How do human lungs work?

The lungs are the major component of the respiratory
system . A good way to understand
the workings of the lungs is to consider their structure,
or anatomy . The human respiratory system
begins at the nose and includes the nasal passages, which
direct air to the back of the throat and into the windpipe,
or trachea. The trachea sits below the voice box
(larynx), and can be felt in the front of your neck as it
descends behind the breastbone (sternum) into the upper
chest. The trachea ends and divides into two branches:
the right mainstem bronchus and the left mainstem
bronchus. The right mainstem bronchus leads air to
and from the right lung, and the left mainstem bronchus
leads air to and from the left lung.

 

 The area where the trachea divides into the right and

left mainstem bronchi is called the carina. After the
split, the right and left mainstem bronchi leading to each
lung subdivide further into smaller and smaller tubelike
passages, via the branching tracheobronchial tree. As
the bronchi continue to subdivide into successively narrower
and narrower bronchi, they ultimately end in the
tiniest subdivision, the bronchiole. Each bronchiole
leads to the lung air sacs, the alveoli.

 The alveoli are highly specialized lung structures. They

are the gas-exchanging lung units. They ensure that
fresh, oxygen-rich (O2) inhaled air enters the body at the
same time that oxygen-poor, carbon dioxide-rich (CO2)
air exits (Figure 1B). Oxygen (O2) is required for life;
oxygen deprivation is rapidly fatal. As inhaled oxygen is
provided to the body’s organs via the lungs, “used” air—
composed mostly of carbon dioxide (CO2)—is excreted
by exhalation. Carbon dioxide is produced by the body’s
metabolism and is considered a “waste product.” Abnormal
accumulation of carbon dioxide in the body and the
bloodstream is detrimental to health and is responsible
for certain forms of respiratory failure.
The process that is responsible for the body’s oxygen
uptake and its carbon dioxide removal (or excretion) is
called respiration. Respiration is the primary, crucial
function of the lungs and of the respiratory system.
Physicians occasionally refer to respiration as “gas
exchange.” The exchanged gases are oxygen (O2) and
carbon dioxide (CO2). Exchange means that CO2 gas
is given up by the body and replaced by a fresh supply
of O2. The exchange takes place in the deepest lung, at
the level of the alveoli. Oxygen and carbon dioxide
exchange takes place along a specialized zone where each
air sac (alveolus) is in intimate contact with fine, minute
blood vessels called capillaries. The capillary bed completely
encircles the alveoli along the alveolar–capillary
membrane. Because of the structure of the alveolar–
capillary membrane, the inhaled oxygen (O2) easily
passes from the alveolus into the capillary blood that
then sends it to our organs. Similarly, the body’s CO2 is
carried through the bloodstream into the blood supply
of the alveolar–capillary membrane where it is given
up to the alveolus, and from there, exhaled by our
lungs with each breath. A normal resting adult breathes

 approximately 12–18 times a minute, children about 20

times a minute, and babies and infants even more frequently.
That number, the number of breaths a person
breathes in 1 minute, is termed the respiratory rate. We
should be unaware of our breathing in health, as respiration
should be automatic, effortless, and of course, painless.
Increases in the respiratory rate, sometimes perceived
as a kind of breathless sensation, can represent a normal
process as during exercise or sports and allows for increased
oxygen delivery to the body. A respiratory rate increase
can also indicate the onset of a medical concern; it may
in particular be a sign of increasing asthma symptomatology.
That is why measurement of respiratory rate,
along with pulse (or heart rate), blood pressure, and
temperature measurements are collectively referred to as
vital signs in medical terminology!