How much fluid can really build up in those tissues? I can see the logic where maybe a little height can be gained because of this, but I would think it minimal and that it would quickly get pressured out once one stands up.
That's correct - it's not a whole lot. Most people have an average height change of about 17 to 18 mm (about .66 to about .7 inches).
Wouldnt you need to take your height seconds after standing up to see any additional height?
No - it takes about 3 hours before you've lost most of your "additional" height for the day (less time for those of higher body weights).
Also, if its gravity related, then once you stand up, more fluid should go to your feet. So wouldnt you gain some height from that? It seems that would counter some of the effect of spine compression / horizontal effect.
The fluid doesn't go to the feet. Fluid that is squeezed out of the disks due to pressure becomes part of the extracellular fluid. This is simply fluid outside of the cells (i.e. interstitial fluid and plasma). The spacing of fluids is such that fluid is held within its compartment. If this were not the case, then yes, the fluid would go to the feet... along with
all of the body's fluid. (Swelling of feet is usually a result of spending lots of time on them - walking, standing, etc. which causes increased blood flow to the feet, leading to increased permeability of blood vessels in the feet, and therefore, increased "deposits" of fluid. Also, when feet swell, it's usually on the top and sides - wouldn't contribute to height). The fluid becoming part of the pre-existing extracellular fluid may contribute to a slight (very slight, actually) increase in "width" but not height. Any fluid that makes it into the blood stream to become part of the plasma would likely be filtered and removed by the kidneys.
It assumes that you sleep on your back, so the spine would be the lowest area to gravity. What if you wake up after sleeping on your belly? Wouldnt gravity pull the fluid out of the spine tissue and send it down to the front areas?
It doesn't have anything to do with what position you sleep in, as long as the back is not vertical. When standing or sitting, the spine becomes a curvy column of sorts. Disks are situated between the vertebrae, so the spine is a stack of vertebrae and disks. Gravity causes the vertebrae to put pressure on the disks, resulting in the fluid being "squeezed out." When lying on the back, stomach, or either side, gravity does not force the weight of the vertebrae onto the disks, but rather onto the soft tissues that lie around the spine (usually fascia and muscle).
Hope this helps. And I hope we're not off-topic here!!
