Lungs Anatomy Biography
(Source google.com)
Alveolar ducts are tiny ducts
that connect the respiratory bronchioles to alveolar sacs, each of which
contains a bunch of alveoli (the balls). They are tiny end ducts of the
branching airways that fill the lungs. Each lung holds approximately 1.5 to 2 million
of them. The tubules divide into two or three alveolar sacs at the distal end.
They are formed from the confluence openings of several alveoli. Distal
terminations of alveolar ducts are atria which then end in alveolar sacs. In
human anatomy, respiratory bronchioles exist proximal to the alveolar ducts.
The epithelial lining consists of smooth muscle knobs covered by nonciliated,
simple cuboidal cells. The smooth muscle constricts under parasympathetic
innervation and relax under sympathetic innervation. The alveolar septum
separates adjacent alveoli in lung tissue. The minimal components of an
alveolar septum consist of the basement membranes of alveolar-lining epithelium
(mostlytype I pneumocytes) and capillary endothelium. Thicker alveolar septa may
also contain elastic fibers, collagen, interstitial cells, smooth muscle cells,
mast cells, lymphocytes andmonocytes. The base of the lung is broad, concave,
and rests upon the convex surface of thediaphragm, which separates the right
lung from the right lobe of the liver, and the left lung from the left lobe of
the liver, the stomach, and the spleen. Since the human diaphragm extends
higher on the right than on the left side, the concavity on the base of the
right lung is deeper than that on the left.
Laterally and behind, the base is
bounded by a thin, sharp margin which projects for some distance into the
costodiaphragmatic recess of the pleura, between the lower ribs and the costal
attachment of the diaphragm. The base of the lung descends during inspiration
and ascends during expiration. The anterior border of the lung is thin and
sharp, and overlaps the front of thepericardium. The anterior border of the
right lung is almost vertical, and projects into thecostodiaphragmatic recess;
that of the left lung presents, below, an angular notch, the cardiac notch, in
which the pericardium is exposed. Opposite this notch the anterior margin of
the left lung is situated some little distance lateral to the line of
reflection of the corresponding part of the pleura. The posterior border of the lung
is broad and rounded, and is received into the deep concavity on either side of
the vertebral column. It is much longer than the anterior border, and projects,
below, into the phrenicocostal sinus. The 'inferior border of the lung
is thin and sharp where it separates the base from thecostal surface and
extends into the costodiaphragmatic recess; medially where it divides the base
from the mediastinal surface it is blunt and rounded. Above and behind the cardiac
impression on the lung is a hilum, a triangular depression named the hilum of
lung, where the structures which form the root of the lung enter and leave the
viscus. These include the pulmonary artery, superiormost on the left lung, the
superior and inferior pulmonary veins, lymphatic vessels and the bronchus,
withbronchial veins and bronchial arteries surrounding it. The pulmonary
ligament droops down from the hilum of the lung and terminates in a free, or
falciform, edge. The rib cage is separated from the lung by a two layered
membranous coating, called apleura. The hilum represents the point where the
parietal pleura (covering the rib cage) and the visceral pleura (covering the
lung) connect. The hilum is where the connection between the mediastinum and
the pleural cavities meet.
An alveolus (plural: alveoli,
from Latin alveolus, "little cavity") is an anatomical structure that
has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary
alveoli are the terminal ends of the respiratory tree, which outcrop from
either alveolar sacs or alveolar ducts, which are both sites of gas exchange
with the blood as well. Alveoli are particular to mammalian lungs. Different
structures are involved in gas exchange in other vertebrates. The alveolar
membrane is the gas-exchange surface. Carbon dioxide rich blood is pumped from
the rest of the body into the alveolar blood vessels where, through diffusion,
it releases its carbon dioxide and absorbs oxygen. The alveoli are located in the
respiratory zone of the lungs, at the distal termination of the alveolar ducts
and atria. These air sacs are the forming and termination point of the
respiratory tract. They provide total surface area of about 100 m2. A typical pair of human lungs
contain about 700 million alveoli, producing 70m2 of surface area. Each
alveolus is wrapped in a fine mesh of capillaries covering about 70% of its
area. An adult alveolus has an average diameter of 200 micrometres, with an
increase in diameter during inhalation. The alveoli consist of an epithelial
layer and extracellular matrix surrounded by capillaries. In some alveolar
walls there are pores between alveoli called Pores of Kohn. The alveoli contain
some collagen and elastic fibres. The elastic fibers allow the alveoli to
stretch as they are filled with air during inhalation. They then spring back
during exhalation in order to expel the carbon dioxide-rich air.
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