Utente:Mikialba/Sandbox2/Polmone

Polmone dei mammiferi

I polmoni dei mammiferi hanno un rivestimento spugnoso e sono alveolati con epitelio, aventi una larga superficie. I polmoni umani fanno parte di questo tipo di polmoni. L'ambiente del polmone è molto umido e quindi facilmente attaccabile da batteri. Molte malattie respiratorie sono proprio dovute ad un'infezione virale o batterica. La respirazione è largamente guidata dal diaframma il quale si trova in fondo al torace. La contrazione del diaframma espande verticalmente la cavità dove il polmone è semichiuso. Il rilassamento del muscolo ha l'effetto opposto.

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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.

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.

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.

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".

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.