hydra is a unicellular animal. it do not have respiratory organs like respiratory system or gills. it respires through its skin by the process of diffusion.
The brain sends a signal to the muscles located around the lungs to push on the lungs to expand. The lungs draw in oxygen (or bad particles like smoke) and it oxygenates the blood. That is...
People need to breathe in order to stay alive. All the cells in your body require oxygen to move, work and break down food, so without oxygen you will literally die.