Lecture 17: Heamodynamics and vascular function

Question 1

Describe MABP = CO x TPR

Answer 1

MABP = Fairly constant
CO = Total blood flow, set by the bodies needs, individual tissue is highly regulated by response to tissue needs.
TPR = Increasing resistance increases upstream pressure to maintain flow.

Question 2

Describe how resistance influences flow?

Answer 2

Resistance increases and reduces flow to the local vascular bed

BUT

Pressure increases in the vessels prior.

Parallel circuits see no change in total flow as blood is shunted elsewhere.

Also, Q=p/tpr…. and resistance in parallel doesn’t change much. So think about the consequences of this.

Question 3

Where does the greatest pressure drop occur?

Answer 3

Small arteries/arterioles, highest resistance (variable) results in largest pressure drop.

Question 4

How is the Change in pressure determined across a vascular system?

Also what is the general equation for pressure?

Answer 4

Change P = Pa-Pv (Pv usually close to 0 and thus ignored)

P = CO x R

Question 5

What are 3 notes about pressures across the vascular system?

Answer 5

1. Major pressure drop occurs in arteries and small arterioles
2. The inverse relationship between blood flow velocity and cross sectional area {V < 1/A]
3. The maximal cross-sectional area and minimal flow rate in the capillaries.

Question 6

What is the ohms low of blood flow?

Answer 6

Blood flow (Q) = Change in pressure / resistance

Question 7

What is blood flow (Q) equal to?

Answer 7

Cardiac output

Question 8

What is another equation for cardiac output?

Answer 8

CO = (Pa-Pv)/TPR

Question 9

What is bloodflow/CO proportional to?

Answer 9

1. Directly proportional to the pressure gradient

2. Inversely proportional to TPR (most impacted by precapillary resistance vessels)

Question 10

Describe resistance in a series;

Answer 10

R(total) = Summation of resistances

i.e aortic stenosis = change in resistance (as its in series)

Greatest contribution from arterial system thus changing its resistance has greatest effect on total

Question 11

Describe resistance in parallel;

Answer 11

R(total) = 1/R + 1/R etc

i.e Restricted renal art. blood flows elsewhere but no change in resistance

Question 12

What are two important principles regarding resistance in parallel?

Answer 12

1. The total resistance of a network of parallel vessels is less than the resistance of the vessel having the lowest resistance. i.e an arrangement of parallel vessels reduces resistance to flow.
2. When many are in parallel, changing the resistance of a small number of vessels will have little effect on the total resistance for the segment

Question 13

How does parallel resistance influence local flow?

Answer 13

Little effect on total flow, but big effect on local flow. i.e can restrict blood to specific tissue and flow will move to the other parallel vessels

Question 14

What is poiseuilles equation?

Answer 14

Q (flow) = P(change) * [ (3.14 x r^4/8nL]

n = blood viscosity
L = Length of vessel 

i.e flow is dependant on pressure, tube geometry, and viscosity of the fluid. [In a glass tube]

Question 15

How can poiseuilles law be rearranged and whats the importance of this?

Answer 15

Q = P(Change) / R

Thus

R = 8nL / 3.14*r^4

i. e Resistance is proportional to
1. Tube length
2. Viscosity of fluid

and inversely proportional to
3. THe radius raised to the power of four

Take home: A small change in the radius of blood vessels has a large effect on resistance.

Question 16

Whats the impact of resistance being inversely proportional to the radius raised to the power of four?

Answer 16

Drugs that dilate the vessels therefore have powerful effect on blood flow

Question 17

What is viscosity?

Answer 17

A measure of the friction between adjacent layers of fluid as they slide over one another, increasing viscosity decreases flow

Question 18

What are the determinants of blood viscosity?

Answer 18

1. Temperature (colder, blood gets thicker)
2. Heamatocrit (the ratio of red cell volume to total cell volume) (inc. this inc. viscosity) (blood loss replaced with saline, which decreases viscosity)
3. Shear rate (velocity)
4. Vessel diameter (very small vessels appear to decrease viscosity(Axial streaming))

Question 19

Describe how shear rate influences blood viscosity;

Answer 19

At very low velocities blood becomes non-newtonian

• Viscosity is no longer independent of shear rate.
• Viscosity increases as cells aggregate (sludging of blood)

Question 20

What 3 things do poiseuilles equation assume?

Answer 20

1. Steady laminar flow
2. Rigid straight tube
3. Newtonian fluid

Question 21

We do not have rigid straight tubes, we have distensible vessels, describe what this means? and describe two differing instances

Answer 21

Increasing pressure results in:

• Distension of blood vessel
• Thus reduced resistance
• Increased blood flow

Decreasing pressure results in:

• Reduced stretch of vessels
• Increased resistance
• Decreased blood flow

Question 22

What does auto regulation mean for Q=P/TPR

Answer 22

Blood flow is not directly proportional to the pressure gradient.

In many vascular beds there are local mechanisms to ensure blood flow remains relatively constant following changes of pressure.

Question 23

What are four things the contractile state of vascular smooth muscle controlled by?

Answer 23

1. Myogenic mechanisms i.e stretch, pH
2. Vascular endothelial cells (NO)
3. Neural
4. Circulating hormones i.e ANG2`

Question 24

Describe why shear stress influence vessels;

Answer 24

Endothelium is very sensitive to shear stress;

Inc. flow = inc. shear stress

• NO release
• Vasodilation

Potential damage

Question 25

Do we have laminar flow?

Answer 25

We can have turbulent flow from;

• High fluid densities
• large tube diam
• high flow velocities
• low fluid viscosities
• abrupt variations in tube dimensions
• Irregularities in the tube walls

Question 26

What is reynolds number? and its equation

Answer 26

A predictor of turbulent flow, when Re is greater than 2000-3000

Re = pvD/n

p = density
v = mean velocity of flow
D = diam of tube

Question 27

What increases Re?

Answer 27

An increase in diam, velocity or decrease in viscosity will lead to increase in Re

Question 28

What is velocity of blood inversely proportional to?

Answer 28

For constant Q,

v = 1/A

Inversely proportional to cross sectional area

Question 29

What is bernoullis principle?

Answer 29

Conservation of energy

States that: Total energy of laminar flow without resistance is constant and equals the sum of

1. Pressure energy
2. Potential energy
3. Kinetic energy

(Some energy lost to resistance)

i.e velocity can affect pressure.

Question 30

How does bournoullis principle come into effect when there is a narrowing of a vessel?

Answer 30

When there is a narrowing of a vessel:

• V increases
• Kinetic energy increases
• Total energy remains constant

BUT P must drop in this narrow part..

Question 31

Describe how stenosis impacts vascular function?

Answer 31

Stenosis = narrowing

Bournoullis principle; Increased velocity and decreased pressure

Increased velocity then into post stenosis widening lumen will in turn create turbulent flow as Re has increased.

Question 32

What is the ability of blood vessels to change tone also known as?

Answer 32

Compliance