bronchitis means inflammation of the bronchialtubes in the lung, and it’s said to be chronic when it causes a productive cough—whichmeans produces mucus—for at least 3 months each year for 2 or more years. chronic bronchitis is actually lumped underthe umbrella of chronic obstructive pulmonary disease (or copd), along with emphysema. these two are different in that chronic bronchitisis defined by clinical features, like a productive cough, whereas emphysema is defined by structuralchanges—specifically enlargement of the air spaces. that being said, they often coexist, probablybecause they share the same major risk factor
— smoking. other risk factors for chronic bronchitisinclude exposure to air pollutants like sulfur and nitrogen dioxide, exposure to dust andsilica, as well as genetic factors like having a family history of chronic bronchitis. with copd, the airways become obstructed,and the lungs don’t empty properly, and that leaves air trapped inside the lungs. for that reason, the maximum amount of airpeople with copd can breath out in a single breath, known as the fvc, or forced vitalcapacity, is lower. this reduction is especially noticeable inthe first second of air breathed out in a
single breath, called fev1—forced expiratoryvolume (in one second), which typically is reduced even more than the fvc. a useful metric therefore is the fev1 to fvcratio, which, since the fev1 goes down even more than fvc, causes the fev1 to fvc ratioto go down as well. alright so say normally your fvc is 5 l, andyour fev1 is 4 l, your fev1 to fvc ratio would end up being 80%. now, someone with copd’s fvc might be 4l instead, which is lower than normal, but the volume of air that he or she can expirein the first second is only 2 l, so not only are both these values lower, but their ratiois lower as well—and this is a hallmark
of copd. all that had to do with air breathed out right? conversely, for air going in, the tlc, ortotal lung capacity, which is the maximum volume of air that can be taken in or inspiredinto the lungs, is actually often often higher because of the air trapping. alright, so chronic bronchitis is a type ofcopd that’s diagnosed based on clinical symptoms, specifically coughing up a lot ofmucus. but why does this happen? well, first off, in the lungs, the walls ofnormal airways have a couple layers to think
about. lining the lumen of the airways you’ve gotthe epithelium, composed of ciliated pseudostratified columnar epithelial cells, which are namedthat because these epithelial cells have hair-like projections called cilia, their nuclei don’talign so it looks like they’re more than one layer even though they’re not, hence,pseudostratified, and because the cells are mostly tall and narrow - or columnar in shape. this layer also contains the occasional gobletcell which makes some of the mucus that lines the airway. going deeper past that layer you’ve gotthe basement membrane and loose connective
tissue, called the lamina propria—whichtogether with the epithelium makes up the mucosa. beyond the mucosa, there is smooth musclefollowed by more connective tissue, and together, these two layers make up the submucosa andthis is where the bronchial mucinous glands live. these are the glands that secrete the majorityof the mucus into the lumen of the bronchi, helping to catch and filter out particlesand pathogens. finally, in the bronchi, but not the bronchioles,there is also a layer of cartilage below the submucosa which stiffens the bronchus andhelps to keep it open.
alright so people who smoke expose their airwaysto all sorts of irritants and chemicals. whatever the irritants are, their effect isto stimulate hypertrophy and hyperplasia of the mucinous glands in the main bronchi, aswell as the goblet cells in the smaller airways - the bronchioles, which increases mucus productionin both locations. since the bronchioles are smaller, even aslight increase in mucus can lead to airway obstruction, so this contributes to the majorityof the air trapping. to make matters worse, though, smoking makesthe cilia short and less mobile, making it harder to move mucus up and out of bronchiolestowards the back of the throat to get swallowed. as a result of having too much mucus and poorlyfunctioning cilia, people with chronic bronchitis
end up relying on coughing to get rid of theirmucus plugs. one measurement, typically done post-mortem,is called the reid index, which is the ratio of the thickness of the bronchial mucinousglands, relative to the total thickness of the airway - from the epithelium to the cartilage. normally, this ratio should be less than 40%,but it can be over 40% for people with chronic bronchitis, because of the hyperplasia andhypertrophy of the glands. even though a decreased reid index goes alongwith chronic bronchitis, the diagnosis is still done clinically and this measurementis not usually used diagnostically. all this mucus in the lungs causes peoplewith chronic bronchitis to wheeze due to narrowing
of the passageway available for air to movein and out, these people also have crackles or rales caused by the popping open of small airways. people with chronic bronchitis also oftenpresent with hypoxemia, low oxygen in the blood, and hypercapnia, increased carbon dioxidein the blood. this is because the mucus plugs in the airwaysblock air flow, right? which causes the partial pressure of co2 toincrease in the lungs. increased pco2 means that the partial pressureof o2 in the lungs goes down, and a lower po2 means less oxygen gets to the blood, causinghypoxemia. this trapped co2 in the lungs also makes itharder for co2 to get out of the bloodstream,
which also explains the hypercapnia. the increased co2 levels in the blood canget so bad that some people develop cyanosis, which is a blue discoloration of the skin,and this is why patients with chronic bronchitis are sometimes referred to as blue bloaters. this is compared to the term pink pufferswhich describes patients with emphysema. alright so in addition to those things, inthe the areas with decreased gas exchange, blood vessels undergo vasoconstriction inan attempt to shunt blood to an area with better exchange; which, if it’s localizedto one area of the lungs, that would work pretty well, but when a large proportion ofthe lungs aren’t exchanging oxygen effectively,
a large proportion of blood vessels startto clamp down. and this has the effect of increasing pulmonaryvascular resistance, and to maintain pulmonary blood flow the body responds by developingpulmonary hypertension. over time, this increases the work neededby the right side of the heart to pump blood to the lungs, and eventually the right sideenlarges, leading to right-sided heart failure, a process called cor pulmonale. and finally another consequence of mucus pluggingin chronic bronchitis is that people can develop lung infections behind the mucus blockagesin the airway, and these infections can worsen the pulmonary and cardiac symptoms.
treatment of chronic bronchitis largely involvesreducing risk factors, like for example stopping smoking, but also managing associated illnesses. supplemental oxygen, as well as certain medicationslike bronchodilators, inhaled steroids, and antibiotics to control secondary infectionsmight also be used. okay, to recap, chronic bronchitis is a typeof chronic obstructive pulmonary disease or copd, where exposure to chemicals and irritants—likewith smoking—stimulates increased mucus production in the airways, which causes aproductive cough that lasts for at least 3 months each year for at least 2 years. thanks for watching, you can help supportus by donating on patreon, subscribing to
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