The respiratory system functions in the exchange of gases
between the body and the environment.
Together with the other organ systems they also contribute to
hemostasis- that is, to maintaining internal operating conditions for all of
the body’s living cells.
- The Nature of Respiration
- The Basis of Gas Exchange
- Respiratory systems rely on the diffusion of
gases down pressure gradients.
- Partial pressures
for each gas in the atmosphere can be calculated; for example, oxygen’s
is 160 mm Hg.
- Gases enter and leave the body by diffusing
across a moist respiratory surface.
- According to Fick’s Law, the more extensive the
surface area and the larger the partial pressure gradient, the faster
the rate will be.
- Factors That Influence Gas Exchange
- Surface-to-Volume Ratio
- As an animal grows, its surface area increases
at a lesser rate than its volume, making diffusion of gases into the
interior a problem.
- Therefore, animals either must have a body
design that keeps internal cells close to the surface (flatworms) or
must have a system to move the gases inward.
- Animals have adaptations to move the air, or
water, over the respiratory surfaces.
- Bony fish move the covers over the gills;
sponges move the flagella on their collar cells; humans move the
muscles of the thorax to expand and contract the chest cavity and move
air in and out of the lungs.
- Transport Pigments
- Hemoglobin is the
main transport pigment; maintain steep pressure gradients across a
- It binds four molecules of oxygen in the lungs
(high concentration) and releases them in the tissues where oxygen is
- Invertebrate Respiration
- Integumentary exchange is used by small invertebrates.
- In animals such as flatworms which have a low
metabolic rate, the epidermis at the body surface is used for
- For terrestrial animals, like the earthworm,
mucus helps keep the surface moist to allow the oxygen to diffuse inward
through the thin epidermis.
- Gills are used by invertebrates that live in aquatic
- A gill has a moist, thin, vascularized
- The highly folded gill walls greatly increase
the respiratory surface.
- Tracheal respiration is used by arthropods in terrestrial settings.
- Tracheal respiration in insects and spiders,
utilizes fine air-conducting tubules to provide gaseous exchange at the
- In most cases no participation by the
circulatory system is needed, neither are any respiratory pigments
- Vertebrate Respiration
- Gills of Fishes and Amphibians
- The internal gills of adult fishes are rows of
slits or pockets at the back of the mouth that extend to the body
surface and water flows into the mouth and pharynx, then over filaments
in the gills.
- Water flows over the gills and blood circulates
through them in OPPOSITE DIRECTIONS.
- This mechanism, called countercurrent flow, is
highly efficient in extracting oxygen from water, whose oxygen content
is lower than air.
- Lungs contain internal respiratory surfaces shaped
as a cavity or sac.
- Simple lungs evolved about 450 million years
ago to assist respiration in oxygen-poor habitats; some evolved into
swim bladders, others into complex respiratory organs.
- Lungs provide a membrane for gaseous exchange
- Air moves by bulk flow into and out of the
- Gases diffuse across the inner respiratory
surfaces of the lungs.
- Pulmonary circulation enhances the diffusion
of dissolved gases into and out of lung capillaries.
- In body tissues, oxygen diffuses from blood à interstitial fluid à cells; carbon dioxide travels the route in
- Human Respiratory System
- Functions of the Respiratory System
- Ventilation alternately moves air into and out
of a pair of lungs inside of which are the tiny air sacs called alveoli
where gas exchange takes place.
- Breathing is necessary for speech.
- Limited amounts of excess heat and water are
- It also adjusts the body’s acid-base balance.
- The respiratory system has mechanisms to deal
with airborne foreign matter than enters the system.
- From Airways to the Lungs
- Air enters or leaves the respiratory system
through nasal cavities where hair and cilia filter out dust and
particles; blood vessels warm; and mucus moistens the air.
- Air moves via this route: pharynx à larynx (route blocked by epiglottis during
swallowing) à vocal cords (space between is glottis) à trachea à bronchi à bronchioles à alveoli.
- Sites of Gas Exchange in the Lungs
- Human lungs are elastic, cone-shaped organs of
gas exchange inside the rib cage, to the left and right of the heart.
