A long nededle verus a short one

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Discussion Overview

The discussion revolves around the effects of needle length and diameter on pressure readings during kidney dialysis. Participants explore the underlying physics principles, including the Venturi effect and Poiseuille's law, while considering the implications for fluid dynamics in medical applications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Stuart Mo notes a difference in arterial and venous pressures when using 1-inch versus 3/5-inch needles during dialysis.
  • Some participants suggest that the internal diameter of the needle may significantly affect the pressure readings, rather than just the length.
  • One participant introduces the Poiseuille effect, indicating that friction between the fluid and needle walls could influence pressure changes.
  • Another participant emphasizes that the pressure drop occurs within the needle due to increased flow rate, particularly in smaller diameter needles.
  • There is a discussion about whether the friction difference is negligible over the short length of the needles, with some arguing that it could still be significant depending on the flow dynamics.
  • One participant questions the application of the Venturi effect, clarifying that the pressure difference is due to resistance to flow rather than the Venturi principle.
  • Concerns are raised about potential cell damage associated with different needle sizes, with a suggestion to consult a biologist for insights.

Areas of Agreement / Disagreement

Participants express differing views on the primary factors affecting pressure changes, with some emphasizing needle diameter and others focusing on length and flow dynamics. The discussion remains unresolved regarding the exact contributions of these factors.

Contextual Notes

Participants note that assumptions about fluid viscosity, flow rates, and needle dimensions are critical to understanding the pressure changes, but these values are not consistently provided or agreed upon.

Who May Find This Useful

This discussion may be of interest to professionals in medical fields, particularly those involved in vascular access and dialysis, as well as students and researchers studying fluid dynamics in biomedical applications.

  • #31
Skrambles said:
The Reynolds number also depends on the diameter of the pipe, in this case the needle. A small change in this variable can have a huge effect on turbidity.

If a small change in diameter can have a huge effect, so can a small change in density. Neither are squared / cubed (etc) so equal changes in either would have the same effect.
The density you gave is almost exactly the same as water.

The point of the above being that you are talking about microscopic changes in diameter internally having a "huge effect", and yet you have a far bigger change in density between your assumption of water and actual blood.
 
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  • #32
the art pressure with the one inch is 240-280 Venus pressure is 240-280 the 3/5 inch is 160-180 Venus is 160-180. this is mercury pressure that shows on the dialysis machine the 400 is 400 hundred milliliters per minutes. once again let me state the only difference is the length of the needle one inch versus 3/5 inch
 
  • #33
smott2011 said:
the art pressure with the one inch is 240-280 Venus pressure is 240-280 the 3/5 inch is 160-180 Venus is 160-180. this is mercury pressure that shows on the dialysis machine the 400 is 400 hundred milliliters per minutes. once again let me state the only difference is the length of the needle one inch versus 3/5 inch

Ok, then the result I got in #25 (6808 pa or about 1 psi) gives 51 torr (mm hg). This puts it in the Poiseuille range. I imagine the pressure 260 torr and 170 torr are above the body's pressure of maybe 120 torr? So that's 140 torr and 50 torr for the one inch and 0.6 inch, respectively. Missing a factor of three - and that could well be in the viscosity coefficient. I used 3.5 times that of water.

For the Poiseuille effect, the pressure drop is proportional to the length. Pressure over length should be the same. For the one inch its 140/1=140. For the 0.6 inch its 50/0.6=83. I would think it would be closer.

I used http://en.wikipedia.org/wiki/Needle_gauge_comparison_chart to figure the ID of both needles, assuming from #17 that OD was 1.81 to1.85mm.
 
  • #34
jarednjames said:
If a small change in diameter can have a huge effect, so can a small change in density. Neither are squared / cubed (etc) so equal changes in either would have the same effect.


The point of the above being that you are talking about microscopic changes in diameter internally having a "huge effect", and yet you have a far bigger change in density between your assumption of water and actual blood.

Re = (\rho * V * d) / \mu

V is average velocity of the flow, and d is the pipe diameter.

1060 kg/m^3 = 1.06 g / mL That's a 6% difference from the number I used.

4.5 centiPoise / 3.5 centiPoise = 1.286 That's a 28% difference in viscosity from what I used.

