(1) nasal cavity
The nasal cavity is the starting point of the respiratory tract and is divided into left and right parts by the nasal septum. In the nasal cavity, the alar bone forms a paper-like maze with mucous membrane attached to its inner wall. There are abundant blood vessels in the nasal mucosa and mucus on the surface, which has the functions of moistening, warming, inhaling air and adsorbing dust in the air. There are olfactory cells in the mucosa behind and above the nasal cavity. This part of nasal mucosa (olfactory mucosa) is very developed, its area is about four times that of human beings, and there are more than 200 million olfactory cells. There are many dense olfactory hairs on the surface of olfactory cells, which greatly increases the contact area with air. The reason why dogs have a keen sense of smell is inseparable from the structural and functional characteristics of their olfactory mucosa.
(2) larynx
The throat is an important organ to ensure the passage of gas, and it is also a vocal organ. Located on the ventral side of the junction of head and neck, the front end communicates with pharynx, and the back end is connected with trachea, which is composed of laryngeal muscle, thyroid cartilage and laryngeal mucosa. Thyroid cartilage becomes the laryngeal cavity through the connection of joints and ligaments. There are a pair of mucosal folds on the lateral wall of the middle part of the laryngeal cavity, which are called vocal cords. Vocal cords are vocal organs, and the barking of dogs is caused by the vibration of vocal cords.
(3) trachea and bronchus
Trachea is a tube behind the larynx, which is a slender tube connected by 40 ~ 45 "C"-shaped cartilage rings. There are many dense cilia on the mucous membrane on the inner surface of the trachea, which regularly swing to the throat to discharge dust particles in the air. The trachea is located on the ventral side of the cervical vertebra, and it is divided into two bronchi after entering the chest, which enter the ipsilateral lung respectively.
(4) Lung
The lung is an important organ for gas exchange in the respiratory system, and it is the place where oxygen inhaled in the air and carbon dioxide in the blood exchange. The lung is located in the chest cavity and divided into two lobes. The lungs are spongy and there are many blood vessels in them, so the lungs of healthy dogs are generally pink, light, soft and elastic. There is a plasma membrane (lung pleura) on the surface of the lung, and its connective tissue enters the lung and divides the lung into many lobules. After the bronchus enters the lung from the hilum, it branches several times to form bronchial tree, bronchioles, terminal bronchioles, respiratory bronchioles and alveoli. Alveola is the place where the lungs exchange gas. The dog's breathing is controlled by the central nervous system and coordinated by diaphragm, intercostal muscle and respiratory muscle. When inhaling, the chest cavity expands, which increases the lung volume and reduces the pressure in the lung. So as to inhale a large amount of air containing high concentration of oxygen, in which oxygen enters blood vessels through alveolar walls and reaches the whole body for metabolism through blood circulation. When exhaling, the chest cavity contracts, the lung volume is compressed, and the carbon dioxide and oxygen in the blood vessels are replaced, so that the gas in the alveoli spreads out of the alveoli and exhales through the respiratory tract. Dogs mainly rely on the respiratory system to regulate their body temperature during exercise or hot summer.