In living organisms, a respiratory system
functions to allow gas exchange
. The gases that are exchanged, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on the organism. In humans
and other mammals
, for example, the anatomical features of the respiratory system
include airways, lungs
, and the respiratory muscles
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.
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 unique anatomical features such as holes on the undersides of leaves known as stomata.
Anatomy of Respiratory System in Vertebrates
For humans and mammals respiration is essential. In humans and mammals, the respiratory system can be subdivided into an upper respiratory tract and a lower respiratory tract based on anatomical features. The upper respiratory tract includes the nasal passages, pharynx and the larynx. While the trachea, the primary bronchi and lungs are parts of the lower respiratory tract. The respiratory system can also be divided into physiological, or functional, zones. These include the conducting zone (the region for gas transport from the outside atmosphere to just above the alveoli), the transitional zone, and the respiratory zone (the alveolar region where gas exchange occurs). (See also respiratory tract.)
Unique Aspects of Comparative Anatomy/Physiology in Mammals
Horses are obligate nasal breathers. That is, they are different from many other mammals in that they do not have the option of breathing through their mouths and must take in air through their nose.
The horse's respiratory system is divided into two sections, the upper respiratory tract and the lower respiratory tract.
The upper respiratory tract includes the nostrils, the nasal passages, pharynx, larynx and the trachea. The lower respiratory tract is made up of the bronchi, bronchioles, and alveoli, all of which reside within the lungs of the horse. See Respiratory system of the horse for a detailed description.
The elephant is the only mammal known to have no pleural space. Rather, the parietal and visceral 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 diaphram, are thought to be evolutionary adaptations allowing the elephant to remain underwater for long periods of time while breathing through its trunk which emerges as a snorkle.
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.
Skin is one of the important respiratory organs in amphibians. It is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. These properties aid rapid gas exchange.
In most fish the respiration takes place through gills. (See also aquatic respiration.) Lungfish, however, do possess one or two lungs. The labyrinth fishes have developed a special organ that allows them to take advantage of the oxygen of the air, but is not a true lung.
Anatomy of Respiratory System in Invertebrates
Sponges and Jellyfish
These animals lack specialized organs for gas exchange, instead taking in gases directly from the surrounding water.
Flatworms and Annelids
Air enters the respiratory system 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 tracheal system wihtin 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) .
Physiology of Respiratory System in Mammals
For more detailed descriptions see also Respiratory physiology
of the lungs is carried out by the muscles of respiration.
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 apneustic 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. Its time period is 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.
Inhalation is driven primarily by the diaphragm. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic volume, which in turn causes a decrease in intrathoracic pressure. 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 further expand the thoracic cavity.
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
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 right side of the heart pumps blood from the right ventricle
through the pulmonary semilunar valve
into the pulmonary trunk
. The trunk branches into right and left pulmonary arteries
to the pulmonary blood vessels
. The vessels generally accompany the airways and also undergo numerous branchings. Once the gas exchange process is complete in the pulmonary capillaries
, blood is returned to the left side of the heart through four pulmonary veins
, two from each side. The pulmonary circulation
has a very low resistance, due to the short distance within the lungs, compared to the systemic circulation
, and for this reason, all the pressures
within the pulmonary blood vessels are normally low as compared to the pressure of the systemic circulation loop.
Virtually all the body's blood travels through the lungs every minute. The lungs add and remove many chemical messengers from the blood as it flows through pulmonary capillary bed. The fine capillaries also trap blood clots
that have formed in systemic veins.
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: 1) respiratory acidosis
, a life threatening condition, and 2) respiratory alkalosis
Upon inhalation, 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) in close proximity 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.
The movement of gas through the larynx
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
Panting in dogs
Coughing and sneezing
The phlegm is removed from the body by coughing, and sneezing.
Development of Respiratory System in Animals
Humans and Mammals
The respiratory system lies dormant in the human fetus
. At birth, the respiratory system has under-developed lungs. This is due to the incomplete development of the alveoli type II cells in the lungs, necessary for the production of surfactant
. The infant lungs do not function due to collapse of alveoli caused by surface tension of water remaining in the lungs, which in normal cases would be prohibited by the presence of surfactant. This condition may be avoided by giving the mother a series of steroid
shots in the final week prior to delivery, which will have weard the development of type II alveolar cells.
Disease and the Respiratory System
Disorders of the respiratory system can be classified into four general areas:
- Obstructive conditions (e.g., emphysema, bronchitis, asthma attacks)
- 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 or respiratory physician.
Respiratory System in Plants
Gas exchange in plants
use carbon dioxide
gas in the process of photosynthesis
, and then exhale oxygen
gas, a waste product of photosynthesis. However, plants also sometimes respire as humans do, using oxygen
and producing carbon dioxide.
Plant respiration is limited by the process of diffusion. Plants take in carbon dioxide through holes on the undersides of their leaves known as stomata (sing:stoma). 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.
- Perkins, M. 2003. Respiration Power Point Presentation. Biology 182 Course Handout. Orange Coast College, Costa Mesa, CA.
- Medical Dictionary