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1. Pulmonary ventilation
Air inside the lungs is exchanged with fresh air on the outside.
2. External Respiration
Fresh air in the lungs is moved into the blood, and used air in the blood is moved into the lungs to be removed.
3. Respiratory Gas Transport
The circulatory system pumps the blood into which the fresh air has been moved throughout the body.
4. Internal Respiration
The cells of your body remove air from your red blood cell and move the carbon dioxide into them.
The subsystem that removes carbon dioxide from the lungs in moves in fresh air from outside is made up of the nasal cavity ( nose ), the pharynx , the larynx , the trachea , the bronchi (and all the smaller branches of the bronchi), and the air sacs, or alveoli, to which the entire external respiration subsystem leads to. The respiration zone consists of the bronchioles (not the large bronchi), the alveolar ducts, and the alveoli, all of which basically make up the lungs. This is where the oxygen and carbon dioxide are exchanged. All other organs in the external respiration subsystem make up the conducting zone . During the trip that air takes through the conducting zone, it is humidified, cleaned, and warmed so that it does not harm any of delicate organs that it passes through. When the air finally reaches the alveoli, it is closer to the air in the tropics, which is the kind of air that your lungs prefer.
The nose is the first and last organ that air passes through. The nose serves some very most important functions. As part of the conducting zone, it cleans the air of dust and other impurities, warms the air if it is too cool, and moistens the air if it is dry. Though not related to respiration, your nose also helps you to speak, and is the organ that gives you the power to smell.
After passing through the external nares (nostrils), it passes through the nasal cavities. Your nasal septum separates the two nasal cavities. Immediately after passing through the nostrils into the nasal cavities, the air begins to be purified, humidified and warmed. The skin of the vestibule, the part of the nasal cavities behind the nostrils, has sebaceous and sweat glands and hair follicles, which catch the dirt or other impurities that may be in the air. The hair growing out of the follicles are called vibrissae.
The olfactory mucosa is what detects scents that you inhale. The serous glands excrete enough lysozyme, an enzyme that destroys bacteria, to keep the air you breathe mostly pure. That is about a quart a day. It will kill the bacteria that is caught by the vibrissae.
In the pharynx, the cilia moves bacteria up away from the lungs so that you can swallow it into your stomach, where the bacteria can do little harm. Because of the shape of your organs, air swirls and twists as it moves down. This make sit virtually impossible for impurities to not make contact with the mucous lining your organs. This will catch most particles larger than 4µm.
The pharynx, most commonly known as the throat, serves duel purposes. Not only does it move the air into your lungs, but it also moves food into your stomach. About five inches long, the pharynx is separated into three distinct regions, chosen by location and function: the nasopharynx, the oropharynx, and the laryngopharynx.
The nasopharynx is located above the part of the pharynx that food enters. At the base of the nasopharynx are the soft palate and its pendulous uvula. When swallowing, there is a dangerous possibility: that food will enter into your nasopharynx and nose. This would disrupt severely your breathing. So, when you swallow, the soft palate and its pendulous uvula point upwards blocking of the nasopharynx so that neither air nor food can pass through it.
The auditory, equally poorly known as the eustachian, connects the middle ear to the wall of the nasopharynx so that ear pressure can be equalized. Infections in the nasopharynx are commonly followed by ear infections because of this.
The mouth leads into the oropharynx. The mucous lining the walls of the oropharynx change slightly to adapt for handling food as well as air. It is here that the two tonsils are located. One is at the entrance from the mouth into the oropharynx, and the other is somewhat deeper.
The laryngopharynx too serves as a common passageway for both food and air. At the base of the laryngopharynx is the esophagus, which directs food and air to where they should be. Sometimes it can get confused and make mistakes. Swallowing air can lead to burping more often. Inhaling food or liquid causes you to cough until it is expelled.
Also known as the voice box, the pharynx is what allows you to speak. The larynx has an inlet at the top that allows substances to pass through it or not. When food is being swallowed, the inlet is closed, forcing food into the stomach. When air is being breathed, the inlet is wide open so that air can enter your lungs.
With the exception of the epiglottis, all larynx cartilage is hyaline cartilage. The Adam's apple is really the laryngeal prominence, where the curved disc shaped thyroid cartilage bond.
The trachea, or windpipe connects the larynx to the bronchi. This organ differs from others in the neck in that it is flexible, stretching to be between four and five inches long, and about one inch in diameter. The trachea is lined with mucous called the mucociliary escalator, which represents the mucous and cilia and carry the foreign substances up to be swallowed.
The trachea is made up of between 16 and 20 cartilage rings in the shape of a "C". Because the trachea is so flexible and twistable, without these cartilage rings, it would collapse under the partial vacuum formed when inhaling. The open part of the "C" shape is covered with the Trachealis muscle, which can stretch itself to prevent tracheal tearing when swallowing large things. When you cough, the muscle also contracts to force air out at a faster speed to dislodge food or other foreign objects stuck.
The trachea branches off into two main bronchi, your left and right primary bronchi, which lead to the left and right lung respectively. Your right lung is slightly wider, shorter, and taller that the left, which makes it more vulnerable to foreign invasion. At this point in breathing, the air has been moistened, purified and warmed.
