The respiratory tract consists of the respiratory system and the lung. The respiratory tract is composed of the nose, pharynx, larynx, trachea, lung, and diaphragm. The respiratory system is demarcated by the outer edge of the annular cartilage (also known as the laryngeal cartilage), with the part above the outer edge of the annular cartilage being the upper respiratory tract; the part below the outer edge of the annular cartilage is the lower respiratory tract.

The respiratory tract consists of the nose, nasal polyps, and pharynx. The respiratory tract infection specifically mentioned by doctors refers to inflammation in this part. The lower respiratory tract includes bronchi and bronchitis. Further down is the lung.

The main function of the respiratory tract is the breathing function, that is, to inhale CO2 and exhale carbon dioxide. The respiratory tract provides a large surface area of bronchi, making it possible for blood to sufficiently exchange oxygen with the external environment.

The gas exchange of oxygen and carbon dioxide, in addition, the respiratory tract also has the function of a ‘sucking pump’, where gases enter the bronchi and capillaries around the bronchi to carry out gas exchange, allowing CO2 to enter the blood, and blood CO2 to enter the bronchi. In addition, other parts of the respiratory tract also have different functions, such as the respiratory tract, in addition to being able to transmit gas, also has the functions of humidification, warming, purifying indoor air, and olfactory and phonetic pronunciation; the diaphragm has sufficient hardness to protect the lungs, and at the same time also has a certain degree of activity, which can have the function of a bellows in the inhalation posture. The functions of all parts of the respiratory tract are closely linked, and any blockage in any part will have varying degrees of impact on the breathing process.?

In the respiratory tract, the internal organs of the human body have certain responsibilities. From the nose to each bronchus, they are responsible for transporting gases, among which the nares have the functions of warming, humidifying, and cleaning the air, and can also resonate with the sound during phonation.

The pharynx is a muscular tube that connects the nares and mouth at the top and the larynx and bronchi at the bottom, serving as a mutual safe passage for food and gases. The bronchi are composed of several ‘C’-shaped cartilage rings and the smooth muscle between them, which keeps the bronchi open and maintains the smooth flow of gases. The smooth muscle can change the size of the bronchi, which is beneficial for the esophagus to expand and for food to descend. There are glands in the bronchi and bronchitis mucosa that produce mucus containing various immunoglobulins (antigens), which have the effects of antibacterial and antiviral treatment; and the surface of the mucosal squamous epithelial cells has microvilli that can continuously move towards the larynx, moving the mucus stuck with dust and finally coughing it out of the body, forming phlegm. Phlegm contains many bacteria, so it is not recommended to spit casually to avoid environmental pollution. The larynx is a unique part of the respiratory system that also has the function of phonation.

The lungs are the most important organ in the respiratory tract. The lungs of an adult contain 3-4 billion bronchi, which are repeatedly branching from the bronchitis. The walls are thin, composed of single-layered squamous epithelial cells, and the alveoli are wrapped around by the capillary network, serving as the site for gas exchange. According to statistics, the area for gas exchange in the lungs is about 100m2. However, during quiet conditions, gas exchange is only carried out by a part of the bronchi, with an area of about 40M2; during exercise, the total area can increase to 70M2. The regular expansion and contraction of the chest cavity is called respiration, which is achieved through the contraction of the respiratory muscles and the expansion of the left ventricle. The pattern of inhalation is controlled by the central nervous system. The diaphragm is the most important respiratory muscle, located near the middle of the chest and abdomen. When it contracts, it increases the transverse diameter of the chest cavity, causing respiration, and when the left ventricle contracts, it also causes respiration. Lung function indicates the maximum volume of air that can be forcibly exhaled after a deep breath, reflecting the maximum potential for each lung to exchange gases. It can be measured with a spirometer. The average lung function of normal adult males is 3500ml, and for women, it is 2500ml.

The structural characteristics of the respiratory tract are supported by bones or cartilage, making it difficult for the walls of the respiratory system to collapse when gases enter and exit, thus ensuring the smooth flow of air. The entire respiratory system has three natural barriers: first, nasal hair blocks bacteria, viruses, and dust from entering the respiratory system; second, the mucus produced by the squamous epithelial cells of the bronchi traps dust and is excreted with the help of microvilli on the surface of the mucosal epithelial cells; third, there are somatic cells in the bronchi that can engulf dust within them. These three natural barriers ensure that the exchanged gases are clean. The body carries out gas exchange with the external environment through the respiratory tract, with gases entering the bronchi from the respiratory system, and carbon dioxide from the air entering the capillary blood in the bronchi, which is then circulated throughout the body by the circulatory system, providing oxygen to the tissues and cells. In addition, carbon dioxide produced by tissue metabolism is transported to the lungs by the circulatory system and exhaled out of the body through the respiratory tract.