The respiratory system consists of the nasal cavity, pharynx, larynx, trachea (windpipe), bronchi, and lungs. The upper respiratory tract includes the nasal cavity, pharynx, and associated structures, and the lower one involves the larynx, trachea, bronchi, and lungs. Respiratory movements are realized by the diaphragm and the muscles of the thoracic wall. All cells of the body perform aerobic metabolism for which oxygen is essential. The respiratory system and the cardiovascular system take oxygen from the air and transport it to individual cells. They then transport carbon dioxide from cells and release it from the body into the air. The respiratory system plays an important role in regulating pH of the body fluids. Respiration is the exchange of oxygen and carbon dioxide between the atmosphere and body cells. Air enters the nasal cavity located inside the external nose and joined the pharynx through the Respiratory System

external nares.

A mucous membrane and cilia warm the air and filter out foreign bodies. Then the air passes into the pharynx. Pharynx (throat) is the common opening of both the digestive and respiratory systems. Inferiorly, the pharynx leads to the separate openings of the respiratory system (opening into the larynx) and the digestive system (i.e. the esophagus). The pharynx can be divided into three regions, the nasopharynx, the oropharynx, and the laryngopharynx. The adenoids and the tonsils are located in the pharynx. Then the air reaches the larynx (voice box). The larynx consists of an outer casing of nine cartilages that are connected to each other by muscles and ligaments. Six of the nine cartilages are paired, and three are unpaired. The largest and most superior cartilage is unpaired thyroid cartilage, or Adam's apple. After that the air passes through the trachea and bronchi. The trachea is a membranous tube that consists of dense connective tissue and smooth muscle. The cilia propel mucus and foreign particles toward the larynx where they can enter the esophagus and be swallowed. The trachea connects the larynx to the primary bronchi. They go to each lung.

The lungs are the principal organs of the respiration and the largest organs of the body. Each lung is conical in shape, its base is on the diaphragm and its apex extends superiorly to a point approximately 2.5 cm superior to each clavicle. The right lung is larger than the left and weights an average of 620 g, whereas the left lung weights 560 g. The right lung has three lobes, and the left lung has two. The alveoli located in the lungs allow for the exchange of gases. The blood absorbs the oxygen from the alveoli and gives carbon dioxide away, which is exhaled (breathed out).


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