We all take expectorating, or coughing, for granted. Whenever there is a tickle in the throat or a great deal of phlegm that you need to get out of your lungs, the cough is an essential part of getting rid of unwanted stuff in the lower tracts of your lungs. But how is it carried out?
The Mechanics of the Cough
Even though the cough may only seem to last for a split-second, it is divided into several separate phases. These all combine to give the various pressure changes in the subglottis (the bit which is a little further down the back of the throat), which will eventually give rise to the cough.
The Inspiratory Phase
This is where you will take a big breath in - the air to be expelled in the actual cough. Here, the pressure in the subglottis is comparatively low. This is then followed by...
The Glottic Phase
The glottis, although sounding like one single entity, is in fact two flaps of muscle that make up the vocal bits of the larynx (or more commonly, the voice box). These flaps close, blocking the only exit for the previously inspired air. Then, the pressure increases in the subglottis as you try to breathe out but with no place for the air to go.
The Explosive Expiratory Phase
When the desired pressure is reached, the glottis opens up, and air is forced out at over 500 mph. Whatever was tickling the airways of the lungs or any phlegm in the lungs should be forced up into the mouth to be spat out or swallowed. The pressure returns to pre-cough levels, and the glottis closes. If whatever is in the airways still hasn't been shifted, then the whole process is repeated until it has.
This may explain how we cough, but not why we cough. For this, we have to look into how our airway nervous system reacts to stuff that really shouldn't be there - in other words, the origin of the reflex cough.
The Reflex Cough
It all starts with the stimulation of two types of receptors: irritant and cough. These themselves are part of a family of stretch receptors that detect a change in the diameter of tubes in the body; ie, blood vessels or airways of the lungs.
These particular stretch receptors are found not on just any airways but are found in the pharynx (the back of the throat) and the bronchi (the divisions of the windpipe).
These receptors would then be stimulated by either:
- Excess phlegm (mucus in the airways)
- An irritating particle or large object that shouldn't be there
- Noxious gases
The signals would then be sent up certain nerves. If the irritation took place in the pharynx, then signals would be sent to the brain via a nerve called the Superior Laryngeal nerve. If instead it was in the bronchi, then signals would be sent via the Vagus nerve.
Either way, signals from the throat indicating that there is something down there which shouldn't be reach the 'Cough Centre' in a part of the brain called the Nucleus Solitary Tract, otherwise known as the medulla oblongata. It is here where the signals are processed, and the signal to cough is given.
The signal to cough is really more of a signal to breathe out violently. In other words, the same muscles that are used in breathing out are used in coughing. Thus signals are sent to the diaphragm via the Phrenic nerve, to the glottis via the Larengeal nerves and to other expiratory muscles via their corresponding spinal nerves (these are nerves coming out of the spinal cord.)
However, this is only one method of coughing. There is another, more deliberate method.
The Voluntary Cough
This is a cough that you consciously initiate. You make a decision to, say, attract someone's attention, so you cough. This decision is made in a part of the brain known as the cerebral cortex, which is in charge of most conscious decisions. You determine how loud you want your cough to be, what pitch and so on. These requirements for the cough are then sent to the Cough Centre, and from then on, it is pretty similar to the reflex cough.
This is then repeated until you attract the person's attention. However, it is not just used as an alternative means of communication, but also a symptom of something much more in the way of illness.
The Darker Side of the Cough
The cough is one of the most common reasons that a person will go and see the doctor about. It is not a disease, but rather an indicator that something else may be wrong. There are two types of cough, determined by how long they last.
Generally, this lasts for less than three weeks and is the one most associated with:
- The common cold
- Influenza, more commonly known as the Flu
Predictably, this lasts for more than three weeks and is not necessarily due to a respiratory (lung-based) problem. For instance, the reasons for this type of cough may be:
- Heart failure
- Chronic bronchitis (swelling of the bronchi)
- 'Smoker's cough'
There are many other reasons far too numerous to mention here.
