Respiration is the
act of breathing:
The respiratory system
functions to deliver the oxygen to the blood -- the transport medium of
the cardiovascular system
-- and to remove oxygen from the blood. The actual exchange of oxygen and
carbon dioxide occurs in the lungs.
inhaling (inspiration): muscle
contraction, which lifts the ribs and pulls them outward, increases
lung volume, allowing
air to rush in (inspired air contains 21% oxygen and essentially no
exhaling (expiration) - muscle
relaxation decreases lung volume and the air passively flows out (expired
air contains 16% oxygen and 4.5% carbon dioxide)
The respiratory centers in
brain stem (pons
and medulla) control
respiration's rhythm, rate, and depth. Primary controlling factors include
1) the concentration of carbon dioxide in the blood (high CO2
concentrations initiate deeper, more rapid breathing) and 2) air pressure
within lung tissue. Expansion of the lungs stimulates nerve receptors (vagus
nerve X) to signal the brain to "turn off" inspiration. When the lungs
collapse, the receptors give the "turn on" signal, termed the Hering-Breuer
inspiratory reflex. Other regulators are: 3) an increase in blood pressure,
which slows down respiration; 4) a drop in blood acidity, which stimulates
respiration; and 5) a sudden drop in blood pressure, which increases the
rate and depth of respiration. Voluntary controls -- "holding one's breath"
-- can also affect respiration, but not indefinitely. Carbon dioxide build-up
soon forces an automatic start-up.
Anatomy of the Respiratory System
image for more detail.
of the diaphragm
The respiratory system consists
of two tracts: The upper respiratory tract includes the nose (nasal
cavity, sinuses), mouth, larynx, and trachea (windpipe). The lower
respiratory tract includes the lungs, bronchi, and alveoli.
The two lungs, one on the
right and one on the left, are the body's major respiratory organs. Each
lung is divided into upper and lower lobes, although the
upper lobe of the right lung contains a third subdivision known as
the right middle lobe. The right lung is larger and heavier than
the left lung, which is somewhat smaller in size because of the predominately
left-side position of the heart.
A clear, thin, shiny coating
-- the pleura
-- envelopes the lungs. The inner, visceral layer of the pleura attaches
to the lungs; the outer, parietal layer attaches to the chest wall
Pleural fluid holds both layers in place, in a manner similar to
two microscope slides that are wet and stuck together. The lungs are separated
from each other by the mediastinum, an area that contains the heart
and its large vessels, the trachea (windpipe), esophagus,
and lymph nodes.
the muscle that contracts and relaxes in breathing, separates the thoracic
cavity from the abdominal cavity.
On inspiration, air enters
the body through the nose and the mouth.
Nasal hairs and mucosa (mucus) filter out dust particles and bacteria and
warm and moisten the air. Less warming, filtering, and humidification occur
when air is inspired through the mouth.
Air travels down the throat,
or pharynx, where two openings exist, one into the esophagus for
passage of food, and the other into the larynx
(voice box) and trachea (windpipe) for
continued airflow. When food is swallowed, the opening of the larynx (the
automatically closes, preventing food from being inhaled. When air is inspired,
the walls of the esophagus are collapsed, preventing air from entering
the stomach. The larynx, which also contain the vocal
cords, is lined with mucus that further warms and humidifies the air.
continues continues down the trachea, which branches into the right and
left bronchi. The main-stem bronchi
divide into smaller bronchi, then into even smaller tubes called
The bronchial structures contain hair-like, epithelial
projections, called cilia, that beat rythmically to sweep debris
out of the lungs toward the pharynx for expulsion. Once in the bronchioles,
the air is at body temperature, contains 100% humidity, and is (hopefully)
end in air sacs called alveoli
-- small, thin-walled "balloons," arranged in clusters. When
you breathe in, enlarging the chest cavity, the "balloons" expand as air
rushes in to fill the vacuum. When you breathe out, the "balloons" relax
and air moves out of the lungs. It is at the alveoli that gas
exchange occurs. Tiny blood vessels, capillaries,
surround each of the alveoli. On inspiration, the concentration of dissolved
oxygen is greater in the alveoli than in the capillaries. Oxygen, therefore,
diffuses across the alveolar walls into the blood plasma. In the reverse
process, carbon dioxide concentration is greater in the blood than the
alveoli, so it passes from the blood into the alveoli and is ultimately
As oxygen diffuses into
the plasma, hemoglobin
in the red blood cell picks up the oxygen, permitting more to flow into
the plasma. The oxygen-carrying capacity of hemoglobin allows the
blood to carry over 70 times more oxygen than if the oxygen were simply
dissolved in the plasma alone. Therefore, the total oxygen uptake depends
on: 1) the difference in oxygen concentration between the blood and alveoli,
2) the healthy functioning of the alveoli, and 3) the rate of respiration.
circulatory circuit describes the process whereby oxygen and carbon
dioxide are delivered to and from the lungs. Oxygen-poor blood travels
to the right atrium via the inferior and superior vena cavae, then to the
right ventricle. The right ventricle subsequently pumps the blood into
the pulmonary artery,
which branches to the right and left lungs. The pulmonary arteries subdivide
until reaching the arteriole, then capillary levels. After gas exchange,
the capillaries recombine to form venules and veins. Ultimately two right
and two left pulmonary
veins carry oxygen-rich blood to the heart for distribution, via the
aorta/systemic circuit, to the rest of the body.
Lung Volumes/ Capacities
The air that the lungs can
hold can be divided into smaller designations called "volumes."
The amount of air a person
breathes in and out at rest is called the Tidal Volume (Vt about
500ml). During such breathing, a person could actually take in more air
or blow more out. The additional amount a person could inhale, such as
during maximum physical activity, is called the Inspiratory Reserve
Volume (IRV 3,000 ml). The additional amount a person could exhale
is called the Expiratory Reserve Volume (ERV 1,000 ml). The
Volume (RV) is the amount of air that stays in the lung even after
"Capacities" are combinations
of two or more volumes.
To Learn More
The Total Lung Capacity
(TLC) is the total amount of air the lungs can contain:
TLC = RV + ERV + Vt +
The Vital Capacity (VC)
is the total amount of air the person can breathe in and out:
VC = ERV + Vt + IRV
Functional Residual Capacity
(FRC) is the total amount of air left in the lungs at the end of a normal
exhalation: FRC = RV + ERV
Joel DeLisa and Walter C. Stolov,
"Significant Body Systems," in: Handbook of Severe Disability, edited
by Walter C. Stolov and Michael R. Clowers. US Department of Education,
Rehabilitation Services Administration, 1981, pages 41-45.
Catherine Parker Anthony and
Gary A. Thibodeau, Textbook of Anatomy & Physiology. St. Louis:
Mosby, 1983, pages 498-548.
Dennis Kunkel, Electron
Marjorie Thompson, BIO 189,
Show, Brown University School of Medicine.