In another thread, over in IMHO, we got into a bit of confusion over this: when you go to a lab to have blood drawn, and they take it in one of those small vacuum tubes, the blood squirts rather energetically into the tube. How much of that is simply ordinary internal blood pressure – the same would happen if the needle went into a tube with air in it, or if the needle simply opened into the environment – and how much of the effect is from the vacuum in the tube? Does the vacuum “suck” the blood out in a stronger spurt/squirt?
I once had a lab tech tell me that it was NOT due to the vacuum, but was entirely from my own internal blood pressure.
It is entirely due to the vacuum in the tubes and the blood rushing in to fill that vacuum. If you allow the tube to fill with air you will not get much, if any, blood flow from a vein. There is not much blood pressure behind the veins, that tends to be the arteries.
I think the results are much richer when using vacuum tubes. Transistors just don’t have the same warmth.
Wait … so those little glass mini-test tubes with the rubber tops are actually vacuum tubes? I thought they were capped just to keep contaminants out, not that the air was evacuated. Neat.
Well, the other name for the tube in which blood is drawn is vacutainer…it is referred to as an “evacuated tube.”
I use butterflies almost exclusively for blood draws. When you have hit the vein a tiny “blood flash” shows that fact. You then attach to vacutainer to the transfer device and blood is pulled into the tube. The blood stops when the pressure is equalized and you can pop off one tube pop another one on. Easy-peasy.
In a pedantic way the tech is right, the flow is entirely due to pressure of your blood. Vacuums cannot of themselves make anything flow. It is pressure from the outside that forces stuff in.
But the majority of this is atmospheric pressure acting on you, not “internal” pressure from your heart’s action. Given the clear difference in flow rate from a simple exposed needle versus one of the tubes I cannot believe the tech was not aware of the contribution of the vaccum. So, one wonders if they were not just being deliberately pedantic and unhelpful.
I thought the fact that blood doesn’t flow without the tube shows the tube is doing something. Also, if the tube had air inside and was sealed, it would be almost impossible to get blood into it.
The thread title made me think of the other type of vacuum tube.
I was at a lab once and a tech was trying to draw blood with an old supply of vacuum tubes. The first 3 or 4 had lost vacuum and he could not get but a few drops into each one.
Then he got one that still had vacuum and it filed right up.
As someone who has to get blood drawn on a frequent basis, I vastly prefer butterflies. The tech has better control, and the needles are smaller. I usually never feel a butterfly, but I might feel the other kind of needle.
This is part of the problem lab techs have with being taken seriously.
The lab person that has the most contact with the public (the phlebotomists) are typically the least trained and educated people working in the lab and sometimes say silly things like what this thread is about. Then the public is left with the impression that lab people don’t know what they’re doing.
Depending on jurisdiction, the person taking blood may have had a weekend course or a year long diploma or anything in between. Heck, they may have been some guy off the street just a few months ago and were trained on the job.
I highly suspect the person in this anecdote honestly believed that blood pressure was all there was to filling the tubes and wasn’t trying to be pedantic.
