www.3DScience.com Zygote Media Group. Respiratory System Andrew McCaskill ICS
In humans 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 lungs, molecules of oxygen and carbon dioxide are passively exchanged, by diffusion , between the gaseous environment and the blood. 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 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: 1. Nostrils 2. Nasal cavity 3. Pharynx (naso-, oro-, laryngo-) 4. Larynx (voice box) 5. Trachea (wind pipe) 6. Thoracic cavity (chest) 7. Bronchi (right and left) 8. Alveoli (site of gas exchange)
Upper respiratory tract/conducting zone 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 Progression
A nostril (or naris , pl. nares ) is one of the two channels of the nose. In humans the nasal cycle is the normal ultradian cycle of each nostril's blood vessels becoming engorged in swelling, then shrinking. During the course of a day they will switch over approximately every four hours or so. Meaning that only one nostril is used at any one time. The nasal cycle is the rhythmic, alternating side-to-side fluctuation in nasal airflow. It is known to be regulated by the autonomic nervous system probably from the centers located in brainstem.
The nasal cavity (or nasal fossa ) is a large air-filled space above and behind the nose in the middle of the face. The nasal cavity is important in warming or cooling and cleaning the air as it is inhaled. The nasal cavity is enclosed by the nasal bone above and by the maxilla and ethmoid bone on the sides. The palate separates the nasal cavity from the mouth.
The nasal cavity is divided in two by a vertical fin called the nasal septum . Cilia and mucus along the inside wall of the nasal cavity trap and remove dust and germs from the air as it flows through the nasal cavity. The cilia move the mucus down the nasal cavity to the pharynx, where it can be swallowed.
The pharynx (plural: pharynges ) is part of the digestive system and respiratory system in humans. Because both food and air pass through the pharynx, special care is necessary to prevent choking or aspiration when food or liquid is swallowed. In humans the pharynx is important in vocalization.
The larynx (plural larynges ), colloquially known as the voicebox , is an organ in the neck of humans involved in protection of the trachea and sound production. The larynx houses the vocal cords, and is situated just below where the tract of the pharynx splits into the trachea and the esophagus. Sound is generated in the larynx, and that is where pitch and volume are manipulated. The strength of expiration from the lungs also contributes to loudness, and is necessary for the vocal cords to produce speech.
During swallowing, the larynx (at the epiglottis) closes to prevent swallowed material from entering the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong cough reflex to protect the lungs
The trachea , or windpipe , is a tube extending from the larynx to the bronchi in humans, carrying air to the lungs. It is lined with ciliated cells which push particles out, and cartilage rings which reinforce the trachea and prevent it from collapsing on itself during breathing. These numerous cartilaginous half-rings, located one above the other along the trachea, have open ends adjacent to the esophagus. The rings are connected by muscular and fibrous tissue, and they are lined inside with a ciliated mucous membrane. Drawn by Theresa Knott
Lower respiratory tract/respiratory zone 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.
A bronchus (plural bronchi) is a caliber of airway in the respiratory tract that conducts air into the lungs . No gas exchange takes place in this part of the lungs. A bronchopulmonary segment is a division of a lung that is separated from the rest of the lung by a connective tissue septum. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. Smooth muscle and hyaline cartilage is present continuously around the bronchi.
An alveolus (plural: alveoli , from Latin alveus , "little cavity"), is an anatomical structure that has the form of a hollow cavity. In the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood . The lungs contain about 700 million alveoli , representing a total surface area of 70-90 square meters, each wrapped in a fine mesh of capillaries. The alveoli consist of an epithelial layer surrounded by capillaries . In some alveolar walls there are pores between alveoli.
