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Old Friday, July 06, 2007
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Default Topic # 8

The Lung




The lung is the essential respiration organ in air-breathing vertebrates, the most primitive being the lungfish. Its principal function is to transport oxygen from the atmosphere into the bloodstream, and to excrete carbon dioxide from the bloodstream into the atmosphere. This exchange of gases is accomplished in the mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs called alveoli. The lungs also have non respiratory functions.



Respiratory function

Energy production from aerobic respiration requires oxygen and produces carbon dioxide as a by-product, creating a need for an efficient means of oxygen delivery to cells and excretion of carbon dioxide from cells. In small organisms, such as single-celled bacteria, this process of gas exchange can take place entirely by simple diffusion. In larger organisms, this is not possible; only a small proportion of cells are close enough to the surface for oxygen from the atmosphere to enter them through diffusion. Two major adaptations made it possible for organisms to attain great multicellularity: an efficient circulatory system that conveyed gases to and from the deepest tissues in the body, and a large, internalized respiratory system that centralized the task of obtaining oxygen from the atmosphere and bringing it into the body, whence it could rapidly be distributed to all the circulatory system.

In air-breathing vertebrates, respiration occurs in a series of steps. Air is brought into the animal via the airways — in reptiles, birds and mammals this often consists of the nose; the pharynx; the larynx; the trachea (also called the windpipe); the bronchi and bronchioles; and the terminal branches of the respiratory tree. The lungs of mammals are a rich lattice of alveoli, which provide an enormous surface area for gas exchange. A network of fine capillaries allows transport of blood over the surface of alveoli. Oxygen from the air inside the alveoli diffuses into the bloodstream, and carbon dioxide diffuses from the blood to the alveoli, both across thin alveolar membranes.

The drawing and expulsion of air is driven by muscular action; in early tetrapods, air was driven into the lungs by the pharyngeal muscles, whereas in reptiles, birds and mammals a more complicated musculoskeletal system is used. In the mammal, a large muscle, the diaphragm (in addition to the internal intercostal muscles), drive ventilation by periodically altering the intra-thoracic volume and pressure; by increasing volume and thus decreasing pressure, air flows into the airways down a pressure gradient, and by reducing volume and increasing pressure, the reverse occurs. During normal breathing, expiration is passive and no muscles are contracted (the diaphragm relaxes).

Another name for this inspiration and expulsion of air is ventilation. Vital capacity is the maximum volume of air that a person can exhale after maximum inhalation. A person's vital capacity can be measured by a spirometer (spirometry). In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease.



Non respiratory functions

In addition to respiratory functions such as gas exchange and regulation of hydrogen ion concentration, the lungs also:
  • influence the concentration of biologically active substances and drugs used in medicine in arterial blood
  • filter out small blood clots formed in veins
  • serve as a physical layer of soft, shock-absorbent protection for the heart, which the lungs flank and nearly enclose.
  • filter out gas micro-bubbles occurring in the venous blood stream during SCUBA diving decompression.




Anatomy

In humans, it is the two main bronchi (produced by the bifurcation of the trachea) that enter the roots of the lungs. The bronchi continue to divide within the lung, and after multiple divisions, give rise to bronchioles. The bronchial tree continues branching until it reaches the level of terminal bronchioles, which lead to alveolar sacks. Alveolar sacs are made up of clusters of alveoli, like individual grapes within a bunch. The individual alveoli are tightly wrapped in blood vessels, and it is here that gas exchange actually occurs. Deoxygenated blood from the heart is pumped through the pulmonary artery to the lungs, where oxygen diffuses into blood and is exchanged for carbon dioxide in the hemoglobin of the erythrocytes. The oxygen-rich blood returns to the heart via the pulmonary veins to be pumped back into systemic circulation.


Bronchi, bronchial tree, and lungs (Cardiac notch labeled at bottom left).Human lungs are located in two cavities on either side of the heart. Though similar in appearance, the two are not identical. Both are separated into lobes, with three lobes on the right and two on the left. The lobes are further divided into lobules, hexagonal divisions of the lungs that are the smallest subdivision visible to the naked eye. The connective tissue that divides lobules is often blackened in smokers and city dwellers. The medial border of the right lung is nearly vertical, while the left lung contains a cardiac notch. The cardiac notch is a concave impression molded to accommodate the shape of the heart. Lungs are to a certain extent 'overbuilt' and have a tremendous reserve volume as compared to the oxygen exchange requirements when at rest. This is the reason that individuals can smoke for years without having a noticeable decrease in lung function while still or moving slowly; in situations like these only a small portion of the lungs are actually perfused with blood for gas exchange. As oxygen requirements increase due to exercise, a greater volume of the lungs is perfused, allowing the body to match its CO2/O2 exchange requirements.

The environment of the lung is very moist, which makes it hospitable for bacteria. Many respiratory illnesses are the result of bacterial or viral infection of the lungs.
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Last edited by Sureshlasi; Saturday, September 01, 2007 at 12:36 AM.
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