Respiration in Human
What do you understand by Respiration?
STRUCTURE
The organs of the respiratory system extend from the nose to the lungs and are divided into the upper and lower respiratory tracts. The upper respiratory tract consists of the nose and the pharynx, or throat. The lower respiratory tract includes the larynx, or voice box; the trachea, or windpipe, which splits into two main branches called bronchi; tiny branches of the bronchi called bronchioles; and the lungs, a pair of saclike, spongy organs. The nose, pharynx, larynx, trachea, bronchi, and bronchioles conduct air to and from the lungs. The lungs interact with the circulatory system to deliver oxygen and remove carbon dioxide.
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| Nasal passage |
The flow of air from outside of the body
to the lungs begins with the nose, which is divided into the left and right
nasal passages. The nasal passages are lined with a membrane composed primarily
of one layer of flat, closely packed cells called epithelial cells. Each
epithelial cell is densely fringed with thousands of microscopic cilia,
fingerlike extensions of the cells. Interspersed among the epithelial cells are
goblet cells, specialized cells that produce mucus, a sticky, thick, moist
fluid that coats the epithelial cells and the cilia. Numerous tiny blood
vessels called capillaries lie just under the mucous membrane, near the surface
of the nasal passages. While transporting air to the pharynx, the nasal
passages play two critical roles: they filter the air to remove potentially
disease-causing particles; and they moisten and warm the air to protect the
structures in the respiratory system.
Filtering prevents airborne
bacteria, viruses, other potentially disease-causing substances from entering
the lungs, where they may cause infection. Filtering also eliminates smog and
dust particles, which may clog the narrow air passages in the smallest
bronchioles. Coarse hairs found just inside the nostrils of the nose trap
airborne particles as they are inhaled. The particles drop down onto the mucous
membrane lining the nasal passages. The cilia embedded in the mucous membrane
wave constantly, creating a current of mucus that propels the particles out of
the nose or downward to the pharynx. In the pharynx, the mucus is swallowed and
passed to the stomach, where the particles are destroyed by stomach acid. If
more particles are in the nasal passages than the cilia can handle, the
particles build up on the mucus and irritate the membrane beneath it. This
irritation triggers a reflex that produces a sneeze to get rid of the polluted
air.
The nasal passages also
moisten and warm air to prevent it from damaging the delicate membranes of the
lung. The mucous membranes of the nasal passages release water vapor, which
moistens the air as it passes over the membranes. As air moves over the
extensive capillaries in the nasal passages, it is warmed by the blood in the
capillaries. If the nose is blocked or “stuffy” due to a cold or allergies, a
person is forced to breathe through the mouth. This can be potentially harmful
to the respiratory system membranes, since the mouth does not filter, warm, or
moisten air.
In addition to their role
in the respiratory system, the nasal passages house cells called olfactory
receptors, which are involved in the sense of smell. When chemicals enter the
nasal passages, they contact the olfactory receptors. This triggers the
receptors to send a signal to the brain, which creates the perception of smell.
Pharynx
Air leaves the nasal passages and flows to the pharynx, a short,
funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx.
Like the nasal passages, the pharynx is lined with a protective mucous membrane
and ciliated cells that remove impurities from the air. In addition to serving
as an air passage, the pharynx houses the tonsils, lymphatic tissues that
contain white blood cells. The white blood cells attack any disease-causing
organisms that escape the hairs, cilia, and mucus of the nasal passages and
pharynx. The tonsils are strategically located to prevent these organisms from
moving further into the body. One tonsil, called the adenoids, is found high in
the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is
located at the back of the pharynx on either side of the tongue. Another pair,
the lingual tonsils, is found deep in the pharynx at the base of the tongue. In
their battles with disease-causing organisms, the tonsils sometimes become
swollen with infection. When the adenoids are swollen, they block the flow of
air from the nasal passages to the pharynx, and a person must breathe through
the mouth.
