The respiratory system can be conveniently subdivided into a conducting zone and a respiratory zone.
The conducting zone comprises:
The nose
The nasopharynx
The larynx, or voicebox
The trachea, an air tube that connects with the bronchi
The right main bronchus and the left main bronchus tubes that carry air to and from the lungs
The bronchioles, branches of the bronchi which distribute air to the alveoli
The terminal bronchioles
The respiratory zone comprises:
The respiratory bronchioles
The alveolar ducts
The alveoli, terminal sacs in which gas exchange occurs
Muscles used for inspiration include:
The diaphragm, which mediates intrathoracic pressure to initiate inspiration.
The external intercostal muscles, during vigorous inspiration
Although expiration is generally a passive process, muscles aiding forced expiration include:
The abdominal muscles
The internal intercostal muscles
Other:
The right and left bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveoli form the right and left lungs respectively.
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 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.
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