What is the effect on gravity underwater?

[jonnythan]

The specific gravity of human blood is about 1.06, which means it's about 6% more dense than pure water, and 3-4% more dense than seawater.

 

[Mick_O]

However, the compelling thing is that the pressure of a 6 foot column of blood is VERY close to the pressure of a 6 foot column of water, but VERY much higher than a 6 foot column of air

Ecaxtly Unless you don't have enough blood in your alcohol. In which case you shouldn't dive with Ziploc bags anyway. Umm.

 

[redacted]

Someone reads TSS

If you're not convinced, picture this. You have a 1 gallon ziploc bag with 1 quart of red-tinted water and no air in it. Hold it vertically on dry land. All the water sinks as far to the bottom as possible, right?

Now take that ziploc diving. The water no longer sinks to the bottom, does it?

Neat analogy. And absolutely accurate if the liquid in the bag has the same specific gravity (sg) as the water you are diving in. But what happens when the specific gravity of the liquid in the bag varies. If the bag contained pure water (sg=1.00) and you were diving in sea water (sg=1.02) then the contents would tend to rise imperceptably. If the contents were gasoline (sg=.74) then they would rise noticably. Milk (sg=1.05) would tend to sink. I could't google up an answer to the specific gravity of blood, but I'd guess it around 1.05 to 1.06. That old blood is thicker than water thing. So it would also tend to sink until the forces exerted by the "bag" containing it countered the sinking force.

 

[knotical]

Could it have any consequences for on-gassing/off-gassing for instance? Could different positions underwater effect this process - ie, vertical, horizontal, head slightly down?

I think your body’s orientation would have little to no practical impact for on-gassing and off-gassing.

A different gravity consideration is that a head-down position will cause your stomach contents to flow toward your esophagus, sometimes causing heartburn, especially among those with GERD or a hiatal hernia. This is true above water too, but I don't see as many people (adults anyway) hanging upside down on land as I do underwater.

 

[Mick_O]

So it would also tend to sink until the forces exerted by the "bag" containing it countered the sinking force.

The bag would sink until it encountered something with a higher density, and then "float" between the two layers.

 

[whitedragon13]

Is gravity related to "sinking"? I would think that part would be mostly related to Archimedes Principle.

 

[Mick_O]

Is gravity related to "sinking"? I would think that part would be mostly related to Archimedes Principle.

Archimedes' Principle is all about gravity. It's gravity that causes the displaced volume of <insert media of choice here> to have a weight, thus keeping ships and divers afloat.
"Gravity is everywhere. It is all around us. Even now, in this very room. You can see it when you look out your window or when you turn on your television. You can feel it when you go to work... when you go to church... when you pay your taxes."

 

[redacted]

The bag would sink until it encountered something with a higher density, and then "float" between the two layers.

Yes, if released. I was assuming the bag was being held at a constant point. Just like the bag was being held up in the air.

 

[simbrooks]

Is gravity related to "sinking"? I would think that part would be mostly related to Archimedes Principle.

Its a dual force system (neglecting drag etc), the force of gravity is pulling the object down (related to an object's mass, acceleration is essentially constant in the ranges we are discussing), whilst the Archimedes principle effects the upthrust countering the gravity (related to the volume of water displaced). We just roll them both into one and talk of density (mass/unit volume) or specific gravity (density compared to pure water) for an easy term to relate whether the object will float or sink.

As for the blood, well i like Jonny's explanation. When it comes to blood flow around the body, the pressure at your feet is going to be higher than the head, so it takes more effort to pump the blood down to your feet in a vertical position compared to a horizontal position where the blood would be countered by water pressure that is almost equal around your entire body (ie your body feels the same pressure pretty much all over as its all around the same depth when horizontal, whereas its at a variety of depths when vertical). Does that make any sense as its a rough paraphrasing/summary of the discussions i have had (offline) with others about this type of subject. In my case there would be a difference of about 6ft of water or 2.5psi if i were vertical compared to horizontal.

 

[O2BBubbleFree]

In my case there would be a difference of about 6ft of water or 2.5psi if i were vertical compared to horizontal.

Interesting. I guess since blood pressure is only about 2.32 PSI (systolic), it follows that circulation would be much better when horizontal.

I mean, if your heart is putting out 2.3 PSI, and your feet are seeing +2.5 PSI of pressure, the blood ain't gonna flow too good. (Rough figures used for illustration. I'm sure it's not 6' from your heart to your feet, or you'd be about 7' tall).