In humans and other animals, for example, the anatomical features of the respiratory systemlungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs include airways,
Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants
also have respiratory systems but the directionality of gas exchange
can be opposite to that in animals. The respiratory system in plants
also includes anatomical features such as holes on the undersides of leaves known as stomata.
Comparative anatomy and physiology
Horses are obligate nasal breathers
which means that they are different from many other mammals because
they do not have the option of breathing through their mouths and must
take in oxygen through their noses.
The elephant is the only animal known to have no pleural space. Rather, the parietalvisceral pleura are both composed of dense connective tissue and joined to each other via loose connective tissue. This lack of a pleural space, along with an unusually thick diaphragm, are thought to be evolutionary adaptations allowing the elephant to remain underwater for long periods of time while breathing through its trunk and which emerges as a snorkel.
The respiratory system of birds differs significantly from that found in mammals, containing unique anatomical features such as air sacs. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli. See Avian respiratory system for a detailed description of these and other features.
The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however, reptiles do not possess a diaphragm. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration.
See also reptiles for more detailed descriptions of the respiratory system in these animals.
Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus
from specialized cells. While the lungs are of primary importance to
breathing control, the skin's unique properties aid rapid gas exchange
when amphibians are submerged in oxygen-rich water.
In most fish respiration takes place through gills. (See also aquatic respiration.) Lungfish, however, do possess one or two lungs. The labyrinth fish have developed a special organ that allows them to take advantage of the oxygen of the air, but is not a true lung.
Anatomy in invertebrates
Air enters the respiratory systems of most insects through a series of external openings called spiracles.
These external openings, which act as muscular valves in some insects,
lead to the internal respiratory system, a densely networked array of
tubes called trachea.
The scientific tracheal system within an individual is composed of
interconnecting transverse and longitudinal tracheae which maintain
equivalent pressure throughout the system. These tracheae branch
repeatedly, eventually forming tracheoles, which are blind-ended,
water-filled compartments only one micrometer in diameter. It is at this level of the tracheoles that oxygen is delivered to the cells for respiration.
Insects were once believed to exchange gases with the environment continuously by the simple diffusion
of gases into the tracheal system. More recently, however, large
variation in insect ventilatory patterns have been documented and
insect respiration appears to be highly variable. Some small insects do
demonstrate continuous respiration and may lack muscular control of the
spiracles. Others, however, utilize muscular contraction of the abdomen
along with coordinated spiracle contraction and relaxation to generate
cyclical gas exchange patterns. The most extreme form of these patterns
is termed discontinuous gas exchange cycles (DGC).
generally possess gills that allow exchange of oxygen from an aqueous
environment into the circulatory system. These animals also possess a
heart that pumps blood which contains hemocyaninine as its oxygen-capturing molecule. Hence, this respiratory system is similar to that of vertebrate fish. The respiratory system of gastropods can include either gills or a lung.
Physiology in mammals
For more detailed descriptions see also Respiratory physiology or Respiration.
In respiratory physiology, ventilation (or ventilation rate) is the
rate at which gas enters or leaves the lung. It is categorised under
the following definitions: Measurement Symbol Equation Description
Minute ventilation = tidal volume * respiratory rate the total
volume of gas entering the lungs per minute. Alveolar ventilation =
(tidal volume - dead space) * respiratory rate  the volume of gas
per unit time that reaches the alveoli, the respiratory portions of the
lungs where gas exchange occurs. Dead space ventilation = dead space *
respiratory rate is the volume of gas per unit time that does not
reach these respiratory portions, but instead remains in the airways
(trachea, bronchi, etc.).
Ventilation occurs under the control of the autonomic nervous system from parts of the brain stem, the medulla oblongata and the pons. This area of the brain forms the respiration regulatory center, a series of interconnected brain cells within the lower and middle brain stem which coordinate respiratory movements. The sections are the pneumotaxic center, the apneaustic center, and the dorsal and ventral respiratory groups.
This section is especially sensitive during infancy, and the neurons
can be destroyed if the infant is dropped and/or shaken violently. The
result can be death due to "shaken baby syndrome".
Inhalation is initiated by the diaphragm and supported by the external intercostal muscles.
Normal resting respirations are 10 to 18 breaths per minute, with a
time period of 2 seconds. During vigorous inhalation (at rates
exceeding 35 breaths per minute), or in approaching respiratory
failure, accessory muscles of respiration are recruited for support. These consist of sternocleidomastoid, platysma, and the scalene muscles of the neck. Pectoral muscles and latissimus dorsi are also accessory muscles.
Under normal conditions, the diaphragm is the primary driver of inhalation. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic
volume and negative pressure (with respect to atmospheric pressure)
inside the thorax. As the pressure in the chest falls, air moves into
the conducting zone. Here, the air is filtered, warmed, and humidified
as it flows to the lungs.
During forced inhalation, as when taking a deep breath, the external intercostal muscles and accessory muscles aid in further expanding the thoracic cavity.
Exhalation is generally a passive process; however, active or forced exhalation is achieved by the abdominal and the internal intercostal muscles. During this process air is forced or exhaled out.
The lungs have a natural elasticity: as they recoil from the stretch
of inhalation, air flows back out until the pressures in the chest and
the atmosphere reach equilibrium.
During forced exhalation, as when blowing out a candle, expiratory
muscles including the abdominal muscles and internal intercostal
muscles, generate abdominal and thoracic pressure, which forces air out
of the lungs.
The major function of the respiratory system is gas exchange between the external environment and an organism's circulatory system. In humans and mammals, this exchange facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation. As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis. If proper ventilation is not maintained, two opposing conditions could occur: respiratory acidosis, a life threatening condition, and respiratory alkalosis.
