Q.Why are high tides and low tides in the sea?

Fluid Redistribution

Where This Idea Belongs

"Fluid redistribution" is the term used for what happens to the fluids inside your body -- blood, lymph, and other tissue fluid -- when the pull of gravity on them is removed or greatly reduced, as it is for an astronaut in orbit. It is one of the everyday, human-scale consequences of weightlessness, the topic that connects gravitation to what actually happens to a person's body in space.

What Gravity Normally Does to Your Fluids

On the ground, gravity is constantly pulling the fluid in your body downward, toward your legs and feet. Your circulatory and nervous systems are tuned to this: valves in your leg veins, blood-pressure sensors in your neck, and reflexes that adjust heart rate all work on the assumption that fluid tends to pool low in the body when you are standing or sitting. This is why, if you stand up very quickly, you can feel briefly light-headed -- your body needs a moment to push enough blood back up to your brain against gravity.

What Happens When the Pull of Gravity Is Removed

In an orbiting spacecraft, everything -- the astronaut and the fluid inside them -- is in continuous free fall around the Earth, so there is effectively no sustained gravitational pull holding fluid down in the legs (this is the same weightlessness that lets objects float freely inside the cabin). Without that downward pull, body fluid is no longer held low. Over the first hours in orbit, roughly a couple of litres of fluid that would normally sit in the legs and lower body shift upward into the chest, neck, and head. This is fluid redistribution: the total amount of fluid in the body does not change, only where it sits.

The Physical Effects

This headward shift produces recognisable, well-documented effects:

• A puffy, rounder-looking face and a feeling of head congestion or fullness (sometimes called "puffy-face" appearance), because more fluid than usual is sitting in the head and upper chest.
• Thinner-looking legs, since less fluid than on Earth is pooling there.
• The body's fluid-control system senses the extra fluid volume in the chest/head as if there were too much total fluid, and can respond by increasing fluid and salt loss (more urination) in the first day or two, which can lower overall blood plasma volume. …