Air is breathed in through the nose to the nasal cavity, where a layer of nasal mucosa acts as a filter and traps pollutants and other harmful substances found in the air. Next, air moves into the pharynx, a passage that contains the intersection between the oesophagus and the larynx. The opening of the larynx has a special flap of cartilage, the epiglottis, that opens to allow air to pass through but closes to prevent food from moving into the airway.

The respiratory system includes the nose, mouth, throat, voice box, windpipe, and lungs.Air enters the respiratory system through the nose or the mouth. If it goes in the nostrils (also called nares), the air is warmed and humidified. Tiny hairs called cilia (pronounced: SIL-ee-uh) protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose through the breathed air.The two openings of the airway (the nasal cavity and the mouth) meet at the pharynx (pronounced: FAR-inks), or throat, at the back of the nose and mouth. The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air.At the bottom of the pharynx, this pathway divides in two, one for food — the esophagus (pronounced: ih-SAH-fuh-gus), which leads to the stomach — and the other for air. The epiglottis (pronounced: eh-pih-GLAH-tus), a small flap of tissue, covers the air-only passage when we swallow, keeping food and liquid from going into the lungs.The larynx, or voice box, is the top part of the air-only pipe. This short tube contains a pair of vocal cords, which vibrate to make sounds.

Learning goal 1: When air is breathed in through the nose, it passes to the lungs through the:larynx → bronchioles → pharynx → tracheapharynx → larynx → trachea → bronchiolesbronchioles → trachea → larynx → pharynxpharynx → oesophagus → trachea → bronchioles

The space at the back of the mouth, that leads either to the airway or the esophagus is the:*1 pointpharynxlarynxnasal cavityconchae

The human gas-exchanging organ, the lung, is located in the thorax, where its delicate tissues are protected by the bony and muscular thoracic cage. The lung provides the tissues of the human body with a continuous flow of oxygen and clears the blood of the gaseous waste product, carbon dioxide. Atmospheric air is pumped in and out regularly through a system of pipes, called conducting airways, which join the gas-exchange region with the outside of the body. The airways can be divided into upper and lower airway systems. The transition between the two systems is located where the pathways of the respiratory and digestive systems cross, just at the top of the larynx.T

The upper airway system comprises the nose and the paranasal cavities (or sinuses), the pharynx (or throat), and partly also the oral cavity, since it may be used for breathing. The lower airway system consists of the larynx, the trachea, the stem bronchi, and all the airways ramifying intensively within the lungs, such as the intrapulmonary bronchi, the bronchioles, and the alveolar ducts. For respiration, the collaboration of other organ systems is clearly essential. The diaphragm, as the main respiratory muscle, and the intercostal muscles of the chest wall play an essential role by generating, under the control of the central nervous system, the pumping action on the lung. The muscles expand and contract the internal space of the thorax, the bony framework of which is formed by the ribs and the thoracic vertebrae. The contribution of the lung and chest wall (ribs and muscles) to respiration is described below in The mechanics of breathing. The blood, as a carrier for the gases, and the circulatory system (i.e., the heart and the blood vessels) are mandatory elements of a working respiratory system (see blood; cardiovascular system).