What is the effect on gravity underwater?

[Kim]

OK guys and girls, here's one that might be interesting.

What is the effect on gravity underwater?

I know that it still exists - throw a stone in the sea and see where it goes - the bottom right? However, now consider blood flow around the body. At the surface our blood flow is affected greatly by gravity. Some medical conditions for instance require raising the feet to a height above the head to improve blood flow.
What about underwater? Is that flow the same or different? If it's different - how? 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 realize that this is quite a technical question but as we have so many experts on the board I'm curious as to whether there has ever been any research in this direction and whether anyone has any answers!

 

[JimC]

Gravity hasn't changed under water, so I would guess that nothing changes.

 

[jonnythan]

Someone reads TSS

Think about it this way. If your heart stopped pumping while you were standing on land, and arterial pressure just went to nothing, what would happen? The blood would get pulled down and pool around your feet. This is because the the pressure in the circulatory system is much higher at the feet due to the weight of the blood.

Now if your heart stopped pumping while you were vertical underwater, the same thing would *not* happen. The blood would stay right where it is. This is because, although the absolute pressure in the circulatory system is much higher at the feet than the head, the ambient water pressure COUNTERACTING that pressure is equal to it, whereas on land the air pressure is negligible.


In the human body, vasoconstriction is what has to generate this coutneracting pressure to keep the blood moving. Underwater, the water does it for you. In short, since the water around you weighs so much more than the air around you, the vertical positioning becomes irrelevant. You don't get a mad head rush when you go head-down in the water the way you do on land, right? Try standing on your head for 10 minutes on land, then in the same position underwater. Which is easier?


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?

 

[Rick Murchison]

... 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?

Beautiful. Best and easiest explanation I've seen on the subject. I'll use it in my classes.
S

 

[DennisS]

What is the effect on gravity underwater?

Put on thirty or forty pounds of lead, deflate BC, jump in, gravity still works underwater.

 

[MaresMan1]

Jonnythan,

Great example of relativity and nicely done.

Dive Smart; Dive Safe
Enjoy the ride
:14:

 

[Mick_O]

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?

THAT depends on temperatures. If the water in the ziploc bag is 4C, it would sink, although slowly, since the weight/density of the 4C water is only *slightly* higher than the surrounding water. Same goes for the little bag inside - your comparison is interesting, but doesn't prove anything. A mass of air would sink or rise for the exact same reason - density differences due to temperatures. It has absolutely nothing to do with gravity.

 

[O2BBubbleFree]

Someone reads TSS

Think about it this way. If your heart stopped pumping while you were standing on land, and arterial pressure just went to nothing, what would happen? The blood would get pulled down and pool around your feet. This is because the the pressure in the circulatory system is much higher at the feet due to the weight of the blood.

Now if your heart stopped pumping while you were vertical underwater, the same thing would *not* happen. The blood would stay right where it is. This is because, although the absolute pressure in the circulatory system is much higher at the feet than the head, the ambient water pressure COUNTERACTING that pressure is equal to it, whereas on land the air pressure is negligible.


In the human body, vasoconstriction is what has to generate this coutneracting pressure to keep the blood moving. Underwater, the water does it for you. In short, since the water around you weighs so much more than the air around you, the vertical positioning becomes irrelevant. You don't get a mad head rush when you go head-down in the water the way you do on land, right? Try standing on your head for 10 minutes on land, then in the same position underwater. Which is easier?


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?

Nice illustration. However, blood has a lower density than water, right? Especially with disolved nitrogen? Isn't this one of the arguements for horizontal ascents? Just curious.

Addited: Wiseguy answer to original question, "Buoyancy is the effect of gravity under water." ;^)

 

[Kim]

THAT depends on temperatures. If the water in the ziploc bag is 4C, it would sink, although slowly, since the weight/density of the 4C water is only *slightly* higher than the surrounding water. Same goes for the little bag inside - your comparison is interesting, but doesn't prove anything. A mass of air would sink or rise for the exact same reason - density differences due to temperatures. It has absolutely nothing to do with gravity.

Which would suggest that warm blood would actually try to rise during a dive. I would suppose though that the differences would not really create anything like the pressure differences on land.

Otherwise though Jonnythan - I also liked your example. Easy to picture and understand.

 

[jonnythan]

Nice illustration. However, blood has a lower density than water, right? Especially with disolved nitrogen? Isn't this one of the arguements for horizontal ascents? Just curious.

Addited: Wiseguy answer to original question, "Buoyancy is the effect of gravity under water." ;^)

I'm not really sure what the specific gravity of blood is off-hand. I'd guess it's slightly higher than water.

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