These coughs can be subdivided into further categories, depending on what you bring up.
The Dry Cough
This starts off as way to get rid of a tickle in the throat, but may typically give rise to a sore throat on repeated attempts on getting rid of the tickle manually. It brings up no phlegm of any kind.
The Clear/Frothy, Non-Smelly Phlegmy Cough
This is a characteristic of viral throat infections ie, the common cold and influenza. Usually accompanied by the symptoms of the infection, like headaches, aching joints, etc. This is the most common reason that people will go to see their doctors to get antibiotics, even though they are pretty useless against viral infections such as these.
The Yellow/Green, Very Smelly Phlegmy Cough
This is a typical sign of a bacterial throat infection; the green sputum (the technical term for spit) with an unwelcoming smell is a sign that something else thinks that your throat and airways are a pleasant place to live and has already taken up residence. This is the type of throat infection that can be treated by antibiotics. Often, to determine the type of organism that has set up house in your throat, the doctor will take a sample of the sputum, by having you spit into a plastic, screw-top container. This is then sent to the labs to see who is the unwelcome squatter.
Final Words - The Smoker's Cough
This type of cough, as suggested by the title, is experienced by smokers of tobacco and other substances that require one to inhale hot gases. This uses the cough mechanism as outlined above, but the original reason why this type of cough is found primarily among smokers, requires an understanding of how the airways normally work to remove small particles.
If you look at a bit of normal airways tissue under the microscope, you will find these features:
Columnar epithelial cells - These are cells which line the airways ('epi-' means surface), and they are, as their name suggests, column shaped; ie, oblong.
Goblet cells - These are the cells which secrete the mucus that traps small particles before they irritate the delicate epithelial cells.
Ciliated epithelial cells - These are the columnar epithelial cells but with small, fine hairs sticking out - the cilia. These have the job of wafting the mucus up to the throat, so that the excess mucus with the bits it has caught can be swallowed and kept out of the airways.
In a smoker's airways, the view from the microscope is very different.
No cilia - this is due to the heat of the smoke that the smoker is inhaling. It kills most of the cilia, so that the 'wafting' process is lost.
Excess mucus secretion - The death of the ciliated cells means that there is more mucus around than usual. This also means that firstly, cancer-precipitating agents in the inhaled tobacco tend to remain in contact with the walls of the airways and irritate them. Secondly, it means that the Goblet cells work overtime to protect the airway walls. This, inevitably, leads to a build up of mucus that isn't being removed.
A change in shape of the epithelial cells - Although not related to the cough process, this is worth mentioning here. The cancer-precipitating, or carcinogenic agents irritate the airway walls, and in order to adapt to the change in environment, the smoker's airways adapt. The columnar epithelia is replaced with squamous epithelia, the cells best described as being flattened yet shaped like a cigar. This change in cell shape (technically known as a change in morphology), although a clever adaptation, is worrying, as it is such changes in cell shape that predispose individuals to cancer.
So now, it is evident why the smoker has to endure the bane of the smoker's cough. There is an excess amount of mucus in the airways, partly due to the death of the mucus-moving cilia, and also to the compensation of the Goblet cells to protect the delicate airway walls. The excess mucus, or phlegm, stimulates the cough receptors, and a reflex cough is initiated. Also, the smoke trapped in the stagnant mucus irritates the airways, stimulating irritant receptors and hence a reflex cough.
If a smoker wants to stop their smoker's cough, the only option is to give up smoking.
On giving up smoking, the airway walls will re-adapt to their new surroundings.
No more particles means less mucus secretion - If there are less irritating particles to trap, then it stands to reason not to secrete so much mucus.
No hot gases - The airways environment is much kinder to cilia after giving up, and there is a return of the mucus-moving cilia, as well as the columnar epithelial cells
More efficient removal of mucus - The airways then return to their normal state, removing mucus with greater efficiency than the crude method of coughing it up.