Control of Blood Flow

Question 1

What three things mainly control TPR?

Answer 1

• Poiseulle’s Law
• myogenic response
• blood viscosity

Question 2

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

Answer 2

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, but greater blood flow.

With the arterioles constricted, there is an increase in TPR. This leads to increased BP upstream, but less blood flow.

Question 3

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 3

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 4

What is Poiseuille’s Law?

Answer 4

It describes the parameters that govern TPR.

Conductance (G) = (πr^4)/(8ηL)

r: radius of vessel
η: blood viscosity
L: vessel length

(blood vessel radius to the power of 4 controls TPR)

Question 5

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

Answer 5

CO = Pa - CVP x (πr^4)/(8ηL)

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 6

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

Answer 6

• capillaries have no smooth muscle/sympathetic innervation, so can’t contract
• capillaries are arranged in parallel, so have a low total resistance, while arterioles are in series, so the total resistance is greater
• capillaries are shorter
• there is less resistance in capillaries due to the bolus flow
• there is less of a pressure drop across capillaries (20-30 mmHg) than arterioles (40-50 mmHg) due to less resistance to blood flow in capillaries

Question 7

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

Answer 7

INTRINSIC (factors entirely within an organ or tissue): local hormones, tissue metabolites, myogenic responses, endothelial factors

EXTRINSIC (factors outside the organ or tissue): neural (eg. sympathetic nervous system), hormonal (eg. adrenaline)

Question 8

What is Baylis’s myogenic response?

Answer 8

It is the property of the myogenic tisue that means that the increased distension of the vessel makes it constrict, while the decreased distension of the vessel makes it dilate.

The stretching of the muscle causes ion channels to open, which then depolarise the cell, leading to muscle contraction.
This means that the vessels maintain blood flow at the same level during changing arterial pressures. This is very important in renal, coronary and cerebral circulation.

Question 9

What is blood viscosity and what does it depend on?

Answer 9

Viscosity is the measure of internal friction opposing the seperation of the lamina.

Flow = P1 - P2 x (πr^4)/(8ηL)

Blood viscosity depends on:

• velocity of blood
• vessel diameter
• haematocrit

Question 10

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

Answer 10

• 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)

Question 11

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

Answer 11

There is a -90 mmHg pressure gradient against the flow from the feet back to the heart. 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, which is clinically important.