During a breathing cycle, air moves in and out of the lungs by bulk flow. The respiratory muscles are responsible for the changes in the shape and volume of the chest cavity that cause the air movements in breathing.
Inspiration or inhalation is an active process that occurs when the chest cavity enlarges because of the contraction of the muscles. The dome-shaped diaphragm is the most important muscle at this stage. At the start of inspiration, the diaphragm contracts and flattens, pressing down on the abdominal contents and lifting the ribcage.
This increases the vertical height of the thoracic cavity Fig 1. At the same time, the external intercostal muscles between the ribs contract and lift the ribcage and pull the sternum forward, thus increasing the front-to-back and side-to-side dimensions Figs The outer parietal layer of the pleura is attached to the diaphragm and the inside of the chest wall, and so moves with those structures. As a result, the inner visceral layer of the pleura, which is attached to the surface of the lungs, follows and the lungs expand, that is their volume increases.
The air in the lungs now has a larger space to fill and so its pressure falls. This produces a partial vacuum, which sucks air into the lungs by bulk flow. Air continues to move into the lungs until the intrapulmonary pressure is the same as atmospheric pressure. During forced inspiration, the accessory muscles in the neck may also be used to elevate the sternum and first two ribs Fig 1.
This, combined with the maximal contraction of the inspiratory muscles, leads to the generation of a much more negative intrapleural pressure for example, cmH2O and more rapid airflow McGeown, Normally about ml 1 pint of air is moved in and out per breath - this is known as the tidal volume.
In healthy people quiet expiration or exhalation is passive and relies on elastic recoil of the stretched lungs as the inspiratory muscles relax, rather than on muscle contraction.
The diaphragm and external intercostal muscles return to their resting position and the volume of the chest cavity and of the lungs decreases. Thus, air is driven out of the lungs by bulk flow, until the atmospheric pressure and pressure within the alveoli are equal. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment. Air rushes out of the lungs due to the pressure gradient between the thoracic cavity and the atmosphere.
This movement of air out of the lungs is classified as a passive event since there are no muscles contracting to expel the air. Each lung is surrounded by an invaginated sac.
The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax. The space between these layers, the intrapleural space, contains a small amount of fluid that protects the tissue by reducing the friction generated from rubbing the tissue layers together as the lungs contract and relax.
If these layers of tissues become inflamed, this is categorized as pleurisy: a painful inflammation that increases the pressure within the thoracic cavity, reducing the volume of the lung. Visceral pleura : A tissue layer called pleura surrounds the lung and interior of the thoracic cavity. Types of breathing in humans include eupnea, hyperpnea, diaphragmatic, and costal breathing; each requires slightly different processes.
There are different types, or modes, of breathing that require a slightly different process to allow inspiration and expiration. All mammals have lungs that are the main organs for breathing. During exhalation, the lungs expel air and lung volume decreases. The various types of breathing, specifically in humans, include:. During eupnea, also referred to as quiet breathing, the diaphragm and external intercostals must contract.
As the diaphragm relaxes, air passively leaves the lungs. This type of breathing is also known as deep breathing. Diaphragmatic breathing : Animation of a diaphragm exhaling and inhaling, demonstrating diaphragmatic breathing.
During inhalation, the diaphragm is contracted which increases the volume of the lung cavity. During exhalation, the diaphragm is relaxed which decreases the volume of the lung cavity. As the intercostal muscles relax, air passively leaves the lungs. This type of breathing is also known as shallow breathing. During hyperpnea, also known as forced breathing, inspiration and expiration both occur due to muscle contractions. In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract.
During forced inspiration, muscles of the neck, including the scalenes, contract and lift the thoracic wall, increasing lung volume. During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm.
This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles primarily the internal intercostals help to compress the rib cage, which also reduces the volume of the thoracic cavity. In animals such as amphibians, there have been multiple ways of breathing that have evolved. In young amphibians, such as tadpoles that do not leave the water, gills are used to breathe.
There are some amphibians that retain gills for life. As the tadpole grows, the gills disappear and lungs grow. These lungs are primitive and not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing via lungs is forced.
The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist. Other animals, such as birds, must face a unique challenge with respect to breathing, which is that they fly. Flying consumes a large amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes.
They have evolved a respiratory system that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, while carbon dioxide diffuses from the blood into the lungs and is expelled during exhalation.
However, the details of breathing between birds and mammals differ substantially. In addition to lungs, birds have air sacs inside their body that are attached to the lungs. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, which allows efficient gas exchange. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low.
This directionality of airflow requires two cycles of air intake and exhalation to completely remove the air from the lungs. Avian respiratory system : a Birds have a flow-through respiratory system in which air flows unidirectionally from the posterior sacs into the lungs, then into the anterior air sacs.
The air sacs connect to openings in hollow bones. Breathing includes several components, including flow-resistive and elastic work; surfactant production; and lung resistance and compliance. Explain the roles played by surfactant, flow-resistive and elastic work, and lung resistance and compliance in breathing. Pulmonary ventilation involves three different pressures:. Atmospheric pressure is the pressure of the air outside the body.
Intraalveolar pressure is the pressure inside the alveoli of the lungs. Intrapleural pressure is the pressure within the pleural cavity. These three pressures are responsible for pulmonary ventilation. Inspiration inhalation is the process of taking air into the lungs.
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