The respiratory system consists of the airways and lungs.
The upper airways include: the nasal cavity, nasopharynx, and the lower ones – the larynx, trachea and bronchi.
Air enters the nasal cavity through the nostrils. The epithelium of the nasal cavity secretes mucus, which sticks together dust particles and destroys microorganisms. The mucous membrane of the nasal cavity is lined with ciliated epithelium. Its cilia remove dust particles along with mucus. The mucous membrane is well supplied with blood, which contributes to warming and moisturizing the air. The olfactory receptors are also located in the nasal cavity.
From the nasal cavity, purified, warmed and humidified air enters the nasopharynx, and then into the larynx.
The larynx is formed by cartilages, the largest of which is the thyroid. An important role is played by the epiglottis – a cartilaginous plate located above the entrance to the larynx. The epiglottis closes the entrance to the larynx when swallowing and prevents food from entering the airways.
The vocal cords are located in the larynx cavity. There is a glottis between them. Sound occurs when air passes through a closed glottis. At the same time, the edges of the ligaments vibrate, and sound vibrations occur. In women and children, the vocal cords are short and thin, so their voices are high. In men, the ligaments are always longer, and the male voice is lower. Only sound arises in the larynx. The formation of articulate speech occurs with the participation of the tongue, lips, teeth, cheeks.
The trachea extends from the lower edge of the larynx. It is a tube formed by 16–20 cartilaginous half rings, lined with ciliated epithelium inside. The soft part (without cartilage) of the trachea is adjacent to the esophagus.
The trachea is divided into two bronchi.
The bronchi enter the lungs and form ever smaller branches, at the ends of which there are alveoli. The walls of the alveoli consist of a single layer of epithelial tissue and are braided by a dense network of blood capillaries. This structure of the alveoli provides gas exchange between the air in the lungs and blood.
The lungs are paired organs. They are located in the chest cavity and fit snugly against its walls.