Download Lung Anatomy - Medical

Lecture I
Lung Anatomy
The lung is the essential organ of respiration in air-breathing vertebrates. Its principal
function is to transport oxygen from the atmosphere into the bloodstream, and excrete
carbon dioxide from the bloodstream into the atmosphere. This it accomplishes with its
mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs
where gas exchange takes place. Lungs also have nonrespiratory functions.
Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius
("of the lungs"),
The respiratory function of the lung
Energy production in living organisms often uses oxygen and produces carbon dioxide.
Hence, life necessitates an efficient means of oxygen delivery to cells and carbon dioxide
excretion from cells. In smaller 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 situated 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
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 tissues via 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 these animals are a rich lattice of alveoli, which provide an enormous
surface area for gas exchange. A network of fine capillaries transports blood over the
surface of alveoli. Oxygen from the air inside the alveoli diffuses into the bloodstream
across the exceptionally thin alveolar membranes, and carbon dioxide moves from the
blood to the alveoli via the same process. 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 decreasing pressure, air is sucked into the
airways, and by reducing volume and increasing pressure, the reverse occurs. During
normal breathing, expiration is passive and no muscles are contracted (the diaphragm
relaxes).
Nonrespiratory functions of the lung
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 the systemic veins
 Serve as a physical layer of soft, shock-absorbent protection for the heart, which the
lungs flank and nearly enclose.
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 diaphragm below, a muscle that by contracting expands
the cavity in which the lung is enclosed. The rib cage itself is also able to expand and
contract to some degree.
As a result, air is sucked into and pushed out of the lungs through the trachea and the
bronchial tubes or bronchi; these branch out and end in alveoli which are tiny sacs
surrounded by capillaries filled with blood. Here oxygen from the air diffuses into the
blood, where it is carried by hemoglobin.
The deoxygenated blood from the heart reaches the lungs via the pulmonary artery and,
after having been oxygenated, returns via the pulmonary veins.
ANATOMY
The lungs are located inside the thoracic cavity, protected by the bony structure of the rib
cage and enclosed by a double-walled sac called pleura. The inner layer of the sac
(visceral pleura) adheres tightly to the lungs and the outer layer (parietal pleura) is
attached to the wall of the chest 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 give way for the heart.
The lungs attach 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 (Latin for "tongue") 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 and runs more vertically than the left. For this reason, it is more
common to aspirate foreign objects into the right lung. 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 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 that has returned to the heart from the
venous system to the lungs to be reoxygenated. The pulmonary veins carry oxygenated
blood back to the heart to go to the arterial system. The right and left pulmonary arteries
arise from the pulmonary trunk and carry "venous" blood to their respective lungs. The
pulmonary veins, two on each side, carry "arterial" 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.
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