The nostrils open into the nasal cavity which is the space inside the nose between the base of the skull and the roof of the mouth. It connects the nostrils to the upper part of the pharynx (throat). The whole cavity is covered with mucous membrane (lining that secretes slimy fluid) and tiny hair called cilia. The nasal cavity also contains the olfactory apparatus or epithelium that supports the sense of smell. The air that passes through the cavity is filtered through the cilia.
The respiratory sinuses are air-filled, mucous-membrane lined cavities within the skull and facial bones surrounding the nose. There are four pairs of sinuses in the body that produce mucous. This mucous is drained out through the nasal cavity. Sinuses help to warm and moisten the inhaled air. They produce mucus that helps to trap dust, microbes and other particles. Sinus cavities help in speech production, too.