Vasculature, Arterial Blood Flow and Peripheral Resistance Flashcards
Laminar fluid flow
Vessels lined with endothelial cells Fluid molecules touching wall adhere move slowly – Next layer slips over these • Next layer over these... –Middle most layers move the most rapid
Turbulence
disrupts flow, increases resistance.
• Poiseuille’s law doesn’t hold true during turbulence
Reynold’s number (Re)
used to indicate whether flow is laminar or turbulent.
Re increases with
– High velocity flow
– Large diameter vessels
– Low blood viscosity
– Abnormal vessel wall
Thixotropic fluids
Flow affects viscosity
– Static blood has 100x the viscosity of flowing blood
Korotkoff sounds
Artificially generated turbulence
– Ausculatory measurements using a sphygnomanometer cuff
LaPlace’s Law
Distending pressure (P) produces an opposing force or tension (T) in the vessel wall, proportional to the radius (R) of the vessel
Practical consequences of LaPlace’s Law - control of blood flow
– Low tension required to oppose blood pressure in arterioles
– Smooth muscle control of arteriole and precapillary sphincters are
the sites of tissue blood flow regulation
Practical consequences of LaPlace’s Law - capillaries
– Can be extremely thin and still withstand the pressure
– Thin walls essential for exchange processes
Practical consequences of LaPlace’s Law - Aneurysm
rupturing of a blood vessel, can’t oppose the pressures within so the region expands and becomes weakened and can’t create the same tension so stretches and bulging occurs (radius then increases and gives brief normalisation of pressure but this quickly balances out) and then ruptures (blows up)
Regulation of blood flow - arterioles
– Control regional distribution (Local and extrinsic controls)
Regulation of blood flow - Metarterioles
– Links arterioles to venules, discontinuous smooth muscle
Regulation of blood flow - Precapillary sphincters
– When a true capillary branches from a metarteriole
– Vasodilation produced by local factors
tissue blood flow - Vasomotion
– Spontaneous oscillating contraction of blood vessels
tissue blood flow - Active and reactive hyperemia
– Local factors associated with metabolic activity of tissues
ACTIVE
– If tissue is highly active, the rate of flow will increase
– E.g.byupto20xin skeletal muscle
REACTIVE
– When blood supply blocked (few s to h)
– Blood flow increases to 4-7x normal
tissue blood flow - Flow autoregulation
– In response to changes in arterial pressure
• Arterial pressure↑, arterioles constrict to reduce flow •Arterial pressure↓, arterioles dilate to increase flow
– Myogenic response
• Stretch-activated Ca2+channels
tissue blood flow - Response to injury
– E.g. endothelin-1 released from endothelial cells •Potent vasoconstriction
Role of the endothelial cells in regulating vascular tone
Nitric oxide vasodilator
Local vasoconstrictor
myogenic response
endothelian 1
local vasodilator
decrease in o2, potassium and carbon dioxide and hydrogen
adenosine
nitric oxide
bradykinin
neural vasocontrciter
sympathetic nerve s
neural vasodilator
no releasing nerves
hormonal vasoconstrictor
adrenaline
angiotensin II
vasopressin
hormonal vasodilator
adrenaline
atrial - natriuretic peptide
Blood flow through capillaries
s intermittent, turning on/off every few seconds or minutes
– At rest, only ~5% of total cardiac output is in the capillaries
Acute regulation of local blood flow
– Rapid changes within seconds or minutes
– Vasodilator theory widely accepted (“local factors”)
• E.g.↑PCO2,↓PO2,↑H+,↑K+,↑lacKcacid,↑adenosine,↑ histamine
Long term regulation of local blood flow
– Change in physical size or number of blood vessels
Specialised flow (lungs)
Decreased alveolar O2 reduces local alveolar blood flow
– Opposite to effect observed in systemic vessels
– Mediator unknown
