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Among quadrupeds, the respiratory system generally includes tubes, such as the bronchi, used to carry air to the lungs, where gas exchange takes place. A diaphragm pulls air in and pushes it out. Respiratory systems of various types are found in a wide variety of organisms. Even trees have respiratory systems.
In humans and other mammals, the respiratory system consists of the airways, the lungs, and the respiratory muscles that mediate the movement of air into and out of the body. Within the alveolar system of the lungs, molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous environment and the blood. Thus, the respiratory system facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation. The system also helps to maintain the acid-base balance of the body through the efficient removal of carbon dioxide from the blood.

In humans and other animals, the respiratory system can be conveniently subdivided into an upper respiratory tract (or conducting zone) and lower respiratory tract (respiratory zone), trachea and lungs.
Air moves through the body in the following order:

The conducting zone starts with the nares (nostrils) of the nose, which open into the nasopharynx (nasal cavity). The primary functions of the nasal passages are to: 1) filter, 2) warm, 3) moisten, and 4) provide resonance in speech. The nasopharnyx opens into the oropharynx (behind the oral cavity). The oropharynx leads to the laryngopharynx, and empties into the larynx (voicebox), which contains the vocal cords, passing through the glottis, connecting to the trachea (wind pipe).

The trachea leads down to the thoracic cavity (chest) where it divides into the right and left "main stem" bronchi. The subdivision of the bronchus are: primary, secondary, and tertiary divisions (first, second and third levels). In all, they divide 16 more times into even smaller bronchioles.
The bronchioles lead to the respiratory zone of the lungs which consists of respiratory bronchioles, alveolar ducts and the alveoli, the multi-lobulated sacs in which most of the gas exchange occurs.

Ventilation of the lungs is carried out by the muscles of respiration.

Ventilation occurs under the control of the autonomic nervous system from the part of the brain stem, the medulla oblongata and the pons. This area of the brain forms the respiration regulatory center, a series of interconnected neurons within the lower and middle brain stem which coordinate respiratory movements. The sections are the pneumotaxic center, the apneustic center, and the dorsal and ventral respiratory groups. This section is especially sensitive during infancy, and the neurons can be destroyed if the infant is dropped or shaken violently. The result can be death due to "shaken baby syndrome." Most plants have relatively few living cells outside of their surface because air (which is required for metabolic content) can penetrate only skin deep. However, most plants are not involved in highly aerobic activities, and thus have no need of these living cells.




right • left • lingula • apex • base • root • cardiac notch • cardiac impression • hilum • borders (anterior, posterior, inferior) • surfaces (costal, mediastinal, diaphragmatic) • fissures (oblique, horizontal)
trachea • carina • bronchi • main bronchus (right, left) • lobar/secondary bronchi (eparterial bronchus) • segmental/tertiary bronchi (bronchopulmonary segment) • bronchiole • terminal bronchiole
respiratory bronchiole • alveolar duct • alveolus • alveolar-capillary barrier
parietal pleura (cervical, costal, mediastinal, diaphragmatic) • pulmonary pleura • pulmonary ligament • recesses (costomediastinal, costodiaphragmatic)
Nostrils
Nasal cavity
Pharynx (naso-, oro-, laryngo-)
Larynx (voice box)
Trachea (wind pipe)
Thoracic cavity (chest)
Bronchi (right and left)
Alveoli (site of gas exchange)
Obstructive conditions (e.g., emphysema, bronchitis, asthma attacks
Restrictive conditions (e.g., fibrosis, sarcoidosis, alveolar damage, pleural effusion)
Vascular diseases (e.g., pulmonary edema, pulmonary embolism, pulmonary hypertension)
Infectious, environmental and other "diseases" (e.g., pneumonia, tuberculosis, asbestosis, particulate pollutants) coughing is of major importance, as it is the body's main method to remove dust, mucus, saliva, and other debris from the lungs. Inability to cough can lead to infection. Deep breathing exercises may help keep finer structures of the lungs clear from particulate matter, etc.
Liquid breathing
Aquatic respiration
Involuntary control of respiration
Gill
Respiratory tract
Major systems of the human body
Muscles of respiration
Perkins, M. 2003. Respiration Power Point Presentation. Biology 182 Course Handout. Orange Coast College, Costa Mesa, CA.
Medical Dictionary
Science aid: Respiratory System A simple guide for high school students
Introduction to Respiratory System
A high school level description of the respiratory system
The Respiratory System Univerity level