Factors Affecting Flow Through Tubes -- 5.1 Flashcards
Define flow
The volume of fluid passing a given point per unit time
Define velocity
Rate of movement of fluid particles along the tube
Describe laminar flow
there is a gradient of velocity from the middle to the edge of the vessel. Velocity is highest in the centre, and fluid is stationary at the edge. The flow in most blood vessels is laminar.
Describe turbulent flow
As mean velocity increases, flow eventually becomes turbulent.
Velocity gradient breaks down – layers of fluid try to move over each other faster than physics will allow.
This greatly increases flow resistance.
Define viscosity
The extent to which fluid layers resist sliding over one another.
Describe the effects of viscosity on rate of low
Higher viscosity leads to slower flow of central layers, so the average velocity will be slower.
Low viscosity – difference between the centre and the edge is greater. There is a smaller difference with high viscosity liquids.
What are the factors that occur in turbulent flow?
- flow resistance is increased greatly so is harder to pump blood around.
- noise is generated which can be heard with a stethoscope
What is flow determined by?
Mean velocity
– the higher the mean velocity, the more flow there is
What occurs when there is more viscosity?
Lower average velocity
What effects does the tube diameter have on flow rate?
At a constant velocity, the wider the tube the faster the middle layers flow.
What is velocity proportional to?
Mean velocity is proportional to the cross sectional area of the tube.
What determine the slope of the gradient of velocity?
The viscosity of the fluid
What are the relationships between mean velocity, viscosity and cross sectional area?
Mean velocity is inversely proportional to viscosity
Mean velocity is directly proportional to cross sectional area
At fixed pressure, what changes are seen in velocity and the tube diameter?
Increased velocity occurs when the diameter increases
At fixed flow, what changes are seen in velocity and the tube diameter?
Increasing the diameter of the tube will decrease the velocity of blood flow
What is Poiseulles Law?
Pressure = Flow x Resistance
What happens to resistance when velocity increases?
Resistance increases as velocity increases
– harder to push “thicker blood” around the vessels
Why does the resistance decrease with he fourth power of the radius?
What is the application of this?
The flow increases by SA x SA which is equivalent to the radius^4
Small radius changes produce large resistance changes – it is more difficult to push blood through smaller vessels
When flow is fixed, what is the relationship between pressure and resistance?
The higher the resistance, the greater the pressure change from one end of the tube to the other
If the pressure is fixed, what is the relationship between the resistance and the flow?
The higher the resistance, the lower the flow
What occurs when you have two vessels connected in series?
For vessels in series, resistances add together
What occurs when you have vessels in parallel?
For vessels in parallel, the effective resistance is lower, as there is more than one path for the current to flow down.
How does the flow change throughout the whole circulation?
It doesnt – the flow is constant throughout the whole circulation.
What kind of resistance to arteries have?
Low resistance, so they have a low pressure drop
What kind of resistance to arterioles have?
High resistance, so have a larger pressure drop
What kind of resistance to vein and venules have?
Low resistance, so have a low pressure drop
Why is arterial pressure high?
High pressure is needed in the arteries in order to drive the blood through the arterioles
– For a given total flow, the higher the arterioles resistance the higher the arterial pressure.
What happens when the heart pumps more blood but the resistance of the arterioles remains the same?
The arterial pressure will rise
Why do capillaries only need a small pressure to maintain blood flow?
They have a parallel conformation and low resistance
What do narrower vessels lead to?
Turbulent flow –> atherosclerosis
Define transmural pressure
The difference in pressure between teh outside of the blood vessel and the inside of the blood vessel
What is a special characteristic of blood vessel walls?
They are distensible
– increased pressure stretches the wall which increases the radius and therefore decreases the resistance
Why is flow easier when the pressure is higher in veins?
Higher pressure causes the diameter of the vessel to increase which lowers the resistance and increase the flow.
What happens when the pressure falls too low in veins?
The walls will eventually collapse and blood flow will cease before the driving pressure falls to zero.
How does distensibility of blood vessel produce capacitance?
As vessels widen, more blood flows in than out, so distensible vessels are able to “store” blood.
– veins are the most distensible
What occurs when blood pressure is low?
There is low pressure, so vessels will collapse and the surrounding tissues will die as they are not receiving oxygen and nutrients.
What is a specific characteristic of blood?
RBCs will congregate in the middle of flow, amking it appear more viscous. The plasma remains at the edges of vessels.
This causes RBCs to move around faster than plasma will.
