis of sympathetic transmission
Long-term exposure leads to oscillations of contract/relax
Sweating
Increased sympathetic activity to sweat glands meditated by ACh
Vasodilation
Increased sympathetic sudomotor activity such that ACh act on endothelium to produce NO which dilates arterioles in extremities
Lewis triple response
Local redness - site of trauma
Local swelling - inflammatory oedema
Spreading flare - vasodilation spreading out from site of trauma
Other functional specialisations of cutaneous circulations
Baroreflex/RAAS/ADH stimulated vasoconstriction of skin blood vessels
Special problems
Prolonged obstruction of flow by compression
Postural hypotension/oedema due to gravity
Prolonged obstruction of flow by compression
Severe tissue necrosis
Bed sores - heels, buttocks, weight bearing areas
Postural hypotension/oedema due to gravity
Often standing for long period in hot weather will decrease CVP and increase capillary permeability
