Lecture 12: Principles Of Fluid Flow 3

Question 1

Describe the difference between velocity and flow

Answer 1

Flow = volume of fluid transferred per unit in time (cm3/sec or ml/sec)
Velocity = distance travelled by fluid per unit in time 

Velocity is proportional to the flow rate, as flow increases so does
velocity -inversely proportional to cross sectional area
Velocity = flow rate(Q)/ cross-sectional area (A)

Question 2

Determinants of fluid flow:
Flow is proportional to
Or inversely proportional to

Answer 2

Proportional to the pressure gradient
Inversely proportional to resistance
Increased pressure difference = increased flow
Increased resistance = decreased flow

Question 3

Fluid flow

1a) Pressure gradient

Answer 3

• fluid flows from high pressure to low pressure
• flow is proportional to the pressure difference between two areas (pressure drop or pressure gradient)
• larger pressure gradient = higher flow
• in cardiovascular system pressure difference is mainly generated by contraction of heart muscle
• gravity also contributes

Question 4

Fluid flow
1b) resistance
What is resistance?
What contributes to resistance?

Answer 4

Resistance = friction that opposes blood flow
-flow is inversely proportional to to resistance
What contributes to resistance?
A) tube length: as tube length increases, resistance increases
B) tube radius: as tube radius decreases, resistance increases. A small change in radius leads to big change in resistance
C) fluid viscosity: force generated by the particles of the fluid to resist any relative motion between them
As viscosity increases resistance increases
D) flow pattern: more turbulent flow leads to increased resistance. Turbulence is likely its high density, low viscosity fluids travelling at high velocities through large diameter tubes. Also caused by irregularities in tube walls

Question 5

Blood flow in the cardiovascular system: (haemodynamics)

What is haemodynamics?

Answer 5

haemodynamics refers to forces generated by the heart and movement of blood through the cardiovascular system

Question 6

haemodynamics
A) resistance to flow in the cardiovascular system
Radius
How do we regulate blood flow by regulating radius?

Answer 6

Most important contributor to resistance in blood vessels is changes in vessel radius (smooth muscle)
Constriction of blood vessels in a particular vascular bed increases resistance which decreases blood flow to that area…but blood as to go somewhere…decreased flow in one region of the body will increase pressure and therefore flow in another region of the body

Question 7

List and describe the Special properties of flow in the cardiovascular system..

Answer 7

Flow in the cardiovascular system has special properties because:
A) blood viscosity is variable: blood viscosity varies depending on proportion of RBCs, RBC shape (normal ones are flexible, sickle cells are not as flexible. Decreased flexability = increase viscosity, blood flow rate (low flow rate = increased viscosity)

B) blood cells are not rigid: blood vessels can stretch and recoil. Arteries dampen pressure changes, artery pressure fluctuations are smaller than ventricular pressure fluctuations.

C) blood (arterial) pressure is pulsatile: variable pressure gradient -> flow decreases between heart beats

D) blood flow is not always laminar: normal changes in cardiovascular changes in artery etc causes turbulenc, eg atherosclerosis cause turbulence.

Question 8

Gravity and the cardiovascular system:

What is hydrostatic pressure?

Answer 8

Hydrostatic pressure is pressure on a vertical column of fluid die to gravity. It is proportional to the height of the column (taller column of fluid -> more pressure)

Blood pressure is influence by both the pressure generated by the heart and pressure exerted by gravity.

Question 9

Gravity and blood flow lying down

Answer 9

All parts of the body are approx at the same height

• hydrostatic pressure constant across circulation
• no hydrostatic pressure gradient therefore gravity doesn’t effect flow
• pressure declines as you move away from the heart as blood passes through resistance of blood vessels

Question 10

Gravity and blood flow standing up

Answer 10

• gravity promotes movement of blood to feet
• gravity opposes return of blood from feet to heart -> decreases venous return
• blood moving into feet due to gravity exerts pressure on blood vessel walls -> higher BP in feet

Question 11

Venous return-defying gravity

What are the three mechanisms to promote venous return

Answer 11

1. Valves: only allow blood flow towards the heart
   - located in peripheral veins
2. Skeletal muscle pump: contraction of muscles squeeze veins
   - valves ensure blood moves towards heart
3. Respiratory pump: diaphragm drops during inhalation
   - decreases pressure around heart, increases venous return, valves stop blood flowing backwards during exhalation