The metric system is a number system that designates one base unit for each type of measurement.  For example, the base unit for length is the meter and the base unit for mass is the gram.

After air enters through the nose, it passes through the nasal cavity which filters, moistens, and warms it. Further filtering takes place in the pharynx, which also helps protect against infection, and then in the trachea which is just past the epiglottis which prevents food from entering the airway.

The biosphere is the global ecological system integrating all living beings and their relationships. This includes their interactions with the lithosphere (the rigid outer part of the earth, consisting of the crust and upper mantle), hydrosphere (all surface water), and atmosphere (the envelope of gases surrounding the planet).

The electromagnetic spectrum covers all possible wavelengths and frequencies of radiation.  From lowest frequency (longest wavelength) to highest frequency (shortest wavelength) radiation: radio waves → microwaves → infrared waves → visible light → ultraviolet light → X-rays → gamma rays.

Radiation occurs when electromagnetic waves transmit heat. An example is the heat from the Sun as it travels to Earth.