Lecture 7.1: Blood Flow

Question 1

What is Haemodynamics?

Answer 1

It is the science that describes the association between blood flow, blood pressure and resistance

Question 2

What drives the flow of blood?

Answer 2

• Pressure gradient drives the flow of
  blood
• Flow is proportional to pressure
  difference, the greater the
  difference the greater the flow

Question 3

What is the relationship between blood flow and velocity?

Answer 3

• Flow along a vessel is constant
• Velocity can vary along the tube
  (depending on the radius of the
  tube)
• For a given flow rate, velocity is
  inversely proportional to cross
  sectional area

Question 4

What is Laminar Blood Flow?

Answer 4

Occurs when blood moves in ordered, parallel layers through the arteries with no obstructions to agitate the layers

Question 5

What is Turbulent Blood Flow?

Answer 5

Occurs when blood flows in a whole host of random pathways, turbulence results in sound production that can be heard with a stethoscope

Question 6

What is the gradient of velocity in a blood vessel?

Answer 6

Maximal at centre, theoretically zero at the edge

Question 7

What factors determine resistance in a blood vessel? (3)

Answer 7

• Vessel Length (L)
• Blood Viscosity (η)
• Radius of the Blood Vessel (r)

Question 8

How does the radius of a vessel change resistance?

Answer 8

• A change in radius alters resistance
  inversely to the 4th power of the
  radius (1/R ∝ R^4)
• A twofold increase in radius.
  produces a 16 fold decrease in
  resistance

Question 9

What 3 things does Poiseuilles Law assum?

Answer 9

1) Vessels are rigid
2) Viscosity remains constant
3) Flow is laminar

Question 10

But blood vessels are not rigid, their walls are…? What is the effect of this?

Answer 10

• Distensible
• Pressure within the vessel
  generates a transmural pressure
  i.e. from ‘inside’ to ‘outside’
• A vessel stretches so resistance
  must fall, easier for blood to flow
• The opposite is that as pressure
  falls, the vessel may collapse

Question 11

Which type of blood vessels are the most distensible?

Answer 11

Veins

Question 12

Distensible vessels are said to have…?

Answer 12

Capacitance

Question 13

What is Normal BP?

Answer 13

Systolic – 120mmHg
Diastolic – 80mmHg

Question 14

What is Viscosity?

Answer 14

• Viscosity is an internal friction
  between the components of a liquid
• It’s a measure of the extent to
  which layers of fluid resist sliding
  over each other

Question 15

What is Blood Flow?

Answer 15

The volume of blood passing a given point per unit time

Question 16

What is Resistance?

Answer 16

Collective term for the various frictional forces impeding the flow of blood

Question 17

What is Velocity?

Answer 17

The rate of movement of fluid particles travelling along a tube

Question 18

What is Q=VA?

Answer 18

Volumetric Flow Rate= Mean Velocity x Cross-Sectional Area of Flow Radius

Question 19

What is the Blood Flow Equation?

Answer 19

Flow = Pressure Difference/Resistance

Question 20

What is the Resistance Equation?

Answer 20

Resistance= [8 x viscosity x length]/ [πr^4]

Question 21

Poiseuilles Law: Q= [∆Pπr^4]/[8nL]
What does each symbol stand for?

Answer 21

Q= volume flux
P= pressure difference
r= radius
n= viscosity
L= length

Question 22

For resistances in series, simply add up the individual resistances (Equation)?

Answer 22

(ΣRT = R1 + R2 + R3)

Question 23

For resistances in parallel, add up the reciprocals in this case (Equation)?

Answer 23

Σ1/RT = 1/R1 + I/R2 + 1/R3

Question 24

What type of vessel is least distensible/has highest resistance?

Answer 24

This means terminal arteries and arterioles are the main sites of resistance

Question 25

What does widespread vasodilation do to TPR and BP?

Answer 25

Leads to decreased TPR and decreased BP

Question 26

Why do Capillaries not have massive resistance despite small size?

Answer 26

• Vast numbers in parallel
• Short length
• Bolus flow of fluid (effectively reduces
  viscosity)