Outer Space / Society
One of the adaptation phenomena of zero gravity, where bodily fluids such as blood become concentrated in the head and the chest, causing a person's face to swell slightly and appear chubbier than it does under 1G. The blood of the cardiovascular system is moved throughout the body almost as much by gravity as it is by the pumping action of the heart; the same applies to the lymphatic system, which has no central pump and moves under low pressure via peristalsis. Therefore, it is said that in weightlessness, the head and the chest contain approximately 2 liters more fluid than they do when under 1G.
This collection of fluid in the upper half of the body also causes the lower half of the body to appear thinner, the phenomenon known as bird leg.