Respiratory system
The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants.
In land animals, the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs. In mammals and reptiles, these are called alveoli, and in birds, they are known as atria. These microscopic air sacs have a rich blood supply, bringing the air into close contact with the blood. A system of airways, or hollow tubes, allow the air sacs to interface with the external environment; the largest of these is the trachea, which branches in the middle of the chest into the two main bronchi, which enter the lungs and branch into progressively narrower secondary and tertiary bronchi, which in turn branch into numerous smaller tubes known as the bronchioles in mammals and reptiles. In birds, the bronchioles are termed parabronchi. The bronchioles, or parabronchi, generally open into the microscopic alveoli and atria. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.
In most fish, and a number of other aquatic animals, the respiratory system consists of gills, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat filaments and lammellae which expose a very large surface area of highly vascularized tissue to the water.
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 includes anatomical features such as stomata, that are found in various parts of the plant.
Mammals
Anatomy
In humans and other mammals, the anatomy of a typical respiratory system is the respiratory tract. The tract is divided into an upper and a lower respiratory tract. The upper tract includes the nose, nasal cavities, sinuses, pharynx and the part of the larynx above the vocal folds. The lower tract includes the lower part of the larynx, the trachea, bronchi, bronchioles and the alveoli.The branching airways of the lower tract are often described as the respiratory tree or tracheobronchial tree. The intervals between successive branch points along the various branches of "tree" are often referred to as branching "generations", of which there are, in the adult human, about 23. The earlier generations, consisting of the trachea and the bronchi, as well as the larger bronchioles which simply act as air conduits, bringing air to the respiratory bronchioles, alveolar ducts and alveoli, where gas exchange takes place. Bronchioles are defined as the small airways lacking any cartilaginous support.
The first bronchi to branch from the trachea are the right and left main bronchi. Second, only in diameter to the trachea, these bronchi enter the lungs at each hilum, where they branch into narrower secondary bronchi known as lobar bronchi, and these branch into narrower tertiary bronchi known as segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi.
Compared to the 23 number of branchings of the respiratory tree in the adult human, the mouse has only about 13 such branchings.
The alveoli are the dead end terminals of the "tree", meaning that any air that enters them has to exit via the same route. A system such as this creates dead space, a volume of air that fills the airways after exhalation and is breathed back into the alveoli before environmental air reaches them. At the end of inhalation, the airways are filled with environmental air, which is exhaled without coming in contact with the gas exchanger.
Ventilatory volumes
The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. The volume of air moved in or out of the lungs under normal resting circumstances, and volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by spirometry. A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below :Not all the air in the lungs can be expelled during maximally forced exhalation. This is the residual volume of about 1.0–1.5 liters which cannot be measured by spirometry. Volumes that include the residual volume can therefore also not be measured by spirometry. Their measurement requires special techniques.
The rates at which air is breathed in or out, either through the mouth or nose or into or out of the alveoli are tabulated below, together with how they are calculated. The number of breath cycles per minute is known as the respiratory rate. An average healthy human breathes 12–16 times a minute.
| Measurement | Equation | Description |
| Minute ventilation | tidal volume * respiratory rate | the total volume of air entering, or leaving, the nose or mouth per minute or normal respiration. |
| Alveolar ventilation | * respiratory rate | the volume of air entering or leaving the alveoli per minute. |
| Dead space ventilation | dead space * respiratory rate | the volume of air that does not reach the alveoli during inhalation, but instead remains in the airways, per minute. |
Mechanics of breathing
In mammals, inhalation at rest is primarily due to the contraction of the diaphragm. This is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity. When it contracts, the sheet flattens, increasing the volume of the thoracic cavity in the antero-posterior axis. The contracting diaphragm pushes the abdominal organs downwards. But because the pelvic floor prevents the lowermost abdominal organs from moving in that direction, the pliable abdominal contents cause the belly to bulge outwards to the front and sides, because the relaxed abdominal muscles do not resist this movement. This entirely passive bulging of the abdomen during normal breathing is sometimes referred to as "abdominal breathing", although it is, in fact, "diaphragmatic breathing", which is not visible on the outside of the body. Mammals only use their abdominal muscles during forceful exhalation and never during any form of inhalation.As the diaphragm contracts, the rib cage is simultaneously enlarged by the ribs being pulled upwards by the intercostal muscles as shown in Fig. 4. All the ribs slant downwards from the rear to the front ; but the lowermost ribs also slant downwards from the midline outwards. Thus the rib cage's transverse diameter can be increased in the same way as the antero-posterior diameter is increased by the so-called pump handle movement shown in Fig. 4.
