5. Vasculature: arterial blood flow and peripheral resistance Flashcards
What is changed and what must be preserved in
ArtP = CO x TPR
TPR = changed
CO = preserved
What occurs in laminar fluid flow
Fluid molecules touching the vessel wall adhere and move more slowly due to friction. The next layer slips over these etc. Middle layers move most rapidly
What velocity profile is presenr during laminar flow?
Parabolic velocity profile
What is turbulent flow?
Occurs when a layer of laminar flow is disrputed - Increasing resistance
What is the purpose of Reynold’s Number (Re)
Used to indicate whether flow is laminar or turbulent
In what situations is turbulence likely?
- High velocity flow
- Large diameter vessels
- Low blood viscosity
- Abnormal vessel wall
What is blood viscosity dependent on?
Speed and movement of fluid; when blood stops flowing it becomes thicker
What are Kortkoff sounds?
Artificially generated turbulence
Explain LaPlace’s Law
Distending pressure (P) produces opposing forces or tension (T) in the vessel wall proportional to the radius (R) of the vessel
T=
PR
Tension =
Ability of the vessel to withstand the forces within it
What vessels are at high risk of turbulence?
- Aorta
- Arteriole
- Capillary
How does LaPlace’s law affect blood flow?
Low tension required in arterioles to oppose BP. Smooth muscle = sites of tissue blood flow regulation
How does LaPlace’s law affect capillaries?
Capillaries can be extremely thin and withstand the pressure
Thin walls = essential for exchange process
How does LaPlace’s law affect anerysm?
In an aneurysm increased pressure means already weakened walls stretch eventually leading to rupture
What is the function of arterioles
Control regional distribution (where blood flows to) by local and extrinsic controls
What is the function of metarterioles
Links arterioles to venules, discontinuous smooth muscle; can bypass capillary beds if required
Prepcapillary sphincters are responsible for…
vasodilation produced by local factors; tap like function
What is hyperaemia?
Build up of local factors and products in the tissue
Give 4 ways the body regulates tissue blood flow
- active and reactive hyperaemia
- flow autoregulation
- vasomotion
- response to injury
How does flow autoregulation work?
Responds to changes in arterial pressure
- Increased ArtP = arterioles constrict to reduce flow
- Decreased ArtP = arterioles dilate to increase flow
Also has a myogenic response; stretch activated Ca2+ channels cause reflex constriction
Explain the function of vasomotion
Spontaneous oscillating contraction of blood vessels; inhert activity of smooth muscle
What occurs in response to injury of endothelial cells?
Endothelin-1 released from endothelial cells causes potent vasoconstriction
What is active hyperaemia? And give an example of where it occurs
If a tissue is highly active the rate of flow will increase and metabolic activity increases
E.g. skeletal muscle in exercise
What is reactive hyperaemia?
When blood supply is blocked so blood flow increases to 4-7x normal
How do endothelial cells regulate vascular tone?
Nitric Oxide causes dilation and relaxation of vessels. Nerves that control NO are under PSNS innervation
Why does the body regulate total arteriolar radius?
Allow blood flow through capillary beds that require it
OR
larger more orchestrated role of increasing TPR by decreasing the overall arteriolar radius
How does neural control cause vasocontriction?
Sympathetic nerve
How does neural control cause vasodilation?
NO-releasing nerves (minimal effect)
How does hormonal control cause vasocontriction?
- Adrenaline
- Angiotensin II
- Vasopressin
- (act on α1 receptors)
How does hormonal control cause vasodilation?
- Adrenaline
- Atrial-natriuretic-peptide
- (act on β2 receptors)
How does local control cause vasocontriction?
- Myogenic response
- Endothelin-1
How does local control cause vasodilation?
- Decreased O2
- K+, CO2, H+
- Adenosine
- Nitric Oxide
- Bradykinin
At rest what percentage of the total cardiac output is in the capillaries?
5%; blood flow is intermittent
What controls acute regulation of local blood flow?
“local factors”
What controls long term regulation of local blood flow?
Change in physical size and number of blood vessels
—> Angiogensis
What is arterial pressure regulation independent to?
Local blood flow control or cardiac output control