- Each lung lies in a thin-walled pleural sac,
which leaves a very thin intrapleural space between the membranes.
- Inside the lungs, respiratory bronchioles bear
outpouchings of their walls called alveoli, which are usually clustered as
- Alveoli provide a tremendous surface area for
gaseous exchange with the blood located in the dense capillary network
surrounding each alveolar sac.
- Breathing—Cyclic Reversals in Air Pressure Gradients
- The Respiratory Cycle
- In inhalation, the diaphragm contracts and
flattens, muscles lift the rib cage upward and outward, the chest cavity
volume increases, internal pressure decreases, air rushes in.
- In exhalation, the actions listed above are
reversed; the elastic tissue in the lungs and chest wall recoils
- Lung Volumes
- The maximum volume that can be moved in or out
after a single maximal inhalation is called the vital capacity, but the
lungs cannot be completely emptied.
- About 500 ml of air enters and leaves with each
breath (tidal volume).
- Breathing and Sound Production
- The vocal cords lie at the entrance to the
larynx and near it are two pairs of horizontal folds; the lower fold is
cords and the glottis is the gap between the vocal cords.
- When air is exhaled through the glottis, the
folds of the cords vibrate to produce sounds which are under regulation
by nerve commands to the elastic ligaments that regulate the glottal
- Gas Exchange and Transport
- Gas Exchange
- Each alveolus consists of a single layer of
epithelial cells through which gases can readily diffuse to and from
interstitial fluid and blood capillaries.
- The partial pressure gradients are sufficient
to move oxygen in and carbon dioxide out of the blood, passively.
- Blood cannot carry sufficient oxygen and carbon
dioxide in dissolved form to
satisfy the body’s demands.
- Hemoglobin is a protein with four heme groups
that bind oxygen.
- Oxygen diffuses down a pressure gradient into
the blood plasma ––> red blood cells ––> binds to hemoglobin (4
molecules of oxygen/hemoglobin to form oxyhemoglobin).
- Hemoglobin gives up its oxygen in tissues where
partial pressure of oxygen is low, blood is warmer, partial pressure of
carbon dioxide is higher, and pH is lower; all four conditions occur in
tissues with high metabolism.
- Carbon Dioxide Transport
the concentration of carbon dioxide is higher in the body tissues, it diffuses
into the blood.
percent is dissolved in plasma, 30 percent binds with hemoglobin to form carbaminohemoglobin, and 60 percent is in bicarbonate
and carbonic acid formation is enhanced by the enzyme carbonic
anhydrase, which is
located in the red blood cells.
- Matching Air Flow With Blood Flow
exchange in the alveoli is most efficient when air flow equals the rate of
nervous system controls oxygen and carbon dioxide levels for the entire body by
adjusting contraction rates of the diaphragm and chest wall muscles.
brain monitors input from carbon dioxide sensors in the bloodstream and from
receptors sensitive to decreases in oxygen partial pressure (carotid bodies and
- Respiration in Unusual Environments
at High Altitudes
- At high altitudes, the partial pressure of
oxygen is lower than at sea level.
- The lungs of permanent residents of high
mountains have more alveoli and blood vessels plus larger ventricles in
the heart and more mitochondria in muscle tissue.
- Acclimatization allows newcomers to high
altitudes to adjust.
- The kidney cells release erythropoietin which induces
production of more red blood cells.
- Increased blood cell numbers creates more
resistance to flow, which in turn makes the heart work harder to pump
- Carbon monoxide is a colorless, odorless gas
produced as a byproduct of combustion.
- It combines with hemoglobin 200 times greater
than does oxygen.
in Diving Animals
- The respiratory system of the whale is modified
to collect and store oxygen by several mechanisms including the use of myoglobin,
an oxygen-binding pigment in muscle.
- If a diver ascends too rapidly, the change in
pressures will force the nitrogen to leave the tissues of the body and
pass into the blood, often as bubbles, causing pain in the joints known
as "the bends" or decompression sickness.