Babies Upside Down: How Buoyancy Protects from Gravity

[compuser123]

Although this has biology in it, I still think it qualifies more as a physics question.

If you take person and hang them upside-down long enough, blood will pool in their head and they will eventually die because of the pressure on their brain.

I know that in the uterus, an unborn baby is upside down for many weeks. What is it about the water (buoyant force?) that keeps blood from pooling in a baby's head? Gravity is still acting downwards.

 

[sophiecentaur]

Babies are floating in water. It's like a 'G suit' and prevents body fluids from flowing 'down' into whichever part of the body happens to be lowest at the time. External hydrostatic pressure is the same as internal hydrostatic pressure.
An underwater diver can spend long periods working in an inverted position without suffering any ill effects. (There are enough other ill effects on their bodies but at least that isn't one of them.)

 

[anorlunda]

Babies are floating in water. It's like a 'G suit' and prevents body fluids from flowing 'down' into whichever part of the body happens to be lowest at the time. External hydrostatic pressure is the same as internal hydrostatic pressure.

Thank you @sophiecentaur . I never thought of that. We feel the pressure in our heads when flying upside down, but there is no such feeling when swimming head down. I suppose that subsurface swimming in a liquid with a density significantly different than water would produce different feelings.

 

[Dale]

Babies are also fairly small, so the hydrostatic column doesn’t make that much pressure.

 

[sophiecentaur]

Babies are also fairly small,

Many women appreciate that fact!

 

[compuser123]

Thanks guys. Your answers are much appreciated.

 

[OldYat47]

The small difference in liquid column probably plays a role. Look at a baby in utero. They are all curled up. The fact that they are floating in amniotic fluid doesn't make a difference (hydraulics say so. Try welding underwater head down for a while). Also, human beings can acclimatize themselves to hanging upside down for extended periods with enough practice. Babies in utero know no other position (except if they're breech).
This is from a pediatric neurologist. Not me, but a friend.

 

[256bits]

A water balloon is also quite small, but it makes a difference if one holds it in air or in water.
Hydrostatic inside/outside balance makes a big difference as @sophiecentaur stated. Membrane stress.

 

[OldYat47]

A free body diagram helps visualize this. Suppose we measure the internal pressure of the balloon, say, an inch from the top of the balloon. There will be some pressure contributed by the tension in the skin of the balloon and there will be some pressure from the 1" of head. If we measure the internal pressure at the bottom of the balloon it will be the same skin tension contribution plus the added head from 1" to the bottom. If that same balloon is submerged so the top water level in the balloon is even with the external water level the results will be the same. Some of the skin tension pressure will be replaced by external water pressure. If the balloon is submerged deeply enough, all the skin tension pressure will be replaced by external water pressure. Submerge the balloon to that depth and what is the difference between the internal pressure 1" down from the top and the bottom? The same difference in head pressure as if the balloon were in the air.

 

[sophiecentaur]

hydraulics say so.

Hydraulics is not the whole story though. It's not just a matter of the pressure inside the body because the distribution of body fluids would be affected. An equilibrium situation will be reached both in air and when floating in water. The actual displacement of body fluids will depend on the elastic deformation of the blood vessels and that will be far more when the body isn't supported by external hydrostatic pressure.

 

[OldYat47]

I guess a way to look at this is using the same water balloon with a baby inside it (or a baby in utero).

 

[sophiecentaur]

I’ll effects due to being upside down are largely due to distribution of body fluids and not particularly to pressure gradient. When immersed in water, the fluid distribution is not affected much because the lower parts of the body will not ‘bulge’.