Encyclopedia
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 gasses is accomplished in the mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs called
alveoli. Lungs also have nonrespiratory functions.
Medical terms related to the lung often begin with
pulmo-, from the
Latin pulmonarius , cognate with the Greek
pleumon .
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 , 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 . Another name for this inspiration and expulsion of air is ventilation.
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.
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.
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 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.
Anatomy
The lungs are located inside the
thoracic cavity, protected by the bony structure of the
rib cage. Each is enclosed by a double-layered sac called pleura. The inner layer of the sac adheres tightly to the lung and the outer layer is attached to the inner wall of the thoracic cavity. The two layers are separated by a thin space called the pleural cavity that is filled with
pleural fluid; this allows the inner and outer layers to slide over each other, and prevents them from being separated easily. The left lung is smaller than the right one, to provide room for the heart.
The lungs are attached to the
heart and trachea through structures that are called the "roots of the lungs." The roots of the lungs are the bronchi, pulmonary vessels, bronchial vessels, lymphatic vessels, and nerves. These structures enter and leave at the hilus of the lung.
The lungs are divided into lobes by the horizontal and oblique fissures. The right lung has three lobes and the left lung has two. A unique feature of the left lung is the cardiac notch, which helps create the lingula of the left lung.
The lungs are connected to the upper airway by the trachea and bronchi. The trachea runs down the neck and divides into left and right bronchi behind the sternal angle . The right main bronchus is shorter, wider and runs more vertically than the left. For this reason, it is more common to aspirate foreign objects into the right lung.
The right bronchus gives rise to the superior lobe bronchus before entering the hilum and dividing into the middle and inferior lobe bronchi. The left bronchus enters the hilum and gives rise to the superior and inferior lobe bronchi.
The bronchi enter the lung and branch out to form the bronchial tree. The bronchi divide into smaller bronchioles, which terminate into alveoli. An alveolus is composed of respiratory tissue and is the site of gas exchange in the lung. The inner walls of the alveoli are covered in surfactant, a fluid which reduces the surface tension of the alveoli, allowing them to expand and recoil with inspiration and expiration and preventing them from collapsing.
The blood supply to the lungs is from two sources: the pulmonary vessels and the bronchial vessels. The bronchial vessels support the nonrespiratory tissue and the pulmonary vessels provide support to the respiratory tissue.
The pulmonary arteries carry deoxygenated blood, which has returned to the heart from the systemic venous system, to the lungs to be reoxygenated. The pulmonary veins carry oxygenated blood back to the heart to go to the systemic arterial system. The right and left pulmonary arteries arise from the pulmonary trunk and carry deoxygenated blood to their respective lungs. The pulmonary veins, two on each side, carry oxygenated blood to the left atrium of the heart.
The bronchial arteries that supply the nonrespiratory tissue of the lung arise from different sources. The left bronchial arteries come off of the thoracic aorta, however, the right bronchial artery has a variable source .
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.
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 , 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.
Arachnid lungs
Spiders have structures called "book lungs", which are not evolutionarily related to vertebrate lungs but serve a similar respiratory purpose.
Crustacean lungs
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.
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 . 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. There are no animals which have both lungs and a gas bladder.
See also
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Further reading
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- Lung Function Fundamentals. http://www.anaesthetist.com/icu/organs/lung/lungfx.htm
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