Each bronchi enters its lung and begins on a series of branches, called the bronchial or respiratory tree. The first of these branches is the lobar (secondary) branch. On the left, there are two lobar branches, while on the right, there are three. Each lobar branches into one lobe. The next branch is called the segmental (tertiary) branch. Each branch continues to branch into smaller and smaller bronchioles. The final branch is called the terminal bronchioles. These bronchioles are smaller than 0.5 mm in diameter.
The first few levels of bronchi are supported by rings of cartilage. Branches after that are supported by irregularly shaped discs of cartilage, while the latest levels of the tree have no support whatsoever.
Respiration begins when the terminal bronchioles lead into the respiratory bronchioles. These bronchioles are covered with thin-skinned air sacs that allow for gasses to pass through them. These sacs, which contain alveoli, are called alveolar sacs, and are at the end of alveolar ducts. The alveoli are very small curves in the sac walls. Your lung has many millions of alveoli, which gives your lungs an incredible surface area for gas exchange. Though fairly impossible to measure exactly, that surface area is approximated to 70 - 80 square meters, or a square between eight and nine meters on each side!
The alveoli are covered in inter linking capillaries through which blood flows. The alveoli and the capillary walls form the respiratory membrane. Your lungs rely simply on diffusion to exchange the gasses, and that moves enough gas to have a steady supply of oxygen in your body. The respiratory membrane is only 0.5 - 1µm thick, so diffusion happens pretty efficiently. For maximum efficiency, the amount of blood passing though a capillary on an alveoli and the amount of gas exchange should match precisely. When there is not enough gas in that alveoli, certain pulmonary vessels tighten, slowing the flow of blood, which causes more blood to flow elsewhere. When there is a lot of gas exchange happening, those vessels widen, allowing for more blood to pass though. A similar process happens to bronchioles. When an alveoli has a lot of carbon dioxide in it, the bronchioles that connects it to the outside air widen, allowing it to leave more quickly.
With the lungs coming in direct contact with the air, you would think that it could supply it's own blood supply. This isn't so. The bronchial arteries, which branch from the aorta, supply the lungs with oxygen, and the bronchial and pulmonary veins take old blood away.
The Pleurae is a thin, double-layered tissue which lines the walls of the lungs and heart«link» . Due to the fact that it produces pleural fluid, the pleurae helps the lungs to glide easily against the rib-lining tissues, the thoracic wall, when the lungs take in air. Also, the pleural is essential to breathing because it serves as potential space. This important function helps the lungs form a vacuum which sucks in air from the atmosphere. In addition, its capability to stretch and divide the lungs into two compartments, a lower lung and a upper lung, allows other organs to move without interfering with respiration.
As mentioned before, respiration is the result of a vacuum formed inside your lungs. Your lungs themselves don't have muscles, so they cannot force themselves to expand. Instead of each alveoli or bronchiole having a muscle, you have one big muscle, the diaphragm. The diaphragm lines the lower part of your chest cavity, sealing it off air-tight from the rest of your body. When you want to inhale, your dome-shaped diaphragm contracts, straightening itself out. This lowers the pressure in your chest cavity causing air outside your lungs to rush in to fill the space. Though air can expand, and does for a short time, the low pressure inside pulls in air to equalize the pressure. Your lungs, connected at the ends by cartilage, expand, stretching the cartilage to allow room for the lungs to hold air. With the ribs expanding outward only a few millimeters, and the diaphragm lowers only a few millimeters, this increases the volume of the chest cavity by about half a liter, which is the average inhaling amount.
The exact opposite is the cause for expiration. When the diaphragm relaxes, moving upwards, the chest cavity becomes less in volume, raising air pressure inside the lungs, forcing air out into the atmosphere. Muscles, such as the diaphragm, cannot push out, but only contract. When you inhale, different tissues in your chest cavity stretch. Relaxing the diaphragm allows them to return to normal size, which raises the pressure, thus forcing air out. That is quiet respiration. If you need to exhale quickly, such as in coughing and sneezing, your abdominal walls can contract, doing the same things that the stretched tissues do.
That elasticity is incredibly important. Sufferers of emphysema can testify to this personally. Their lungs of lost most of their original elasticity, so they must actively breath out instead of it happening automatically. The triples or even quadruples the amount of energy required to breath, which is a reason so many emphysema sufferers have little energy most of the time.
Dangers of surface tension
Blood, like any liquid, has a tendency to form a surface that is reluctant to break and mix with other substances. The danger comes in that if the blood pushes on the alveoli instead of passing through it, it could easily collapse every sac in your lungs (which is what happens in IRDS, or infant respiratory distress syndrome, because the premature infants have commonly not developed far enough to hold the sacs patent, or open continuously). Though this is not fatal, having to completely reinflate your lungs is very draining. To break the surface tension of the blood, a surfactant is released. A surfactant acts like the soap you wash your hands and clothes with. The soap breaks the surface of the water allowing it to wash more effectively and pass through the minute openings between fiber strands.
Internal respiration is the exchange of carbon dioxide and oxygen in the cells of the body. It happens in much the same way as gas exchange in the lungs, diffusion. Red blood cells carry the oxygen to the body, and brings back the carbon dioxide to the lungs. See the Circulatory System for more details.
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