Control of blood flow Flashcards
what is blood flow?
blood flow is the difference in pressure (pressure gradient) divided by resistance (TPR)
what does TPR control?
blood flow and blood pressure
if you increase resistance and keep pressure the same, what happens?
flow decreases
what 3 things control TPR?
o Poiseuille’s law
o Myogenic responses
o Blood viscosity
which vessel affects TPR?
arterioles
-they can control blood flow to different parts of the body
the vasodilation of arterioles leads to what?
- decrease in TPR
- decreased blood pressure upstream
- greater flow downstream
vasoconstriction of arterioles leads to what?
- increase in TPR
- increased blood pressure upstream
- less flow downstream
Hypertension of arterioles leads to what?
- over constriction of arterioles
- higher arterial BP
- less capillary flow – under perfusion
why are changes in the arteriole blood flow a good thing? give examples
you can control where the blood goes
e. g. postprandial (when you’ve just eaten), you want blood flowing around the intestines, helping to remove the products of digestion and help us absorb nutrients from the meal we’ve eaten. Don’t need blood going to the legs, SO:
- Dilate superior mesenteric artery and increase blood flow to digestive canal and intestine
- Constrict the common iliac as won’t need blood to legs – more in digestion than muscle.
what does Poiseuille’s Law describe?
parameters that govern TPR
what factors affect TPR according to poiseuilles law?
- Viscosity – more viscous the more resistance
- Length of vessel – longer has more resistance
- Radius – a bigger radius means less resistance
- Density- a high density means more resistance
explain the r^4 effect?
If you double the radius, it will be 16x more conductive
how does a small change in radius affect TPR?
a small change in radius has huge effect on TPR
how do vasoconstricters/vasodilators work?
They produce small changes in vessel radius by affecting smooth muscle. This has large effects on blood flow.
what are the main vessels involved in TPR?
arterioles
what are changes in ventricular pressure moderated by?
compliance of aorta and major arteries
when ventricular pressure drops what do arterioles do?
the arterioles contract and spring back to keep blood flow even
so how do arterioles change their size?
there are circular layers of smooth muscle around the arterioles which can contract and relax
arteriole radius is tightly controlled by what?
sympathetic nerves which provide constant tone
TPR is controlled by 3 parameters- name them:
- Radius - r4 effect
- Pressure difference across vessels
(P1-P2) - Length - arterioles are long vessels
why can’t capillaries change their radius size like arterioles?
there is no smooth muscle/sympathetic innervation in capillaries
what arrangement are capillaries in and how does this affect their resistance?
capillaries are arranged in parallel, so they have a low total resistance