1. The Nature Of Flow

Question 1

Define blood?

Macroscopic and microscopic level also.

Answer 1

Blood is a structural solution that consists of specialised cells.
On the macroscopic level it is a liquid.
Microscopic level - suspension of solid particles.

Question 2

How can we separate blood?

Answer 2

Blood can be separated via centrifugation.

Spin a sample of blood at high speed, which leads to it separating into layers based on density.

Question 3

What is the result of centrifugation?

Answer 3

Blood separates into layers based on density.

Red blood cells are at the bottom, followed by the buffy coat containing platelets and wbc and then finally the plasma.

Question 4

What are the percentages of particles in blood?

Answer 4

Plasma accounts for 59-54%
Buffy coat accounts for 1%
RBC account for 40-45%

Question 5

What is the function of oxygen?

Answer 5

Transport oxygen and CO2 to and from respiring tissues

99% of cells in blood by volume

Question 6

Function of platelets

Answer 6

Involved in haemostats, thrombosis and wound healing

10x more abundant than rbc in blood

Question 7

Function of WBC ?

Answer 7

Involved in immunity and protection. Do not actually function in blood but use it as a transport medium.

Question 8

What is the main rheological property that affects blood flow in different vessels?

Answer 8

Vessel size.
Capillaries are the smallest <10Um
Arterioles ~100Um
Small arteries -mm

Question 9

What is the average size of a RBC

Answer 9

8Um

Question 10

Can RBC flow through all vessels?

Answer 10

Yes, because of their size RBC can flow through small arteries and arterioles with ease. However, a degree of deformation is required when flowing through capillaries.

Question 11

Define shear rate?

Answer 11

The difference in rate of flow of one layer compared to another, creating a velocity gradient.

Question 12

Define shear stress?

Answer 12

The drag that is experienced by a liquid, making it resistant to movement.

Question 13

Define viscosity?

Answer 13

The thickness of a fluid. The more viscous a fluid is the greater the resistance and the lower the shear rate.

Question 14

What model is used to example shear rate ?

Answer 14

Two plate model.
Two plates are positioned parallel to each other, one is fixed and the other has a constant flow applied to it.
The liquid flowing in the middle of the plates will experience laminar flow, and move the fastest. while that closest to the fixed plate will move the slowest.

Question 15

Define Poiseuille’s flow

Answer 15

Steady laminar flow of a Newtonian fluid through a straight, cylindrical rigid tube.

Question 16

Factors that aid in poiseuilles flow

Answer 16

No acceleration 
Blood is parallel to the wall 
No tube expansion 
Driving pressure is balanced by drag 
There is a constant velocity, no matter the flow rate.

Question 17

Is Poiseuille’s flow seen in blood?

Answer 17

No because blood is a non-Newtonian liquid, so viscosity changes with flow rate.
Blood moves at a steady state
The vessel walls are distensible and can only assume there is a steady state.

Question 18

What is the vascular component in blood theology?

Answer 18

Resistance to flow

Question 19

How are flow, resistance and pressure linked ?

Answer 19

Flow rate = pressure differences/ resistance to flow.

High pressure difference means there is a low resistance to flow and a high flow rate.
Low pressure difference means there is a high resistance to flow and a low flow rate.

Question 20

Resistance is directly proportional to length. Increasing resistance leads to an increase in length which decreases flow rate.

Answer 20

Resistance is inversely proportional to r4. An increase in r4 decreases resistance.

Question 21

Characteristics of blood flow in circulation

Answer 21

Blood is non-Newtonian - so viscosity is dependent on shear rate.
Vessel are straight, tapered and curved.
Walls are distensible
Flow is not simple laminar due to entry effects and turbulence.
Blood flow is pulsating

Question 22

What are the intrinsic rheological factors that control viscosity?

Answer 22

Major factors - hematocrit (packed red cell volume) and plasma viscosity
Minor factors - red cell aggregation and red cell deformation

Question 23

What are the extrinsic factors that affect blood viscosity ?

Answer 23

Flow conditions so shear rate which affects blood viscosity

Question 24

What is the normal plasma viscosity ?

Answer 24

Between 1.1 to 1.25 mPa.s

Question 25

What proteins can alter plasma viscosity?

Where are they made and their levels?

Answer 25

Albumin made in the liver 40g/l
Immunoglobulin made by lymphocytes 25g/l
Fibrinogen made in the liver 3g/l

Question 26

Why is fibrinogen at such a low concentration

Answer 26

Main function is in blood clotting and red cell aggregation but levels can be raised in pathology.

Question 27

What is the effect of high shear rates on red cells

Answer 27

High shear rates lead to red cell deformation, as the cells align with flow. This leads to decreased shear stress and viscosity

Question 28

What happens to RBC at low flow rates ?

Answer 28

They undergo red cell aggregation mediated mainly by fibrinogen leading to increased viscosity, shear stress and decreased shear rate

Question 29

What are the characteristics of hyperviscosity syndromes ?

Answer 29

Spontaneous bleeding from mucous membranes
Visual disturbances due to retinopathy
Neurological symptoms ranging from headache and vertigo to seizures and coma.

Question 30

What causes hyperviscosity syndromes ?

Answer 30

Increases in plasma viscosity due to
1. Hyperglobulinaemia - elevated immunoglobulins.
E.g myeloma >2.5m0a.s
Macroglobulinaemia - up to 10 mPa.s
2. Increase in viscosity due to abnormal cell numbers
Polycythemia vera (increased HCT)
Hyperleukotic leukemia
3. Abnormalities in RBC - affecting deformability
Sickle cell disease
Spherocytosis