The enlargement of the thoracic cavity's vertical dimension by the contraction of the diaphragm, and its two horizontal dimensions by the lifting of the front and sides of the ribs, causes the intrathoracic pressure to fall. The lungs' interiors are open to the outside air and being elastic, therefore expand to fill the increased space, pleura fluid between double-layered pleura covering of lungs helps in reducing friction while lungs expand and contract. The inflow of air into the lungs occurs via the respiratory airways. In a healthy person, these airways begin with the nose. It ends in the microscopic dead-end sacs called alveoli, which are always open, though the diameters of the various sections can be changed by the sympathetic and parasympathetic nervous systems. The alveolar air pressure is therefore always close to atmospheric air pressure at rest, with the pressure gradients because of lungs contraction and expansion cause air to move in and out of the lungs during breathing rarely exceeding 2–3 kPa.
During exhalation, the diaphragm and intercostal muscles relax. This returns the chest and abdomen to a position determined by their anatomical elasticity. This is the "resting mid-position" of the thorax and abdomen when the lungs contain their functional residual capacity of air, which in the adult human has a volume of about 2.5–3.0 liters. Resting exhalation lasts about twice as long as inhalation because the diaphragm relaxes passively more gently than it contracts actively during inhalation.
The volume of air that moves in or out during a single breathing cycle is called the tidal volume. In a resting adult human, it is about 500 ml per breath. At the end of exhalation, the airways contain about 150 ml of alveolar air which is the first air that is breathed back into the alveoli during inhalation. This volume air that is breathed out of the alveoli and back in again is known as dead space ventilation, which has the consequence that of the 500 ml breathed into the alveoli with each breath only 350 ml is fresh warm and moistened air. Since this 350 ml of fresh air is thoroughly mixed and diluted by the air that remains in the alveoli after a normal exhalation, it is clear that the composition of the alveolar air changes very little during the breathing cycle. The oxygen tension remains close to 13–14 kPa, and that of carbon dioxide very close to 5.3 kPa. This contrasts with composition of the dry outside air at sea level, where the partial pressure of oxygen is 21 kPa and that of carbon dioxide 0.04 kPa.
During heavy breathing, as, for instance, during exercise, inhalation is brought about by a more powerful and greater excursion of the contracting diaphragm than at rest. In addition, the "accessory muscles of inhalation" exaggerate the actions of the intercostal muscles. These accessory muscles of inhalation are muscles that extend from the cervical vertebrae and base of the skull to the upper ribs and sternum, sometimes through an intermediary attachment to the clavicles. When they contract, the rib cage's internal volume is increased to a far greater extent than can be achieved by contraction of the intercostal muscles alone. Seen from outside the body, the lifting of the clavicles during strenuous or labored inhalation is sometimes called clavicular breathing, seen especially during asthma attacks and in people with chronic obstructive pulmonary disease.
During heavy breathing, exhalation is caused by relaxation of all the muscles of inhalation. But now, the abdominal muscles, instead of remaining relaxed, contract forcibly pulling the lower edges of the rib cage downwards . This not only drastically decreases the size of the rib cage, but also pushes the abdominal organs upwards against the diaphragm which consequently bulges deeply into the thorax. The end-exhalatory lung volume is now well below the resting mid-position and contains far less air than the resting "functional residual capacity". However, in a normal mammal, the lungs cannot be emptied completely. In an adult human, there is always still at least 1 liter of residual air left in the lungs after maximum exhalation.
The automatic rhythmical breathing in and out, can be interrupted by coughing, sneezing, by the expression of a wide range of emotions and by such voluntary acts as speech, singing, whistling and the playing of wind instruments. All of these actions rely on the muscles described above, and their effects on the movement of air in and out of the lungs.
Although not a form of breathing, the Valsalva maneuver involves the respiratory muscles. It is, in fact, a very forceful exhalatory effort against a tightly closed glottis, so that no air can escape from the lungs. Instead, abdominal contents are evacuated in the opposite direction, through orifices in the pelvic floor. The abdominal muscles contract very powerfully, causing the pressure inside the abdomen and thorax to rise to extremely high levels. The Valsalva maneuver can be carried out voluntarily but is more generally a reflex elicited when attempting to empty the abdomen during, for instance, difficult defecation, or during childbirth. Breathing ceases during this maneuver.