Red blood cells move because of osmosis, the movement of water from an area of high water concentration (inside the cell) to an area of low water concentration (outside the cell). The cell membrane is flexible and acts like a semi-permeable membrane, allowing small molecules like water to pass through but keeping larger molecules out. As the cell loses water to the surrounding environment, it shrinks and becomes more rigid. This causes the cell to become more spheroidal and anisotropic, resulting in an increase in the cell’s buoyancy and an increase in its velocity, which is the basis of its movement.