Specialised Circulations Flashcards
Special requirements of cutaneous circulation?
- Defence against environment
- Temp regulation
- Lewis triple response to trauma
How does skin regulate temp?
Radiation (skin vs ambient temp)
Conduction - skin onto other object
Convection - removal by air/water
Sweating (latent heat of evaporation)
What does skin temp depend on?
Skin blood flow
Ambient temp
How’s the skin a poikilo-thermic organ?
temp ranges from 0 -40oC briefly w/o damage for short periods of time
Special structural feature of skin?
Arterio-Venous Anastomoses (AVAs)
What are arterio-venous anastomoses (AVAs)?
Direct connections of arterioles + venules – expose blood to regions of high SA
What controls arterio-venous anastomoses (AVAs)?
Sympathetic vasoconstrictor + sudomotor vasodilator fibres driven by temp regulation nerves in hypothalmus
Special functional features of skin?
- Responsive to ambient + core temperatures
- Severe cold causes ‘paradoxical cold vasodilatation’
- Core temp receptors in hypothalamus control sympathetic activity to skin –> blood flow
Effect of increased ambient temp?
vaso + venodilation helping heat loss
Effect of decreased ambient temp?
vaso + venoconstriction conserving heat
Why’s there cold-induced vasoconstriction?
Conserves heat because:
- abundance of α2 receptors
- ↓ AC/cAMP/PKA on VSMCs in skin
- bind NA at lower temp than α1 receptors
What pathway is α2 receptors?
Gi so reduces AC/cAMP/PKA
Why’s there paradoxical cold vasodilatation
Protect from skin damage:
- caused by paralysis of sympathetic transmission
- long-term exposure leads to oscillations of contract/relax
How does core temp regulate blood flow to skin?
- increased core temp
- stimulate warmth receptors in anterior hypothalamus
- sweating + vasodilation
Describe sweating from increased core temp
increased sympathetic activity (Ach) to sweat glands
Describe vasodilation from increased core temp
increased sympathetic sudomotor activity (Ach act on endothelium to produce NO) to arterioles in extremities
Baroreflex/RAAS/ADH-stimulated vasoconstriction of skin blood vessels
-haemorrhage, sepsis, acute cardiac failure
-drop in BP
-so blood directed to vital organs/tissues during
-mediated by sympathetic vasoconstrictor fibres + A +
ADH + Ang II
Why’s a patient pale cold skin in shock?
Baroreflex/RAAS/ADH-stimulated
vasoconstriction of skin blood vessels
What happens if somebody with a haemorrhage is warmed up too quickly?
- warm up body too quickly
- reduce cutaneous vasoconstriction
- blood flow to skin not vital organs/tissues
- dangerous
eg of emotional communication?
blushing (sympathetic sudomotor nerves)
What’s the Lewis triple response to skin injury?
Local redness: Site of trauma
Local swelling: Inflammatory oedema (wheal)
Spreading flare: Vasodilatation spreading out from site of trauma
Importance of Lewis triple response to skin injury?
Increased delivery of immune cells, antibodies to site of damage to deal with invading pathogens
Effect of prolonged obstruction of flow by compression + eg
Severe tissue necrosis
‘bed sores’; heals, buttocks, weight bearing areas
How to avoid prolonged obstruction of flow by compression?
Shifting position causing reactive hyperaemia (on removal of compression)
High skin tolerance to ischemia
Effect of postural hypotension / oedema due to gravity?
Standing for long periods in the hot reduces CVP
(hypotension) + increases capillary permeability (oedema)
Feel faint, rings on fingers can be tighter
Effect of Raynaud’s disease?
Sustained vasoconstriction to cold in fingers
Local tissue ischemia
Special requirements of pul circulation?
Gaseous exchange
Area for metabolic function
Why’s gaseous exchange a special requirement of pul circulation?
Receives entire CO from RV
Needs low pressure to do this
Needs to occur very fast – not to be ‘diffusion-limited’
Why’s an area for metabolic function a special requirement of pul circulation?
Receiving entire CO from RV means the pul circulation is a good system to produce or remove substance
Which special factors provides a huge O2 diffusion capacity?
-High capillary density : Over 100 m2 area of capillaries in lungs Continuous sheet of blood -Short distance between capillary + alveolar : 0.3 µm so fast diffusion
Special problems with pul circulation?
Gravity
Chronic HPV
Pulmonary oedema
How’s gravity a problem with pul circulation?
Upright : low pul arterial pressures at lung apex
MPA pressure – 15 mmHg, Apex – 3 mmHg, Base – 21 mmHg
Poor perfusion at apex –> vessel collapse
Standing person slightly impaired blood oxygenation
How’s chronic HPV a problem with pul circulation?
High altitude for long periods or respiratory disease eg COPD
- hypoxia
- vasoconstriction
- pul hypertension
- right ventricular failure
How’s pul oedema a problem with pul circulation?
Thinness of capillary-alveoli means potential for stress + leak eg mitral valve stenosis
- increased pressure in LA
- increased pulmonary capillary pressures
- increased filtration
- oedema
- poor lung function
Special requirements of skeletal muscle circulation?
Exercise
Controls arterial pressure
Why’s exercise a special requirement of skeletal muscle circulation?
Delivery of O2 and nutrients eg glucose must equal work
Blood flow must be proportional to exercise intensity
Why’s arterial p a special requirement of skeletal muscle circulation?
- skeletal muscle is 40% of body mass
- vascular resistance contributes to TPR
- vasodilation + vasoconstriction of skeletal muscle circulation has sig changes in BP
Special structural feature of skeletal muscle + eg?
Capillary density differs in diff muscles:
Postural muscles capillary denisty > phasic muscles eg soleus always active > calf, forearm
Why does capillary density differs in diff muscles?
Endurance training increases capillary growth,
at a rate proportional to numbers of mitochondria per fibre
Special functional features of skeletal muscle circulation?
- High vascular tone
- Metabolic vasodilatation
- High expression of β2-adrenoceptors on VSM
- O2 extraction
Effect of high vascular tone on skeletal muscle circulation?
- Reduces BF at rest enabling vasodilatation during exercise to increase BF
- Reduces BF to capillaries at rest enabling capillary recruitment (vasodilatation) to increase SA + BF for exchange
Effect of metabolic vasodilation on skeletal muscle circulation?
Metabolic products (K+, adenosine, phosphate, H2O2) produce vasodilatation Relates exercise intensity to increased blood flow
Effect of high expression of β2-adrenoceptors on VSM on skeletal muscle circulation?
Stimulation of β2 by A –> vasodilatation
Effect of O2 extraction on skeletal muscle circulation?
- during exercise increased from 25-30% to 80-90%
- increased blood flow
- increased area for exchange
- reduced distance for exchange
- muscle cells using lots more O2
Special problems of skeletal muscle circulation?
- Mechanical interference
- Increased capillary pressure during exercise
- Leg arteries major areas for atheroma
Effect of mechanical interference?
-muscles contract
-reduce blood flow in intra-muscle vessels
-good in rhythmic exercise eg running
BUT
-sustained contraction eg carry bagss
-poor O2 supply
-anaerobic respiration
-build up of lactate
-muscle fatigue
Effect of increased capillary pressure during exercise?
-during exercise increased BF to muscles
-increased capillary pressure
-increased filtration of plasma volume into muscles
-oedema
eg pumped muscle reduces plasma volume by 10%
eg of leg arteries major areas for atheroma?
Ischemia leg pain, ulcers, gangrene
