The respiratory system comprises of the nostrils, the nasal cavity, the nasopharnyx, the bronchi, the bronchioles, the trachea and the alveoli. It can be further divided into two: the upper respiratory tract and the lower respiratory tract. The upper respiratory tract is made up of the nasal passages, pharynx and the larynx. The lower respiratory tract is made up of the trachea, the primary bronchi and lungs. The major function of the respiratory system is the exchange of gases within the body specifically it delivers oxygen to the blood and removes carbon dioxide from the blood.
The gaseous exchange occurs in tiny sacs located within the lungs which are known as alveoli. These sacs are thin walled and are in close proximity to the pulmonary capillaries (circulatory system). The alveoli are about 0.2 micrometers thin. The exchange of gases occurs through the transfer of gases from the epithelial cells of the alveoli to the endothelial cells of the pulmonary capillaries and vice versa. The exchange of gases is facilitated by pressure difference. Gaseous exchange requires frequent inhalation and exhalation for it to be sustained.
Inhalation is a process that occurs when the diaphragm contracts, the rib cage moves downwards hence increasing the volume of the thorax and decreasing the pressure within the thoracic cavity. This results in movement of air from the atmosphere into the lungs through the nostrils. On the other hand, exhalation occurs when internal intercostal muscles and abdominal muscles contract hence generating substantial pressure that results in the expulsion of the air from the lungs.
The respiratory system closely relates with the circulatory system in the sense that gases are exchanged between the two systems. While the respiratory system delivers oxygen to the circulatory system and removes carbon dioxide the circulatory system transports the oxygen to the other organ systems and delivers carbon dioxide from these systems to the respiratory system for expulsion.
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West, & John, B. (2007). Respiratory physiology-- the essentials. Baltimore: Williams & Wilkins.