Human respiratory system and respiratory organs

The process of respiration includes: (1) moving air in and out of the lungs, (2) exchanging gases between air and blood, (3) exchanging gases between blood and body cells, and (4) the use of oxygen and production of carbon diox­ide by the cells.
Respiratory Organs 
The respiratory organs are the nose (nasal cavi­ties), pharynx, larynx, trachea, bronchi, and lungs. This list­ing also reflects the sequence of organs through which air passes into the body.
Nose 
The nose contains two nostrils, openings that enter the nasal cavities. The nasal cavities are narrow canals separated from one another by a median septum. The surface area of the cavity walls is increased by bone projections called con­chae (kong'ke) (sing., concha [kong'kah]). The conchae are covered by a ciliated mucous membrane that filters, warms, and humidifies the air as it passes through the nose. Olfactory cells are located in the upper medial portion of each nasal cavity, in the lining of the conchae.
The lacrimal (tear) glands in the upper lateral corner of the orbit drain into the nasal cavities by way of tear ducts. This is why crying produces a runny nose.
The paranasal sinuses are air-filled spaces within the skull. Each is named for the bone where found. The paranasal sinuses open into the nasal cavities and are also lined with a mucous membrane. Inflammation of this membrane is called sinusitis. If the passageways to the si­nuses are blocked, a partial vacuum is created, causing a si­nus headache.
The nasal cavities are separated from the oral cavity (mouth) by the hard and soft palates, and they empty into the nasopharynx. When we swallow, the uvula, a posterior extension of the soft palate, moves back and blocks the na­sopharynx so that food does not ordinarily enter the nose. 
Pharynx 
The pharynx (far'inks), commonly called the throat, con­nects the nasal and oral cavities to the larynx. The pharynx has three parts: (1) The nasal cavities open into the na­sopharynx, (2) the oral cavity (mouth) opens into the oropharynx, and (3) the laryngopharynx opens into the lar­ynx.
The auditory (eustachian) tubes lead from the na­sopharynx to the middle ears. The middle ears contain air, and the auditory tubes keep the pressure in the middle ears equal to that in the nasopharynx. The pharyngeal tonsils, or adenoids, are situated in this part of the pharynx. The oropharynx is the middle portion of the pharynx. The palatine tonsils, located in the posterior lateral wallcan be observed if the tongue is depressed. There are also lingual tonsils at the base of the tongue. The tonsils are part of the immune system and help protect us from disease. Sometimes, they become diseased and are surgically removed.
The laryngopharynx is a passageway for both air and food. Air that enters from either the nose or oral cavity is sent on to the larynx. Food enters the esophagus, a tube that leads to the stomach.
The nasal cavities, vvhich filter, vvarm, and humidify incoming air, open into the pharynx. Food and air passages join in the pharynx, vvhich conducts air to the larynx and food to the esophagus.
Larynx 
The larynx (lar'inks), or voice box, can be envisioned as a triangular box whose apex, the thyroid cartilage (Adam's ap­ple), is located at the front of the neck. At the top of the larynx is a variable-sized opening called the glottis. When food is being swallowed, a flap of tissue called the epiglottis covers the glottis so that no food passes into the larynx. If by chance, food or some other substance does gain entrance to the larynx, reflex coughing usually occurs, expelling the substance.
The larynx is called the voice box because the vocal cords are inside the larynx. The vocal cords are mucous membrane folds supported by elastic ligaments stretched across the glottis. When air passes through the glottis, the vocal cords vibrate, producing sound. At the time of puberty, the growth of the larynx and the vocal cords is much more rapid and accentuated in the male than in the female, causing the male to have a more promi­nent Adam's apple and a deeper voice. The voice "breaks" in the young male due to his inability to control the longer vocal cords. These changes cause the male voice to have a lower pitch.
The high or low pitch of the voice is regulated when speaking and singing by changing the tension on the vocal cords. The greater the tension, as when the glottis becomes more narrow, the higher the pitch. When the glottis is wider, the pitch is lower. The loudness, or in­tensity, of the voice depends upon the amplitude of the vi­brations (that is, the degree to which vocal cords vibrate).
The vocal cords are stretched across the glottis inside the larynx, often called the voice box. When swallowing, the glottis is covered by the epiglottis.
Lung cancer usually begins in a bronchus. Callusing and a loss of cilia due to irritants lead to tumor formation.
Asthma occurs when airflow is obstructed due to wide­spread narrowing of the airways. Severe episodes are asso­ciated with bronchospasms caused by contractions of the smooth muscle in the walls of the bronchioles. A typical asthma attack begins with breathlessness and difficulty in forcing air out of the lungs. Usually, there is wheezing, but with severe attacks, wheezing may not be audible. An at­tack, which may last several hours, is followed by pro­longed coughing with mucus production.
Lungs 
Within the lungs, the alveoli are surrounded by a network of capillaries. Both the alveolar wall and the capillary wall consist of a layer of simple squamous epithe­lium and a basement membrane. These two walls together are called the respiratory membrane because this is where gas exchange occurs.
The alveolar wall contains surfactant-secreting cells.
Surfactant lowers surface tension and prevents alveoli from collapsing. Some newborn babies do not produce
each lung is broad and concave to fit upon the convex sur­face of the diaphragm. The other lung surfaces follow the contours of the ribs and of the organs present in the tho­racic cavity.
The left lung, which is somewhat smaller than the right, has a superior lobe and an inferior lobe. The right lung has a superior lobe, a middle lobe, and an inferior lobe. Each lobe contains a specific number ofbronchopul­monary segments, each of which receives a tertiary bronchus.
Air moves from the nasal cavities to the pharynx, to the trachea. to the bronchi. to the bronchioles, and finally, to the approximately 300 million alveoli in the lungs. Gas exchange occurs at the alveoli.