L21. Blood Vessels - Flow and Control Flashcards
(15 cards)
Blood flow?
F = change in P/R
- The Poiseuille Relationship:
For laminar flow of fluid in a rigid tube, the volume flow is proportional to:
- the pressure difference between the ends of the tube (P2-P1) = change in P
- the radius4 (r4)
and is inversely proportional to:
- the length of the tube (l)
- the viscosity of the fluid (n)
Series flow has more resistance compared to parallel flow which has less resistance
Blood vessel differences?
Veins: wide lumen, few layers of smooth muscle and connective tissue, few elastic layers
Arteries: small lumen, many layers of smooth muscle and connective tissue, several elastic layers
Pulse pressure?
The difference between systolic and diastolic pressure (120-80 = 40mmHg in the example) is called the pulse pressure
- The most important factors determining the magnitude of pulse pressure are:
1. Stroke volume
2. Speed of ejection of stroke volume
3. Arterial compliance
Arteriosclerosis?
A decrease in arterial compliance (stiffening of the arteries)
Mean arterial blood pressure calculation?
Mean BP at brachial artery = diastolic pressure + 1/3 [pulse pressure]
Arterioles - pressure adjustment?
- Arterial pressure:
MABP = CO x TPR (total peripheral resistance)
CO is the cardiac output (ml/min)
TPR is the total resistance to flow calculated from the resistance of all blood vessels in the body; takes into account series and parallel flow - Arterial pressure can be adjusted by altering CO and/or TPR
- Arterioles important for adjusting TPR to maintain MABP
E.g. MABP falls (due to reduced CO), reflex actions raise TPR (by vasoconstriction of the small arteries and arterioles) and restore pressure to normal
Arterioles - blood flow distribution?
Regional blood flow = arterial pressure/regional resistance
- Since arterial pressure normally is regulated to a fairly constant value, a change in regional blood flow is achieved by altering regional resistance (resistance in appropriate arterioles and small arteries)
- Regional blood flow is regulated by vessel radius so that the regional blood flow and mean arterial pressure are ideal
Blood flow intrinsic mechanisms?
- Myogenic response
- Paracrine factors (metabolites, NO, autacoids)
- Physical factors (temperature, pressure)
Blood flow extrinsic mechanisms?
- Vasodilator nerves
- Sympathetic vasoconstrictor nerves
- Endocrine factors (adrenaline, angiotensin, vasopressin, etc.)
Extrinsic control of blood vessels?
Why have extrinsic controls?
- To regulate overall TPR, hence control blood pressure
- To allow brain to alter blood flow selectively to individual organs (e.g. vasodilation of secreting salivary glands)
What are the extrinsic controls?
- Vasomotor nerves - (vasoconstrictor, vasodilator)
- Hormones (adrenaline, vasopressin, angiotensin II)
–> Sympathetic vasoconstrictor fibres (noreadrenergic) are by far the most widespread and important extrinsic control normally
Sympathetic vasoconstrictor nerves - key facts?
- Controlled by the brainstem (vasomotor centre)
- Innervate most arterioles and veins of the body
- Terminate at edge of tunica media, in strings of synaptic varicosities
- Varicosities release dense-cored vesicles, containing noradrenaline (NAd) and ATP
- NAd activates alpha adrenoreceptors on vascular myocyte
- Alpha adrenoreceptors always cause vasoconstriction
- The fibres are tonically active (1 impulse/sec) therefore a fall in activity, or alpha blockers like phentolamine, causes vasodilation
- Sympathetic activity drive to each tissue is controlled by distinct group of neurons
Reduced sympathetic activity?
Leads to vasodilation
–> increase in blood pressure –> activates baroreflex –> inhibits sympathetic activity –> reduced blood pressure
E.g. Exercise - increase in body temperature –> reduced sympathetic nerve activity to the skin –> vasodilation –> increased blood flow
Increased sympathetic activity?
Leads to vasoconstriction
–> tissue blood flow is reduced –> contraction of local resistance vessels –> total peripheral resistance is increased –> increase blood pressure
E.g. Capillary pressure is reduced –> contraction of local resistance vessels –> allows the interstitial fluid to be absorbed into the plasma
If resistance remains the same, blood flow would be higher if systolic/diastolic blood pressure was 180/120 than if it was 130/70
True or false?
False
What is the function of the aorta and large arteries?
These vessels reduce the fluctuations in flow and pressure generated by the intermittent ejection of the stroke volume; this is accomplished by the highly elastic walls of these vessels
