The process of exchange of O2 (from the atmosphere) with CO2 (produced by cells) is called breathing. This process is commonly known as respiration.
Mechanisms of breathing vary among different animal groups. It usually depends on the habitat and level of organization.
The human respiratory system is composed of following organs:
Pharynx: There is a pair of external nostrils which open out above the upper lips. The nostrils lead to a nasal chamber through the nasal passage. The nasal chamber opens into nasopharynx. Nasopharynx is a part of pharynx. Pharynx is the common passage for food and air.
Larynx: Nasopharynx opens through glottis of the larynx into the trachea. Larynx is a cartilaginous box which helps in sound production. Due to this, larynx is also called the sound box. There is a thin elastic cartilaginous flap; called epiglottis. The epiglottis covers the glottis during swallowing. This prevents the entry of food into the larynx.
Trachea: Trachea is a straight tube which extends up to the mid-thoracic cavity. The trachea divides at the level of 5th thoracic vertebra into right and left primary bronchi.
Bronchi: Each bronchus undergoes repeated divisions to form secondary and tertiary bronchi and bronchioles. They finally end up in very thin terminal bronchioles. The tracheae, bronchi and the initial bronchioles are supported by incomplete cartilaginous rings. Each terminal bronchiole gives rise to a number of very thin alveoli. An alveolus is an irregular-walled and vascularised bag-like structure.
Lungs: The lungs are composed of the branching network of bronchi, bronchioles and alveoli. Each lung is covered by a double-layered pleura. The pleura is filled with pleural fluid. The pleural fluid reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining. The inner pleural membrane is in contact with the lung surface.
The conducting part is constituted by the external nostrils, pharynx, larynx, bronchi and the terminal bronchioles. The conducting part transports the atmospheric air to the alveoli. This part also clears the air from foreign particles, humidifies and brings the air to body temperature.
The exchange part of the respiratory system is composed of the alveoli and their ducts. Actual diffusion of O2 and CO2 (between blood and atmospheric air) takes place in the exchange part of the respiratory system.
Thoracic Chamber: The lungs are situated in the thoracic chamber. This is anatomically an air-tight chamber. The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm. Any change in the volume of the thoracic cavity would be reflected in the lung cavity. This is possible because of the typical anatomical setup of lungs in thorax. Such an arrangement is essential for breathing, because we cannot directly alter the pulmonary volume.
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