Both phlegm and snot, or nasal mucus to use its more polite medical term, are names given to mucus produced in the respiratory system. Nasal mucus, as the name suggests is produced at the back of the nose, while phlegm is used to describe the mucus from the rest of the respiratory system and is typically used to describe the gloop that is coughed up. Contrary to what you might think, mucus is vital for good health. It helps to protect the lungs and also prevents tissues from drying out. It's a water-based liquid that contains proteins, carbohydrates, salt and some cells. The primary type of protein are the mucins, which have a sugar coating that enables them to absorb large quantities of water. This gives mucus its characteristic consistency and wetting properties. Apart from the mucins, the other proteins present hold the key to the protective functions of mucus. These include antibodies that kick start the body’s defences against invading pathogens (disease causing agents such as bacteria, viruses and fungi) and antiseptic enzymes, such as lysozyme, that can directly kill bacteria. Mucus can also protect the lungs because it is sticky meaning that it can trap particles of dust, dirt, bacteria and pollen that could otherwise cause irritation and infection. But once the particles are firmly lodged like this they need to be disposed of and this is achieved by coughing, sneezing and nose-blowing. The average sneeze can propel a mucus missile and its microbial passengers at up to 100 miles per hour, hence the saying "coughs and sneezes spread diseases", and as well as sneezing there is of course nose blowing. But much to the disgust of many a reader, the vast majority of our mucus is in fact eaten! Our airways are lined with millions of tiny hairs, called cilia. These beat in synchrony to produce waves of movement, a bit like how a Mexican wave moves around a football stadium. These waves sweep the mucus to the back of the throat where it is swallowed. Stomach acid then takes care of most of the things inside that could be infectious. But if the mucus dries out and hardens before it can be ferried to the throat it can produce an unsightly bogey! Mucus is secreted by mucous membranes, of which there are many on the human body. Mucous membranes are a lining of cells, continuous with the skin, which protect the cavities and canals of the body that come into contact with the outside world, including our lips, ears, nostrils, mouth, digestive tract, genitals and anus. Within the mucous membrane is a specialised cell type which, due to its characteristic shape, is known as a goblet cell. Goblet CellThe sole function of the goblet cell is to produce mucus, although other cells can make it too. The mucin proteins are packaged in small sacks, called vesicles, which then make their way to the edge of the cells. They subsequently fuse with the membrane (plasma membrane) that surrounds the cell, enabling the contents to be released. As the mucin is released and soaks up water it can expand up to 600 times, which is potentially a lot of snot! Indeed, it is estimated that a healthy nose will pump out more than a pint of mucus a day, although the amount can vary enormously; for example when you cry most of your tears will run into the nose and mix with the mucus, increasing its volume. This is why crying is often accompanied by a runny nose. Also, as one of the roles of mucus is to remove particles, mucus production is stimulated by airway infection and irritation. For example, hayfever sufferers may experience a runny nose as the body attempts to flush out the problem pollen that's triggering the effect. Rhinitis like this occurs when the body’s immune system responds to an allergen such as pollen by releasing the inflammatory chemical histamine, which instructs the mucous membranes to increase their mucus production.
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