1. The Nature Of Flow Flashcards
Define blood?
Macroscopic and microscopic level also.
Blood is a structural solution that consists of specialised cells.
On the macroscopic level it is a liquid.
Microscopic level - suspension of solid particles.
How can we separate blood?
Blood can be separated via centrifugation.
Spin a sample of blood at high speed, which leads to it separating into layers based on density.
What is the result of centrifugation?
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.
What are the percentages of particles in blood?
Plasma accounts for 59-54%
Buffy coat accounts for 1%
RBC account for 40-45%
What is the function of oxygen?
Transport oxygen and CO2 to and from respiring tissues
99% of cells in blood by volume
Function of platelets
Involved in haemostats, thrombosis and wound healing
10x more abundant than rbc in blood
Function of WBC ?
Involved in immunity and protection. Do not actually function in blood but use it as a transport medium.
What is the main rheological property that affects blood flow in different vessels?
Vessel size.
Capillaries are the smallest <10Um
Arterioles ~100Um
Small arteries -mm
What is the average size of a RBC
8Um
Can RBC flow through all vessels?
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.
Define shear rate?
The difference in rate of flow of one layer compared to another, creating a velocity gradient.
Define shear stress?
The drag that is experienced by a liquid, making it resistant to movement.
Define viscosity?
The thickness of a fluid. The more viscous a fluid is the greater the resistance and the lower the shear rate.
What model is used to example shear rate ?
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.
Define Poiseuille’s flow
Steady laminar flow of a Newtonian fluid through a straight, cylindrical rigid tube.
Factors that aid in poiseuilles flow
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.
Is Poiseuille’s flow seen in blood?
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.
What is the vascular component in blood theology?
Resistance to flow
How are flow, resistance and pressure linked ?
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.
Resistance is directly proportional to length. Increasing resistance leads to an increase in length which decreases flow rate.
Resistance is inversely proportional to r4. An increase in r4 decreases resistance.
Characteristics of blood flow in circulation
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
What are the intrinsic rheological factors that control viscosity?
Major factors - hematocrit (packed red cell volume) and plasma viscosity
Minor factors - red cell aggregation and red cell deformation
What are the extrinsic factors that affect blood viscosity ?
Flow conditions so shear rate which affects blood viscosity
What is the normal plasma viscosity ?
Between 1.1 to 1.25 mPa.s
