Respiratory system

The respiratory system is the biological system of any organism that engages in gas exchange. Even trees have respiratory systems, taking in carbon dioxide and emitting oxygen constantly.

Respiratory system in humans and animals

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 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.

Anatomy

The respiratory system can be conveniently subdivided into a conducting zone and a respiratory zone.

The conducting zone starts with the the nares (nostrils) of the nose, which open into the nasopharynx (nasal cavity, which in turn opens into the oropharynx (behind the oral cavity). The oropharynx leads to the larynx (voicebox), which contains the vocal chords, and connects to the trachea (wind pipe) which leads down to the thoracic cavity (chest) where it divides into the right and left "main stem" bronchi, which continue to divide several times into every smaller bronchioles finally reaching the terminal bronchioles.

The terminal bronchiols lead to the respiratory zone of the lungs which consists of respiratory bronchioles which lead toalveolar ducts which finally terminate in 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. Inspiration is initiated by the diaphragm and supported by the external intercostal muscles. During vigorous inspiration (at rates exceeding 35 breaths per minute), or in approaching respiratory failure, accessory muscles of respiration are recruited for support. These consist of sternocleidomastoid, platysma, and the strap muscles of the neck. Expiration is generally a passive process, however active or forced expiration is achieved by the abdominal and the internal intercostal muscles.

Circulation

The pulmonary blood vessels generally accompany the airways and also undergo numerous branchings. The pulmonary circulation has a very low resistance compared to the systemic circulation, and for this reason, all the pressures within the pulmonary blood vessels are low.

Functions

The major function of the respiratory system is gas exchange. Respiration consists of a mechanical cycle of inspiration and expiration, with gaseous exchange occurring in between.

Inspiration 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. Here, the air is filtered, warmed, and humified as it flows to the lungs.

Expiration, on the other hand, is typically a passive process. The lungs have a natural elasticity; as they recoil from the stretch of inspiration, air flows back out until the pressures in the chest and the atmosphere reach equilibrium.

A simple model of how the lungs are inflated can be built from a bell jar.

During forced inspiration, as when taking a deep breath, the external intercostal muscles and accessory muscles further expand the thoracic cavity.

During forced expiration, 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.

Upon inspiration, 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. Oxygen diffuses 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. Expiration follows, thereby ridding the body of the carbon dioxide and completing the cycle of respiration.

Other:

In an average resting adult, the lungs take up about 250ml of oxygen every minute while excreting about 200ml of carbon dioxide.

The movement of gas through the larynx, pharynx and mouth allows us to speak, or phonate.

The respiratory tract is constantly exposed to microbes due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.

Virtually all the body's blood travels through the lungs every minute. The lungs add and remove many chemical messengers from the blood as it flows through pulmonary capillary bed . The fine capillaries also trap blood clots that have formed in systemic veins.

Diseases of the respiratory system

Diseases of the respiratory system can be classified into four general areas:

• Obstructive Diseases (e.g., Emphysema, Bronchitis, Asthma)
• Restrictive Diseases (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)

Respiratory system in plants

Plant respiration is limited by the process of diffusion. Even a baobab tree is mostly dead because air can penetrate only skin deep. However, most plants are not involved in highly metabolic activities like hunting, i.e. they do not need the energy necessary for predators, and thus their breathing is limited.

Tissue engineering

In tissue engineering, respiration is an essential problem. The small depth of diffusion respiration sufficient to support the metabolism of an average human cell is less than a millimetre (0.04 in). Various substances can be used to enhance this depth, essentially having a haemoglobising role.

Sources

• Perkins, M. 2003. Respiration Power Point Presentation. Biology 182 Course Handout. Orange Coast College, Costa Mesa, CA.

See also:

• Liquid breathing
• aquatic respiration
• Involuntary control of respiration

External links

• Introduction to Respiratory System