All the body
cells metabolically consume oxygen, and discharge carbon dioxide.
To cover this need, respiration takes place internally (at the cellular
level) and externally (ventilation/breathing). Ventilation involves
the inhalation of atmospheric air into the lungs via the nose and
mouth through branching passageways, and the exhalation of carbon
dioxide. The lung key function is to bring air and blood into intimate
contact in the alveolar air sacs so that oxygen can enter the blood,
and carbon dioxide can leave. At rest, humans breathe about twelve
times a minute, bringing in approximately a pint of air. Exercise
and certain diseases result in a marked increase of breathing. The
respiratory system also is vital in maintaining normal blood pH
and body temperature.
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and air passages
The lungs are
paired organs that lie on either side of the heart and fill up the
thoracic (chest) cavity. Inferior to (below) the lungs is the diaphragm,
a broad thin muscle that separates the thoracic cavity from the
abdominal (gut) cavity. On the medial (inner) surface of each lung
is the hilus, where blood vessels, nerves, and bronchi (air passages)
enter the lungs.
The lungs differ
in size and shape. Because the heart is slightly larger on the left
side, the left lung has a cardiac notch (indented border). The left
lung is also slightly smaller than the right. Each lung is divided
into lobes (partitions) by fissures. The right lung has three lobes:
lower, middle, and upper. These horizontal and oblique fissures
create these lobes. The left lung has upper and lower lobes that
are divided by the oblique fissure.
Air enters the
body through the mouth or nose. In the nose, thick hairs lining
the nostrils prevent small objects from entering the nasal cavity.
This cavity is lined with cells that produce mucus. Small foreign
matter that enters the nasal cavities is trapped in the mucus, while
tiny cilia (small hair-like projections) push the mucus to the pharynx
(throat), where it is swallowed and digested in the stomach or expectorated.
From the pharynx,
the air passes to the larynx, which is called the voice box because
it contains the vocal cords. To prevent food or liquid from entering
the larynx, the epiglottis (a small flap of tissue) closes over
the opening of the larynx during deglutition (swallowing). If this
process works improperly, a cough reflex expels the foreign material.
When air travels
past the larynx, it enters the trachea (windpipe). The trachea is
a strong tube containing rings of cartilage that prevents it from
collapsing. The mucosa that lines the airway warms and moistens
the air before it reaches the trachea.
Within the lungs,
the trachea branches into a left and right bronchus, which divide
into increasingly smaller branches called bronchioles. The smallest
bronchioles end in a cluster of air sacs, collectively called an
acinus. The acinus comprises individual air sacs called alveoli.
Alveoli are like small balloons that inflate and deflate with air
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occurs in the lungs between the alveoli and a capillary network
within the alveolar wall. Capillaries are microscopic blood vessels
that exchange material between the blood and body tissues. In the
lung capillaries, blood from tissues where cellular metabolism is
occurring is called deoxygenated blood because it contains many
carbon dioxide molecules and few oxygen molecules.
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process has two parts: inspiration (inhaling) and expiration (exhaling).
During inspiration, the diaphragm contracts, moves downward, and
causes the thoracic cavity volume to increase. Because the lungs
are closely associated with the interior chest wall, they expand
as the thoracic cavity expands. When the diaphragm relaxes (upward
position), the thoracic volume decreases and the lungs partially
deflate. This process is called expiration. The elastic recoil of
the expanded thoracic wall and lungs also helps expiration.
the alveoli contain many oxygen molecules. The alveoli are in close
contact with the capillary network. This proximity enables the minuscule
oxygen molecules to diffuse (pass freely) from the alveolus to the
bloodstream, flowing from a region of higher concentration to a
region of lower concentration. In the bloodstream, the oxygen attaches
to red blood cells and is transported to the rest of the body. Likewise,
carbon dioxide diffuses from the bloodstream into the alveolus where
it is transported out of the body during exhalation.
the pleurae (pleural membranes) help the lungs to expand and contract.
These membranes are sacs that tightly cover the lungs and the chest
inside wall. Between these two linings is a space called the pleural
cavity that contains a thin layer of fluid. This fluid allows the
lungs to move freely against the thoracic cavity inside.
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