Control of blood flow Flashcards
Total peripheral resistance
Darcy’s law - Flow=Pa-CVP/TPR
- TPR controls blood flow and blood pressure: increase in resistance means need to increase pressure to keep same flow.
- What controls TPR: Darcy’s and Poiseulle’s law, Myogenic response and blood viscosity.
G - conductants = reciprocal of TPR. G=1/TPR
=== Flow = Pa-CVP x G
Resistance affects both flow and blood pressure
- Decrease in TPR -> decrease blood pressure upstream, but greater capillary flow.
- Increase in TPR -> Increased blood pressure upstream, but less capillary flow.
Hypertension = widespread over constriction of arterioles annd leads to a higher arterial BP but less capillary flow (perfusion)
Changes in blood flow in response to changes in need
Brain areas controlling sympathetic nervous activity to various areas of the body
Sedentary:
* Superior mesenteric dilated increased flow to intestines.
* Common iliac constricted, Decreased flow to legs.
Excercising:
* Superior mesenteric constricted, decreased flow to intestines.
* Common iliac dilated, increased flow to legs.
Poiseuille’s law - total peripheral resistance
Poiseuille’s law - Describes parameters that govern TPR
- Resistance = 8η L / π r^4
- Conductance (G) = π^4 r^4 / 8η L
- Flow = Pa - CVP x r^4 / 8η L (combined Poiseulle and Darcy law = Flow = Pa- CVP xG)
r= radius of vessel
η = blood viscosity
L = vessel length
Blood vessel radius to power of four controls TPR.
The r^4 effect
Change in r^4 = 16 (2x2x2x2)
Vasoconstrictions and dilators make small changes to vessel radius, affecting smooth muscle has large effect on blood flow.
In below diagram:
Vessel 2 equals 1/16 of resistance of vessel 1
16 times greater flow in vessel 2 as flow is proportional to r^4.
Arterioles are the main vessels involved in TPR
- Arterioles have largest pressue drops.
- Arteriole radius is tightly controlled by sympathetic nerves providing constant tone dilation and constriction.
TPR is controlled by 3 parameters:
* Radius - r^4
* Pressure difference across vessels - P1-P2
* Length (L) - arterioles are also long vessels
TPR is not controlled by capillaries
Flow = Pa - CVP x r^4 / 8η L
* Flow - cardiac output or blood flow
* Pa - pressure drop, less pressure drop across capillaries due to less resistance to blood flow in capillaries
* Radius (r^4) - No sympathetic innervation/smooth muscle in capillaries so cannot alter radius
* Length (L) - Individual capillaries are short
* Viscosity (η) - less resistance in capillaries because bolus flow reduces viscosity.
- Capillaries are in parallel, so have low resistance.
- Arterioles are in series so resistance is higher.
TPR - control of loacal blood flow
Local blood flow through organs/tissues is controlled by changes in radius of arterioles supplying a given organ/tissue.
Control mechanisms of arteriole radius: intrinsic/extrinsic.
- Intrinsic: factors entirely within an organ or tissue - response to local factors.
- Extrinsic: factors outside organ/tissue - nervous and hormonal control of blood vessels
Bayliss myogenic response
Stretching of the vessel causes ion channels to open which then depolarise and causes smooth muscle to contract.
Viscosity (η)
Venous return and venous blood pressure
- Veins:
* Thin-walled, collapsible, voluminous
* contain 2/3 of blood volume
* Contractile - contain smooth muscle, innervated by sympathetic nerves but thinner than arterial muscle and more compliant - form blood reservoir. - Volume of blood in veins and contractility:
* Contraction of vessels - expels blood intro central veins
* Increases venous return/CVP/end diastolic volume
* increases SV (Starling’s law)
Venous return - pressure-volume curve of veins
Bernoulli’s law explains arterial blood flow
Bernoulli theory: Mechanical energy of flow is determined by pressure, kinetic and potential energies, ρ-fluid mass.
Energy = pressure + kinetic +potential
venous return
