Control of Blood Flow

Question 1

What three things mainly control TPR?

Answer 1

• Darcy’s and Poiseuille’s Law.
• myogenic response.
• blood viscosity

Question 2

What is conductance?

Answer 2

• It is the reciprocal of resistance.
• Conductance (G) = 1 / TPR.

Question 3

Describe how the arterioles use contraction and relaxation to affect different blood parameters.

Answer 3

• In a normal situation, the arteries have a greater BP than the arterioles.
• The pressure drop between the arteries and arterioles causes blood flow.
• With the arterioles dilated, there is a decrease in TPR. These leads to decreased BP upstream (where it is ejected from the heart), but greater blood flow downstream.
• With the arterioles constricted, there is an increase in TPR. This leads to increased BP upstream, but less blood flow downstream.
• If there is overconstriction of aterioles it can lead to hypertension leading to higher arterial BP but less capillary flow (downstream flow). This can lead to underperfusion of vital organs (even though we have high BP).

Question 4

Describe how there are changes in blood flow to different area of the body in response to changes in demand, with the examples of being sedentary and while exercising.

Answer 4

• When SEDENTARY: -
  
  • The superior mesenteric is dilated (increasing the blood flow to the intestines).
  • The common iliac is constricted (decreasing the blood flow to the legs).
• When EXERCISING: -
  
  • The superior mesenteric is constricted (decreasing the blood flow to the intestines).
  • The common iliac is dilated (increasing the blood flow to the legs).

Question 5

What is Poiseuille’s Law?

Answer 5

• It describes the parameters that govern TPR.
• Resistance (R) = (8ηL) / (πr⁴).
• (Since conductance is the reciprocal of resistance)
• Conductance (G) = (πr⁴) / (8ηL)
  
  • r: radius of vessel
  • η: blood viscosity
  • L: vessel length
  • (blood vessel radius to the power of 4 controls TPR).
• Vasoconstrictors or dilators produce small changes in vessel radius by affecting smooth muscle have large effects on blood flow (As its radius⁴).
  
  • E.g. 1⁴ = 1, But 2⁴ = 16. So there is a x16 change in blood flow.

Question 6

What is Poiseuille’s and Darcy’s Law combined?

Answer 6

• CO = Pa - CVP x G
• Replace G (conductance) = (πr⁴) / (8ηL).
• CO = Pa - CVP x [(πr⁴) / (8ηL)].
• If radius goes up the conductance increases and the flow increases.
• If the length or the density increases the conductance decreases and the flow decreases.
• Now we have expanded the idea of TPR to take into account length, viscosity and radius. It illustrates why the radius of the vessel is such an important determinant in changing blood flow.

Question 7

Arterioles are the main vessels involved in TPR. Why do arterioles control TPR and not capillaries?

Answer 7

TPR not controlled by capillaries because capillaries are not resistance vessels. [CO = (Pa - CVP) x (πr⁴ / 8ηL)].

• Pressure drop (Pa-CVP) is small because of less resistance to blood flow in capillaries (Arterioles have largest pressure drop of 40-50mmHg amongst vessels).
• Radius- not altered due to no sympathetic innervation / smooth muscle in capillaries (Whereas arteriole radius tightly controlled by sympathetic nerves providing constant tone- we both dilate and constrict).
• Length- individual capillaries are short.
• Less resistance in capillaries because bolus flow reduces viscosity.
• Capillaries in parallel so total resistance is R_total = 1/R1 + 1/R2 etc. Whereas, arterioles are in series so R_total = R1 + R2 etc. so total resistance is greater.

Question 8

What are the two control mechanisms of arteriole radius (including examples)?

Answer 8

​Local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissue.

• INTRINSIC (factors entirely within an organ or tissue): local hormones, tissue metabolites, myogenic responses, endothelial factors [constantly produces nitric oxide to dilate the vessels].
• EXTRINSIC (factors outside the organ or tissue): neural (eg. sympathetic nervous system), hormonal (eg. adrenaline).

Question 9

What is Bayliss myogenic response?

Answer 9

• It is the property of the myogenic tissue that means that during increased pressure there is increased distension (enlargement) of the vessel and so the Bayliss response makes it constrict, while the decreased distension of the vessel (during decreased pressure) makes it dilate.
• This means that the vessels maintain blood flow at the same level during changing arterial pressures.
• The stretching of the muscle causes ion channels to open, which then depolarise the cell, leading to muscle contraction.
• This is very important in renal, coronary and cerebral circulation.

Question 10

What is blood viscosity and what does it depend on?

Answer 10

• Viscosity (η) is the measure of internal friction opposing the seperation of the lamina.
• Higher viscosity means more friction means lower flow.

Flow = (Pa - CVP) x (πr⁴) / (8ηL).

Question 11

Describe properties of veins and how they are important for controlling CVP.

Answer 11

• Veins are thin-walled, collapsible, voluminous vessels, and so can act as a blood reservoir of 2/3rd of the blood volume.
• They are also innervated by sympathetic nerves, so they can control their radius.
• The contraction of these vessels expels blood into the central veins, which increases venous return/CVP/end diastolic volume, which in turn increases stroke volume (Starling’s law).
• Typical venous pressures
  
  • Limb vein, heart level :- 5-10 mmHg.
  • Central venous pressure (entering heart) :- 0-7 mmHg
  • Foot vein, standing :- 90 mmHg.

Question 12

How does Bernoulli’s law explain the blood flow from the heart to the feet?

Answer 12

When there is a low pressure the veins collapse and when there is a high pressure the veins distend.

• There is a -90 mmHg pressure gradient against the flow from the heart to the feet.
• This means that he blood has to flow against the pressure gradient (to flow from the heart to the feet).
• The ejected blood has greater kinetic energy at the heart than the feet (more velocity).
• Also, there is greater potential energy at the heart than at the feet (more height).
• The greater kinetic/potential energies overcome the pressure gradient to maintain flow.
• However, the flow to the feet is easily compromised (as even a slight change can cause this balence to be overthrown), which is clinically important.