The windpipe branches out into two bronchi, which divide further into smaller and smaller branches (bronchioles). Finally, the air enters the pulmonary alveoli in the lungs, through whose thin membrane oxygen passes over into the capillaries and in exchange carbon dioxide is released from the blood to the lungs. In a relaxed state, an adult breathes with a frequency of 12 times per minute, a child 20 times per minute, and an infant 30 to 40 times per minute. When breathing, air flows into the body either through the mouth (oropharynx) or the nose (nasopharynx).
When breathing through the nose, the air is first gets cleaned by nasal hairs and mucous membranes, it is humidified and warmed. Next, the air flows through the pharynx, past the larynx and the vocal chords into the windpipe (trachea). During an average lifetime a person breathes about 5,000,000 m³ of air. In a relaxed state, a person exhales about 19 W, 3 W sensible heat, and 22 g zvwater into their surroundings per hour. The surface area of the human lungs is about 100 m², or 1,111 ft². Perceptions of odors are transmitted directly from the nose to those areas of the brain, which are part of the limbic system and therefore responsible for the regulation of emotions. Hence, the brain functions on the premise that odors are always important for emotions. If we are surrounded by a certain smell for an extended period of time the perception of that particular smell finally disappears (adaptation), without hampering our ability to smell other odors. Usually women are more sensitive to smells than men.