Gas Exchange The major function of the respiratory system is gas exchange . As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis. If proper ventilation is not maintained two opposing conditions could occur: 1) respiratory acidosis, a life threatening condition, and 2) respiratory alkalosis. http://www.bbc.co.uk/schools/gcsebitesize/teachers/biology/activities.shtml
Inhalation Inhalation is initiated by the diaphragm and supported by the external intercostal muscles. Normal resting respirations are 10 to 18 breaths per minute. Its time period is 2 seconds. During vigorous inhalation (at rates exceeding 35 breaths per minute), or in approaching respiratory failure, accessory muscles of respiration are recruited for support. These consist of sternocleidomastoid and platysma muscles of the neck. Inhalation is driven primarily by the diaphragm. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic volume, which in turn causes a decrease in intrathoracic pressure. As the pressure in the chest falls, air moves into the conducting zone. Drawn by Theresa Knott
Exhalation Exhalation is generally a passive process. The lungs have a natural elasticity; as they recoil from the stretch of inhalation, air flows back out until the pressures in the chest and the atmosphere reach equilibrium. During forced exhalation , as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure, which forces air out of the lungs.
Review: Upon inhalation, gas exchange occurs at the alveoli, the tiny sacs which are the basic functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2 micrometres), and are permeable to gases. The alveoli are lined with pulmonary capillaries, the walls of which are also thin enough to permit gas exchange. All gases diffuse from the alveolar air to the blood in the pulmonary capillaries, as carbon dioxide diffuses in the opposite direction, from capillary blood to alveolar air. At this point, the pulmonary blood is oxygen-rich, and the lungs are holding carbon dioxide. Exhalation follows, thereby ridding the body of the carbon dioxide and completing the cycle of respiration. In an average resting adult, the lungs take up about 250ml of oxygen every minute while excreting about 200ml of carbon dioxide. During an average breath, an adult will exchange from 500 ml to 700 ml of air. This average breath capacity is called tidal volume.
Disorders of the respiratory system can be classified into four general areas: Obstructive Conditions (e.g., Emphysema, Bronchitis, asthma attacks) Restrictive Conditions (e.g., Fibrosis or other genetic conditions) Vascular Diseases (e.g., Pulmonary Hypertension) Infectious, Environmental and Other "Disease"s (e.g., Pneumonia, Tuberculosis, Asbestosis, Particulate Pollutants) Disorders of the respiratory system are usually treated internally by a Pulmonologist . 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.
Acute respiratory distress syndrome (ARDS) is a severe inflammatory disease of the lung. Usually triggered by other pulmonary pathology, the uncontrolled inflammation leads to impaired gas exchange, alveolar flooding and/or collapse, and systemic inflammatory response syndrome. In asthma , the bronchioles, or the "bottle-necks" into the sac are restricted causing the amount of air flow into the lungs to be greatly reduced. Emphysema is another disease of the lungs, whereby the elastin in the walls of the alveoli is broken down (elevated by cigarette smoke) The resulting loss of elasticity in the lungs leads to prolonged times for exhalation, which occurs through passive recoil of the expanded lung. This leads to a smaller volume of gas exchanged per breath. Chronic bronchitis occurs when an abundance mucus is produced by the lungs . The production of this substance occurs naturally when the lung tissue is exposed to irritants. In chronic bronchitis, the air passages into the alveoli, the broncholiotes, become clogged with mucus. This causes increased coughing in order to remove the mucus, and is often a result of extended periods of exposure to cigarette smoke. Cystic fibrosis is a genetic condition caused by the dysfunction of a transmembrane protein responsible for the transport of chloride ions. This causes huge amounts of mucus to clog the bronchiolites , similar to chronic bronchitis. The result is a persistent cough and reduced lung capacity. Lung cancer is a common form of cancer causing the uncontrolled growth of cells in the lung tissue . It is often difficult to prevent once started, due to the sensitivity of lung tissues. Pneumonia is an infection of the alveoli, which can be caused by both viruses and bacteria . Toxins and fluids are released from the virus causing the effective surface area of the lungs to be greatly reduced . If this happens to such a degree that the patient cannot draw enough oxygen from his environment, then the victim may need supplemental oxygen.