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Respiratory system
The lungs flank the heart and great vessels in the chest cavity. (Source: Gray's Anatomy of the Human Body, 20th ed. 1918.)
Air enters and leaves the lungs via a conduit of cartilaginuous passageways — the bronchi and bronchioles. In this image, lung tissue has been dissected away to reveal the bronchioles
, 20th ed. 1918.)</small>

The lung is the essential respiration organ in air-breathing vertebrates. 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 nonrespiratory functions.

Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius ("of the lungs"), cognate with the Greek pleumon ("lung").


[edit] 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, internalised 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; 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.

[edit] Nonrespiratory 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.

[edit] Mammalian lungs

The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung. The environment of the lung is very moist, which makes it a hospitable environment for bacteria. Many respiratory illnesses are the result of bacterial or viral infection of the lungs.

Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm vertically expands the cavity in which the lung is enclosed. Relaxation of the diaphragm has the opposite effect. The rib cage itself is also able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.

Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. 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 generations of divisions, give rise to bronchioles. Eventually the bronchial tree ends in alveolar sacs, composed of alveoli. Alveoli are essentially tiny sacs in close contact with blood filled capillaries. Here oxygen from the air diffuses into the blood, where it is carried by hemoglobin, and carried via pulmonary veins towards the heart.

Deoxygenated blood from the heart travels via the pulmonary artery to the lungs for oxidation.

[edit] Avian lungs

Many sources state that it takes two complete breathing cycles for air to pass entirely through a bird's respiratory system. This is based on the idea that the bird's lungs store air received from the posterior air sacs in the 'first' exhalation until they can deliver this air to the posterior air sacs in the 'second' inhalation.

This is not possible because bird lungs are essentially sets of fixed volume, open ended tubes. They are like drinking straws. If you blow into one end of a drinking straw then the air comes out the other side. It is not stored, waiting for you to suck it out from the other end some time later. This type of lung construction is called circulatory lungs as distinct from the bellows lung possessed by most other animals (see above).

Avian lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as parabronchi, connected at either ends by the dorsobronchi and ventrobronchi. Air flows through the honeycombed walls of the parabronchi and into air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion, a process of crosscurrent exchange.

This complex system of air sacs ensures that the airflow through the avian lung is always travelling in the same direction - posterior to anterior. This is in contrast to the mammalian system, in which the direction of airflow in the lung is tidal, reversing between inhalation and exhalation. By utilizing a unidirectional flow of air, avian lungs are able to extract a greater concentration of oxygen from inhaled air. Birds are thus equipped to fly at altitudes at which mammals would succumb to hypoxia.

[edit] Reptilian lungs

Reptilian lungs are typically ventilated by a combination of expansion and contraction of the ribs via axial muscles and buccal pumping. Crocodilians also rely on the hepatic piston method, in which the liver is pulled back by a muscle anchored to the pubic bone (part of the pelvis), which in turn pulls the bottom of the lungs backward, expanding them.

[edit] Amphibian lungs

The lungs of most frogs and other amphibians are simple balloon-like structures, with gas exchange limited to the outer surface area of the lung. This is not a very efficient arrangement, but amphibians have low metabolic demands and also frequently supplement their oxygen supply by diffusion across the moist outer skin of their bodies. Unlike mammals, which use a breathing system driven by negative pressure, amphibians employ positive pressure. Note that the majority of salamander species are lungless salamanders and conduct respiration through their skin and the tissues lining their mouth.

[edit] Invertebrate lungs

Some invertebrates have "lungs" that serve a similar respiratory purpose but are not evolutionarily related to vertebrate lungs. Some arachnids have structures called "book lungs" used for atmospheric gas exchange. The Coconut crab uses structures called branchiostegal lungs to breathe air and indeed will drown in water, hence it breathes on land and holds its breath underwater. The Pulmonata are an order of snails and slugs that have developed "lungs".

[edit] Origins

The first lungs, simple sacs that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and into the gas bladders of today's fish. The lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills). The evolutionary origin of both are thought to be outpocketings of the upper intestines. This is reflected by the fact that the lungs of a fetus also develop from an outpocketing of the upper intestines and in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the higher orders. (This is an instance of correlation between ontogeny and phylogeny.) There are no animals which have both lungs and a gas bladder.

[edit] See also

[edit] Further reading

Look up Lung in
Wiktionary, the free dictionary.
Respiratory system - edit
Nose | Nasal cavity | Pharynx | Larynx | Trachea | Lungs | Bronchi | Alveoli | Conducting zone | Respiratory zone
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Cardiovascular system
Blood  |  Heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → Heart → Pulmonary arteries → Lungs → Pulmonary veins → Heart

Lungs and related structures}"> |
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lungs: right, left, lingula, apex, base, root, cardiac notch, cardiac impression, hilum, borders (anterior, posterior, inferior), surfaces (costal, mediastinal, diaphragmatic), fissures (oblique, horizontal)

airway: trachea, carina, bronchi, main bronchus (right, left), lobar/secondary bronchi (eparterial bronchus), segmental/tertiary bronchi (bronchopulmonary segment), bronchiole, alveolar duct, alveolus

pleurae: parietal pleura (cervical, costal, mediastinal, diaphragmatic), pulmonary pleura, pulmonary ligament, recesses (costomediastinal, costodiaphragmatic)


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