Uponalation, gas exchange occurs at the alveoli,
the tiny sacs which are the basic functional component of the lungs.
The alveolar walls are extremely thin (approx. 0.2 micrometres). These
walls are composed of a single layer of epithelial cells (type I and type II epithelial cells) close to the pulmonary capillaries which are composed of a single layer of endothelial cells.
The close proximity of these two cell types allows permeability to
gases and, hence, gas exchange. This whole mechanism of gas exchange is
carried by the simple phenomenon of pressure difference. When the
atmospheric pressure is low outside, the air from lungs flow out. When
the air pressure is low inside, then the vice versa.
Lung Defense Mechanisms
Airway epithelial cells can secrete a variety of molecules that aid
in lung defense. Secretory immunoglobulins (IgA), collectins (including
Surfactant A and D), defensins and other peptides and proteases,
reactive oxygen species, and reactive nitrogen species are all
generated by airway epithelial cells. These secretions can act directly
as antimicrobials to help keep the airway free of infection. Airway
epithelial cells also secrete a variety of chemokines and cytokines
that recruit the traditional immune cells and others to site of
Metabolic & Endocrine Functions of the Lungs
In addition to their functions in gas exchange, the lungs have a
number of metabolic functions. They manufacture surfactant for local
use, as noted above. They also contain a fibrinolytic system that lyses
clots in the pulmonary vessels. They release a variety of substances
that enter the systemic arterial blood and they remove other substances
from the systemic venous blood that reach them via the pulmonary
artery. Prostaglandins are removed from the circulation, but they are
also synthesized in the lungs and released into the blood when lung
tissue is stretched. The lungs also activate one hormone; the
physiologically inactive decapeptide angiotensin I is converted to the
pressor, aldosterone-stimulating octapeptide angiotensin II in the
pulmonary circulation. The reaction occurs in other tissues as well,
but it is particularly prominent in the lungs. Large amounts of the
angiotensin-converting enzyme responsible for this activation are
located on the surface of the endothelial cells of the pulmonary
capillaries. The converting enzyme also inactivates bradykinin.
Circulation time through the pulmonary capillaries is less than 1 s,
yet 70% of the angiotensin I reaching the lungs is converted to
angiotensin II in a single trip through the capillaries. Four other
peptidases have been identified on the surface of the pulmonary
The movement of gas through the larynx, pharynx and mouth allows humans to speak, or phonate. Vocalization, or singing, in birds occurs via the syrinx, an organ located at the base of the trachea. The vibration of air flowing across the larynx (vocal chords), in humans, and the syrinx, in birds, results in sound. Because of this, gas movement is extremely vital for communication purposes.
in dogs and some other animals provides a means of controlling body
temperature. This physiological response is used as a cooling mechanism.
Coughing and sneezing
Irritation of nerves within the nasal passages or airways, can induce coughing and sneezing. These responses cause air to be expelled forcefully from the trachea or nose, respectively. In this manner, irritants caught in the mucus which lines the respiratory tract are expelled or moved to the mouth where they can be swallowed.
Development in People
Humans and mammals
Further information: Development of human lung
The respiratory system lies dormant in the human fetus during pregnancy.
At birth, the respiratory system becomes fully functional upon exposure
to air, although some lung development and growth continues throughout
childhood. Pre-term birth can lead to infants with under-developed lungs. These lungs show incomplete development of the alveolar type II cells, cells that produce surfactant.
The lungs of pre-term infants may not function well because the lack of
surfactant leads to increased surface tension within the alveoli. Thus,
many alveoli collapse such that no gas exchange can occur within some
or most regions of an infant's lungs, a condition termed respiratory distress syndrome. Basic scientific experiments, carried out using cells from chicken lungs, support the potential for using steroids as a means of furthering development of type II alveolar cells. In fact, once a pre-mature birth is threatened, every effort is made to delay the birth, and a series of steroid shots is frequently administered to the mother during this delay in an effort to promote lung growth.
Disorders of the respiratory system can be classified into four general areas:
- Obstructive conditions (e.g., emphysema, bronchitis, asthma)
- Restrictive conditions (e.g., fibrosis, sarcoidosis, alveolar damage, pleural effusion)
- Vascular diseases (e.g., pulmonary edema, pulmonary embolism, pulmonary hypertension)
- Infectious, environmental and other "diseases" (e.g., pneumonia, tuberculosis, asbestosis, particulate pollutants): Coughing is of major importance, as it is the body's main method to remove dust, mucus, saliva, and other debris from the lungs. Inability to cough can lead to infection. Deep breathing exercises may help keep finer structures of the lungs clear from particulate matter, etc.
The respiratory tract is constantly exposed to microbes due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.
Disorders of the respiratory system are usually treated internally by a pulmonologist.
Plants use carbon dioxide gas in the process of photosynthesis, and exhale oxygen2 (carbon dioxide) and 6 H2O (water) and that makes 6 O2 (oxygen) and C6H12O6
(glucose). Respiration is the opposite of that. However, plants also
sometimes respire as humans do, taking in oxygen and producing carbon
dioxide gas as waste. The chemical equation of photosynthesis is 6 CO
Plant respiration is limited by the process of diffusion. Plants take in carbon dioxide through holes on the undersides of their leaves known as stoma or pores. However, most plants require little air.
Most plants have relatively few living cells outside of their surface
because air (which is required for metabolic content) can penetrate
only skin deep. However, most plants are not involved in highly aerobic activities, and thus have no need of these living cells.