The diameter I used, based on the radius Rap provided, is 13.72 x 10^-4 meters, or 1.372 mm. Because this number is so small, it would not take much of a change to make a large percentage difference.

The Reynolds number is a dimensionless number used to approximate the turbidity of a given flow. Also, if the flow is turbulent, then the Hagen-Poiseuille equation is not applicable. It is meant for laminar flow only.
 
  • #35
Skrambles said:
The Reynolds number is a dimensionless number used to approximate the turbidity of a given flow. Also, if the flow is turbulent, then the Hagen-Poiseuille equation is not applicable. It is meant for laminar flow only.

Using Q=A*v where Q is the flow rate (400 ml/minute) and A is the cross sectional area of the needle, I get velocity in the needle as 4.508 m/sec. Using the expression for the Reynolds number, I get Re=1871, which is in the laminar flow regime. I don't know, but I would guess turbulence and blood would not be a good combination clot-wise.

DUH! It's probably a combination of the Poiseuille and Venturi effects.

\Delta P = \frac{\rho Q^2}{2}(1/A^2-\1/A_i^2) + \frac{8\mu L Q}{\pi r^4}

Where Ai is the cross sectional area of the tube or pipe leading to the needle, L is length of needle. Assuming Ai>>A, we can ignore 1/Ai. Using density 1060 kg/m^3, mu=3.5e-3 kg/m/sec, I get for L=1 inch Delta P = 51.06+80.67 =131.73 torr. For L=0.6 inch, I get 30.64+80.67=111.30 torr. As per #33, I would expect 140 and 50 torr. I wonder if this is within the limits of the uncertainty for our values of the various parameters.
 
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  • #36
The coupling between the needle and tube may be a significant factor here. I would guess that they are using a luer-lock, and if so then there would not be a nice smooth taper between the tube and needle. The needle could also be slightly occluded, which would increase the pressure needed to maintain the flow rate.
 
  • #37
Skrambles said:
The Reynolds number is a dimensionless number used to approximate the turbidity of a given flow. Also, if the flow is turbulent, then the Hagen-Poiseuille equation is not applicable. It is meant for laminar flow only.

Spoken like a textbook. Now keep reading and you'll see 1800 is well inside the laminar range. Not sure how you came to the turbulent conclusion above.

Rap is correct (assuming our numbers are good).
 
  • #38
jarednjames said:
Spoken like a textbook. Now keep reading and you'll see 1800 is well inside the laminar range. Not sure how you came to the turbulent conclusion above.

Rap is correct (assuming our numbers are good).

Actually, 1800 is NOT well inside the laminar range, it is in the transitional regime between fully laminar and fully turbulent flow. Perhaps it would help you to actually read a textbook before being so condescending.

It's quite funny that you're a PF Contributor considering you have contributed almost nothing to this thread.

My job is in medical device development, and I have about two dozen syringes on my desk at this very moment. I have to think about this type of problem on an almost daily basis.

If you would like to insult me some more, please do so through a private message.
 
  • #39
Skrambles said:
Actually, 1800 is NOT well inside the laminar range, it is in the transitional regime between fully laminar and fully turbulent flow. Perhaps it would help you to actually read a textbook before being so condescending.

Transitional reynolds flow is approximately 2300 to 4000 (although these numbers can vary within around 200 of the stated values depending on where you look), turbulent is above 4000.

I have never seen a textbook state 1800 as being in the transitional stage.

http://www.princeton.edu/~asmits/Bicycle_web/transition.html
While the transition from laminar to turbulent flow occurs at a Reynolds number of approximately 2300 in a pipe, the precise value depends on whether any small disturbances are present. If the experiment is very carefully arranged so that the pipe is very smooth and there are no disturbances to the velocity and so on, higher values of Re can be obtained with the flow still in a laminar state. However, if Re is less than 2300, the flow will be laminar even if it is disturbed. Thus 2300 is the value the Re below which turbulence will not occur in a pipe.

As someone who works with needles (no sarcasm, completely serious), how smooth are they internally? Perhaps that has a bearing on the flow?

I completely agree with you that I feel it's the connections that cause the problem, not the needle itself. That could certainly induce turbulent flow.
 
