The nose allows you to make scents of what's going on in the world around you. Just as your eyes give you information by seeing and your ears help you out by hearing, the nose lets you figure out what's happening by smelling.
It does this with help from many parts hidden deep inside your nasal cavity and head. Olfactory is a fancy word that has to do with smelling. The olfactory epithelium contains special receptors that are sensitive to odor molecules that travel through the air.
These receptors are very small — there are about 10 million of them in your nose! There are hundreds of different odor receptors, each with the ability to sense certain odor molecules. Research has shown that an odor can stimulate several different kinds of receptors. The brain interprets the combination of receptors to recognize any one of about 10, different smells. When the smell receptors are stimulated, signals travel along the olfactory nerve to the olfactory bulb.
The olfactory bulb is underneath the front of your brain just above the nasal cavity. Signals are sent from the olfactory bulb to other parts of the brain to be interpreted as a smell you may recognize, like apple pie fresh from the oven.
Identifying smells is your brain's way of telling you about your environment. Have you ever smelled your toast burning? In an instant, your brain interpreted the smell and a problem and you knew to check on your toast. You learned to associate a certain smell with burning and now your brain remembers that smell so you recognize it. Your sense of smell also can help you keep safe. For example, it can warn you not to eat something that smells rotten or help you detect smoke before you see a fire.
Most people just think of the tongue when they think about taste. But you couldn't taste anything without some help from the nose! The ability to smell and taste go together because odors from foods allow us to taste more fully. The primary function of the lungs is breathing. The structure of the lung has evolved to perform this task very efficiently. The basic structure of the lungs is a series of hollow tubes, called airways.
Airways branch and get smaller, ending in tiny air sacs called alveoli. Alveoli are covered in small blood vessels named capillaries, which function to exchange gases in the blood [ 1 ].
The lungs can be compared to a tree that has a large, main trunk with branches becoming smaller the further away they are from the trunk. The alveoli are like the leaves. There are two main regions in the lung: the conducting zone and the respiratory zone Figure 1. Air is carried into the lungs through the conducting zone and brought to the alveoli-containing respiratory zone. The alveoli from one person have the same surface area as a tennis court, which is plenty of area to allow for gas exchange.
The conducting zone is the part of the lung that air flows through without any gas exchange, because the tubes are too thick for the gases to move across. The conducting zone is lined with hair-like structures called cilia that are covered in mucus , which helps trap potentially dangerous materials. Cilia are mobile, tiny, finger-like projections on the surface of airway cells. Cilia line the airways and help move mucus up and out of the lungs [ 5 ]. Cilia are about 6—7 micrometers tall, or roughly a tenth the width of a human hair [ 3 , 5 ].
There are many types of particles that could enter the lungs and cause damage. Some of these particles are pollutants, such as emissions from gas-powered vehicles, carbon monoxide from fireplaces, toxins from vaping or smoking, and aerosols like hairspray. Particles can get trapped in various areas of the lung. Short hairs, like the kind on top of your head, line the insides of the nostrils and are covered with mucus. These mucus-covered hairs help trap larger particles as they enter the nose.
Particles that enter the conducting zone can hurt the airway cells, which may reduce the movement of their cilia and lead to a buildup of mucus that cannot be cleared from the airways [ 2 ]. Mucus-clogged airways may not allow air to travel as effectively, like the way leaves in a gutter interfere with water flow.
Smaller particles can sometimes get all the way to the alveoli. Damage to the alveoli makes breathing considerably more difficult, because oxygen will not diffuse as well into the blood. This reduced efficiency of gas exchange may cause the rest of the body to be hypoxic, which means low in oxygen.
Only the conducting zone, from the trachea to the bronchioles, has the ability to move mucus using cilia, so alveoli cannot rely on this mechanism to get rid of these small particulates [ 3 ].
The mucociliary escalator is inside of the conducting airways and is made up of mucus and cilia, which moves the mucus up and out of the lungs where it can be expelled by coughing or swallowing Figures 1 , 2 [ 5 ]. As the first line of defense, airway mucus is made up of different components that help it trap particles and germs [ 2 , 4 ].
But sometimes the mucus is produced in excess and changes in nature. This results in the urge to cough and expectorate this mucus as sputum. Sputum expectoration is not normal and there is always an underlying pathological cause. Mucus is secreted from two distinct areas within the lung tissue. In the surface epithelium, which is part of the tissue lining of the airways, there are mucus-producing cells called goblet cells.
The connective tissue layer beneath the mucosal epithelium contains seromucous glands which also produce mucus. The respiratory tract produce about two litres of mucus a day from these glands Martini, , and this is composed of water, carbohydrates, proteins and lipids. The high water content helps to humidify the passing inspired air.
Mucus contains glycoproteins or mucins as well as proteins derived from plasma, and products of cell death such as DNA. Mucus is sticky and this helps to trap dust particles, bacteria and other inhaled debris. Mucus also contains natural antibiotics, which help to destroy bacteria - the epithelial cells secrete a substance called defensis.
Mucus also contains lysozyme, which is an antibacterial enzyme. Cilia in the nose move the mucus formed there towards the throat where it is swallowed and digested in the stomach. In cold weather, this process slows and the mucus sometimes gathers in the nose and drips or dribbles out - a winter runny nose. Particles larger than 4mm in diameter usually become trapped in mucus in the nose and rarely get any further down the airways.
The nasal mucosa has many sensory nerve endings and large particles irritate these nerves, stimulating a sneeze - a violent burst of air - which expels the particles along with mucus. Further down the airways, cilia in the trachea and bronchi also waft the mucus towards the pharynx to be swallowed. This movement, against the force of gravity, is sometimes called the mucus escalator. Normally, this upward movement is not noticeable, except when we clear our throats. However, if larger quantities of mucus build up, the cough receptors may be stimulated and air and mucus will be forcibly expelled from the trachea.
Moving down the airway, the mucosal epithelium gets thinner and changes in nature. There are only a few cilia and no mucus-producing cells in the bronchioles, so any airborne debris is removed by macrophages in the alveoli or coughed out.
Irritation of the respiratory system causes both inflammation of the air passages and a notable increase in mucus secretion. A person may become conscious of swallowing the mucus or the inflammation may trigger a coughing reflex so that they expectorate these secretions as sputum. It seems that the inflammation of the mucosa is responsible for sputum production rather than any of the other changes that occur in diseased lung tissue Jeffrey Maestrelli et al, Expectorated sputum contains lower respiratory tract secretions, as well as secretions from the nose, mouth and pharynx, and cellular debris and micro-organisms Rubin, In some disease processes, the sputum changes in nature and colour.
Sputum production is associated with many lung disease processes and sputum may become infected, stained with blood or contain abnormal cells. Smoking - Smoking has many effects on the airways. Inhaled smoke destroys the cilia that are important for moving mucus to the throat for swallowing. As a result, mucus accumulates in the bronchioles and irritates the sensitive tissues there, causing a cough.
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