Vasculature: arterial blood flow, peripheral resistance Flashcards
What is the Poiseuille equation?
Flow is proportional to Delta p (pressure at each end of pipe) times radius of vessel to the power of four over viscosity times length
This is apparently simplified to Flow = delta p times radius of vessel cubed over K
or
Flow = delta p over resistance
Where Arterial pressure = cardiac output x total peripheral resistance
How does fluid flow through the vasculature?
In a parabolic velocity profile (peak in centre as vessels are lined with endothelial cells - fluid molecules on this wall adhere and move more slowly - next layer slips over these - next layer over these…–middle most layers move the most rapid
What is the impact of turbulence in fluid flow through vasculature?
Turbulence disrupts flow, increases resistance.
• Poiseuille’s law doesn’t hold true during turbulence
Reynold’s number (Re) is used to indicate whether flow is laminar or turbulent.
What is Reynolds number
Reynold’s number (Re) is used to indicate whether flow is likely to be laminar or turbulent.
– For a given system, there will be a “critical value” for Re, above which turbulence is highly likely.
Re = ((velocity of flow) x (radius of vessel)) / viscosity
What implications does Reynolds number have on turbulence?
Turbulence is therefore likely with (because Re increases with): – High velocity flow – Large diameter vessels – Low blood viscosity – Abnormal vessel wall
What are thixotropic fluids?
Fluids that are solid or more viscous when static and less viscous when agitated
As blood is a thixotropic fluid, how will it be affected by flow?
Flow affects viscosity
– Static blood has 100x the viscosity of flowing blood
When may turbulence be used by physicians?
Korotkoff sounds
Artificially generated turbulence
– Ausculatory measurements using a sphygnomanometer cuff
What is 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:
T = PR
What are the practical consequences of LaPlace’s Law?
1) 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
2) Capillaries
– Can be extremely thin and still withstand the pressure
– Thin walls essential for exchange processes
3) Aneurysm
Discuss how vasculature leading into capillaries can regulate blood flow
Arterioles
– Control regional distribution (Local and extrinsic controls)
Metarterioles
– Links arterioles to venules, discontinuous smooth muscle
Precapillary sphincters
– When a true capillary branches from a metarteriole
– Vasodilation produced by local factors
How is regulation of tissue blood flow achieved?
Active and reactive hyperemia
– Local factors associated with metabolic activity of tissues
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
Vasomotion
– Spontaneous oscillating contraction of blood vessels
Response to injury
– E.g. endothelin-1 released from endothelial cells
• Potent vasoconstriction
What is the difference between active and reactive hyperaemia?
Active hyperaemia
– If tissue is highly active, the rate of flow will increase
– E.g.by up to 20x in skeletal muscle
Reactive hyperaemia
– When blood supply blocked (few s to h)
– Blood flow increases to 4-7x normal
What causes vasoconstriction of the arteriolar radius?
Neural cause - sympathetic nerves
Hormonal - Adrenaline, angiotensin II, vasopressin
Local - Myogenic response, endothelin-1
What causes vasodilation of the arteriolar radius?
Neural cause - NO-releasing nerves
Hormonal - Adrenaline, Atrial-natriuretic peptide
Local - K+, CO2, H+, Adenosine, Nitric oxide, Bradykinin, Decrease in O2
Discuss blood flow through the capillaries
Blood flow through capillaries is intermittent, turning on/off every few seconds or minutes
– At rest, only ~5% of total cardiac output is in the capillaries
There are acute and long term regulations of local blood flow
Discuss the acute regulation of local blood flows through capillaries
– Rapid changes within seconds or minutes
– Vasodilator theory widely accepted (“local factors”)
• E.g. ↑PCO2, ↓PO2, ↑H+, ↑K+, ↑lactic acid, ↑adenosine, ↑ histamine
Discuss the long term regulation of local blood flows through capillaries
– A change in physical size or number of blood vessels
Discuss capillary flow and pressure
Large number of capillaries therefore large cross-sectional area
Velocity of blood flow through capillaries is the slowest
– Allows time for diffusion and exchange of nutrients and waste
What are the main points of the basic principles of circulatory function?
The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need.
The cardiac output is controlled mainly by the sum of all the local tissue flows.
Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control.
How is capillary structure different in the skin?
There are capillary loops in the dermis that feed up to the epidermis. They feed back into veins which travel deep to the subcutaneous tissue where they join a venous plexus
This venous plexus also takes blood from arteries in the subcutaneous tissue via arteriovenous anastomosis
This is controlled in thermoregulation
How is capillary structure different in the lungs?
Decreased alveolar O2 reduces local alveolar blood flow
– Opposite to effect observed in systemic vessels
– Mediator unknown
How is capillary structure different in the kidneys?
See: glomerulus
Blood enters and leaves glomerular capillaries at high pressure to cause fluid to filter into the tubule (Going into bowman capsule arterioles are afferent, after they are efferent)
Learning outcomes
To explain how physical factors affecting the cardiovascular system (e.g. flow, pressure, tension, vessel radius, blood viscosity, and the velocity of flow) are interrelated and the practical haemodynamic implications of these.
To describe how blood flow through the microcirculation is regulated at the tissue level, via both short-term (acute) and long-term mechanisms.
To identify systems with specialised blood flow requirements and describe the function of these specialised flows.
Other than via endothelin-1, what is a way that endothelial cells can regulate vascular tone?
In an increase in sheer stress in a blood vessel, a receptor dependant activation can occur for endothelial(?) nitrous oxide synthase or eNOS. This produces nitrous oxide from O2 + L-Arginine
**NOTNECESARRY**JUSTGETTHEGIST**
Once produced in endothelial cells, NO diffuses across the vascular smooth muscle cell membranes and activates the enzyme soluble guanylate cyclase (sGC), which catalyzes the conversion of guanosine triphosphate (cGTP) into cyclic guanosine monophosphate (cGMP).
cGMP, in turn, activates protein kinase G (PKG), which promotes multiple phosphorylation of cellular targets lowering cellular Ca2+ concentrations and promoting vascular relaxation.
The only example of parasympathetic impact on vasculature - in genitals