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  • #40
That 2300 to 4000 range is commonly used in textbooks, but with a little digging you will find published values that are much different. Those numbers are also meant to apply only to Newtonian fluids flowing through a rigid pipe that has a length several factors greater than its inner diameter. Blood is definitely not a Newtonian fluid, and the length of the needle in these cases is not enough to meet the second requirement.

http://books.google.com/books?id=sz...page&q=critical reynolds number blood&f=false

This author puts the critical Reynolds number at 1000 for a smooth, rigid, cylindrical tube.

http://wiki.nus.edu.sg/display/Hemolysis/Turbulence

"A transition state occurs between Re values of 2000 to 4000, and above 4000 the flow is said to be turbulent. However these are general guidelines and whether the flow is turbulent or not depends on other factors."

http://ep.physoc.org/content/44/1/110.full.pdf

"The velocity of flow in the vascular system is sufficiently high for turbulence to occur and Coulter and Pappenheimer [1949] found by experiment that the critical Re for bovine blood flowing in a straight tube was between 1800 and 2100."

Here's an interesting paper on turbulence in flexible tubes:

http://chemeng.iisc.ernet.in/kumaran/publications/1999-1997/vkumaran_jfm357_1998.pdf

"An attempt was made to observe the onset of turbulence by the injection of a dye stream into the tube. The authors reported that the dye stream in the centre of the tube becomes chaotic at a Reynolds number between 570 and 870. However, the authors cautioned that this might overestimate the transition Reynolds number since the turbulence appears to originate at the walls and then grows inwards to engulf the laminar core."

The needle being used is probably connected to the pump with surgical tubing, and this may also be a source for turbulence in the system. The only way to know for sure would be through experimentation.
 
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  • #41
Interesting stuff.

So would the needle be the narrowest part of the system or would it's diameter not vary much from the tubing?
 
  • #42
Skrambles said:
That 2300 to 4000 range is commonly used in textbooks, but with a little digging you will find published values that are much different. Those numbers are also meant to apply only to Newtonian fluids flowing through a rigid pipe that has a length several factors greater than its inner diameter. Blood is definitely not a Newtonian fluid, and the length of the needle in these cases is not enough to meet the second requirement.

http://books.google.com/books?id=sz...page&q=critical reynolds number blood&f=false

This author puts the critical Reynolds number at 1000 for a smooth, rigid, cylindrical tube.

http://wiki.nus.edu.sg/display/Hemolysis/Turbulence

"A transition state occurs between Re values of 2000 to 4000, and above 4000 the flow is said to be turbulent. However these are general guidelines and whether the flow is turbulent or not depends on other factors."

http://ep.physoc.org/content/44/1/110.full.pdf

"The velocity of flow in the vascular system is sufficiently high for turbulence to occur and Coulter and Pappenheimer [1949] found by experiment that the critical Re for bovine blood flowing in a straight tube was between 1800 and 2100."

Here's an interesting paper on turbulence in flexible tubes:

http://chemeng.iisc.ernet.in/kumaran/publications/1999-1997/vkumaran_jfm357_1998.pdf

"An attempt was made to observe the onset of turbulence by the injection of a dye stream into the tube. The authors reported that the dye stream in the centre of the tube becomes chaotic at a Reynolds number between 570 and 870. However, the authors cautioned that this might overestimate the transition Reynolds number since the turbulence appears to originate at the walls and then grows inwards to engulf the laminar core."

The needle being used is probably connected to the pump with surgical tubing, and this may also be a source for turbulence in the system. The only way to know for sure would be through experimentation.

Again - it seems to me that turbulence would be a bad thing when piping blood. It would make the probability of clotting much higher. Just on that basis, I would expect the design, whatever it was, would be good enough to prevent turbulence.
 
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  • #43
jarednjames said:
Interesting stuff.

So would the needle be the narrowest part of the system or would it's diameter not vary much from the tubing?

I don't know for sure what tubing would be used for dialysis, but I seriously doubt it would be that narrow inside.

From what I've been reading there is turbulence that occurs naturally in the vascular system, especially near the heart. I'm not a hematologist so I don't know what effects these conditions might have on blood. Frankly, I don't even like to see blood (especially my own).
 
  • #44
So how do we determine if the flow is turbulent?