Larynx
Air moves from the pharynx to the larynx, a
structure about 5 cm (2 in) long located approximately in the middle of the
neck. Several layers of cartilage, a tough and flexible tissue, comprise most
of the larynx. A protrusion in the cartilage called the Adam’s apple sometimes
enlarges in males during puberty, creating a prominent bulge visible on the
neck.
While the primary role
of the larynx is to transport air to the trachea, it also serves other
functions. It plays a primary role in producing sound; it prevents food and
fluid from entering the air passage to cause choking; and its mucous membranes
and cilia-bearing cells help filter air. The cilia in the larynx waft airborne
particles up toward the pharynx to be swallowed.
Food and fluids from the
pharynx usually are prevented from entering the larynx by the epiglottis, a
thin, leaflike tissue. The “stem” of the leaf attaches to the front and top of
the larynx. When a person is breathing, the epiglottis is held in a vertical
position, like an open trap door. When a person swallows, however, a reflex
causes the larynx and the epiglottis to move toward each other, forming a
protective seal, and food and fluids are routed to the esophagus. If a person
is eating or drinking too rapidly, or laughs while swallowing, the swallowing
reflex may not work, and food or fluid can enter the larynx. Food, fluid, or
other substances in the larynx initiate a cough reflex as the body attempts to
clear the larynx of the obstruction. If the cough reflex does not work, a
person can choke, a life-threatening situation. The Heimlich maneuver is a
technique used to clear a blocked larynx (see First Aid). A surgical
procedure called a tracheotomy is used to bypass the larynx and get air to the
trachea in extreme cases of choking.
Trachea,
Bronchi, and Bronchioles
Air passes from the larynx into the trachea, a
tube about 12 to 15 cm (about 5 to 6 in) long located just below the larynx.
The trachea is formed of 15 to 20 C-shaped rings of cartilage. The sturdy
cartilage rings hold the trachea open, enabling air to pass freely at all
times. The open part of the C-shaped cartilage lies at the back of the trachea,
and the ends of the “C” are connected by muscle tissue.
The base of the trachea
is located a little below where the neck meets the trunk of the body. Here the
trachea branches into two tubes, the left and right bronchi, which deliver air
to the left and right lungs, respectively. Within the lungs, the bronchi branch
into smaller tubes called bronchioles. The trachea, bronchi, and the first few
bronchioles contribute to the cleansing function of the respiratory system, for
they, too, are lined with mucous membranes and ciliated cells that move mucus
upward to the pharynx.
Alveoli
The bronchioles divide
many more times in the lungs to create an impressive tree with smaller and
smaller branches, some no larger than 0.5 mm (0.02 in) in diameter. These
branches dead-end into tiny air sacs called alveoli. The alveoli deliver oxygen
to the circulatory system and remove carbon dioxide. Interspersed among the
alveoli are numerous macrophages, large white blood cells that patrol the alveoli
and remove foreign substances that have not been filtered out earlier. The
macrophages are the last line of defense of the respiratory system; their
presence helps ensure that the alveoli are protected from infection so that
they can carry out their vital role.
The alveoli number about 150 million per lung and
comprise most of the lung tissue. Alveoli resemble tiny, collapsed balloons
with thin elastic walls that expand as air flows into them and collapse when
the air is exhaled. Alveoli are arranged in grapelike clusters, and each
cluster is surrounded by a dense hairnet of tiny, thin-walled capillaries. The
alveoli and capillaries are arranged in such a way that air in the wall of the
alveoli is only about 0.1 to 0.2 microns from the blood in the capillary. Since
the concentration of oxygen is much higher in the alveoli than in the
capillaries, the oxygen diffuses from the alveoli to the capillaries. The
oxygen flows through the capillaries to larger vessels, which carry the
oxygenated blood to the heart, where it is pumped to the rest of the body.
Carbon dioxide that has
been dumped into the bloodstream as a waste product from cells throughout the
body flows through the bloodstream to the heart, and then to the alveolar
capillaries. The concentration of carbon dioxide in the capillaries is much
higher than in the alveoli, causing carbon dioxide to diffuse into the alveoli.
Exhalation forces the carbon dioxide back through the respiratory passages and
then to the outside of the body.
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