Control of blood flow Flashcards
What is G?
G= conductance which is the reciprical of TPR
Equation for cardiac output in relation to conductance and pressure gradient
CO=Pressure gradient x Conductance
What does TPR control?
TPR controls blood flow and blood pressure
What does an increase in resistance mean?
Increase in resistance means you need to increase pressure to keep the same flow
What controls TPR?
o Poiseulle’s Law
o Myogenic response
o Blood viscosity
Blood flow equation
Blood flow(CO)=Pressure gradient/TPR
What is hypertension?
Over constriction of arterioles
What does hypertension lead to and cause?
Higher arterial BP but less capillary flow causing under perfusion
What causes blood flow?
Pressure drop between arteries and arterioles
What does a decrease in TPR lead to?
Decrease in TPR, decreased blood pressure upstream but greater flow due to vasodilation
What does an increase in TPR lead to?
Increase in TPR, increased blood pressure upstream but less flow due to vasoconstriction
Sedentary VS Exercise of Superior mesenteric
Dilated at sedentary-Increased flow to intestine
Constricted during exercise leading to a decreased flow to intestines
Sedentary vs Exercise of Common iliac
Constricted at sedentary-Decreased blood flow at rest
Dilated during exercise-Increased flow to legs
What does Poiseuille’s law describe?
Describes parameters that govern TPR
What does Poiseuille’s law illustrate?
It illustrates why the radius of a vessel is such an important determinant in changing blood flow
What is the r^4 effect?
Double radius size from 1mm to 2mm can cause a change to r^4 by 16 times
What does the r^4 effect show?
A small change in size of radius can have a massive impact
What do vasoconstrictors or dilators produce?
Produce small changes in vessel radius by affecting smooth muscle and have large effects on blood flow
What are the main vessels involved in TPR
Arterioles Are The Main Vessels Involved in TPR
What have the largest drop?
Arterioles have the largest drop
What is arteriole radius tightly controlled by?
Arteriole radius is tightly controlled by sympathetic nerves providing constant tone
o They can dilate and constrict
What are the 3 main parameters TPR is controlled by?
o Radius (r4) o Pressure difference across vessels (P1-P2) o Length (L)
Comment on arterioles structure
Arterioles are long vessels
What is TPR not controlled by?
Not controlled by capillaries
How are capillaries arranged and so whats total resistance?
Capillaries are arranged in parallel, so have a low total resistance as RTotal = 1/R1 + 1/R2
How are arterioles arranged?
In contrast, arterioles are in series
RTotal = R1 + R2 etc so total resistance is greater
What is local blood flow through individual organs/tissues controlled by?
Local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissue
What are the 2 main factors of arteriole radius?
Intrinsic and Extrinsic
What are examples of intrinsic factors?
Local hormones
Tissue metabolites
Myogenic properties of muscle
Endothelial factors
What are examples of extrinsic factors?
Neural
Hormonal
What does increased distension of vessel do?
Increased distension of vessel makes it constrict
What does baylis’s myogenic response maintain?
The response maintains blood flow at the same level during changing arterial pressures
What is Baylis’s response very important in?
It is very important in renal, coronary and cerebral circulation
What does stretching of muscle cause?
Stretching of the muscle causes ion channels to open, which then depolarize, leading to muscle contraction
What does blood vessel do at higher pressures?
At higher pressures when vessel is stretched, it contracts to reduce flow
What are the three factors to which blood flow depends on?
Viscosity of blood
Vessel diameter
Haematocrit
What is viscosity a measure of?
Viscosity is a measure of internal friction opposing the separation of the lamina
Blood viscosity
Refer to table
Where is 60% of blood volume at rest?
60% of blood volume at rest is in systemic veins and venules
What does the venous system function as?
Functions as blood reservoir
Structure of veins
- Thin-walled, collapsible, voluminous vessels
What do veins contain?
- Contains 2/3rd of blood volume
What do veins contain and innervated by what?
contain smooth muscle which is innervated by sympathetic nerves
What does smooth muscle allow veins to be?
contractile
What does the contraction of veins do and increase?
Contraction of vessels o Expels blood into central veins Increases venous return/CVP/End-Diastolic Volume Increases SV (Starling’s Law)
Why is venous pressure high at feet and helped by what?
Venous pressure high at feet so pressure for return to the heart
o Also helped by thoracic pump and skeletal muscle contraction
What does stimulation of sympathetic nerve cause?
Stimulation of sympathetic nerves, causing venoconstriction, shifting blood centrally
What does stimulation of sympathetic nerve increase?
Increases venous return, CVP and End-Diastolic Pressure
Increased CVP increases preload and so increases SV
What is Bernoulli’s theory?
mechanical energy of flow is determined by pressure, kinetic, potential energies (ρ = fluid mass)
Bernoulli’s law whilst an individual is standing?
- -90 mmHg pressure gradient against flow back to the heart from the feet
- Ejected blood has greater KE at the heart than the feet (more velocity, V)
- Also, greater potential energy at heart than feet (more height, h)
- Greater kinetic/potential energies overcome pressure gradient to maintain flow
