On Earth, gravity does half of the work required to circulate blood and other fluids between the upper and lower body. So in microgravity, our internal sea of fluid tends to move towards the upper body. Each leg loses about a litre of fluid, leaving it much thinner. Astronauts call this condition 'bird legs'. As fluids migrate to the upper body, the effect on the lungs and nasal passages is similar to having a cold, and it lasts for the duration of the flight.
Astronauts also experience the discomfort of a swollen head.
Wiring On the hairs in our inner ear rest tiny clumps of calcium crystals, called otoliths. This allows us to detect motion such as the acceleration of an elevator. Other sensors, called semicircular canals, register any rotation by monitoring the movement of a tiny pool of fluid. Information from these biological gyroscopes supplements messages from the muscles, joints and eyes to help the brain orient and direct the body. In microgravity, however, the otoliths have no weight, the fluid in the semicircular canals moves freely, and the muscles and joints no longer have to support any weight.