Blood volume reduced on standing

[Sarah Tee]

I have seen a couple of mentions of blood volume decreasing when the body is upright because plasma (?) gets squeezed out of the capillaries into the surrounding tissues by gravity/pressure.

I’m imagining squeezing oatmeal through pantyhose!

Haven’t seen it mentioned in the dysautonomia literature though. Has anyone else heard of this?

In normal people, it mustn’t make much difference, but if you already have low blood volume and pooling in the lower body and legs then it all adds up.

I found this article and it seems to describe this shift:

https://pubmed.ncbi.nlm.nih.gov/11538894/

[edriscoll]

I haven't come across what you are asking about.  But it sounds like two separate things that I have read about and experienced.  The first, as you mentioned relating to the decrease of blood pressure when standing (OI) and the blood pooling in the lower extremities.  But separate from that, I have had discussions with my doctors about the "leaking" of capillaries into surrounding tissue that can cause swelling in the extremities.  I haven't read or heard anything about this "leaking" or squeezing of capillary blood being related to blood volume upon standing or related to OI/dysautonomia.  I do love the imagery of oatmeal squeezing through panty hose.  

Very interesting article.  Thanks for sharing. 

[Sarah Tee]

Rediscovered the reference for this one:

Quote

… standing produces a substantial increase in the transmural capillary pressure present in the dependent areas of the body, which causes a rise in fluid filtration into tissue spaces. This process reaches a steady state after ≈30 minutes of upright posture and can produce a decline in plasma volume of up to 10%.

It was Dr Grubb in this older article:

https://www.ahajournals.org/doi/full/10.1161/circulationaha.104.482018

Worth a read.