The airways and small air sacs of the lungs(alveoli) are constantly exposed to microorganisms such as bacteria and viruses. We inhale small amounts of these organisms on a regular basis. Normally these organisms are readily dealt with several defense mechanisms in the respiratory system, some of which include
1The cough reflex, which helps expel mucus and foreign substances
2
The cells lining the respiratory passages, which provide barrier and trap the microorganisms by their secretions
3The hairy cells of the respiratory passages, which help expel the microorganisms trapped in secretions
4Secreted proteins and other factors by lung cells, which attack the microorganisms
5Caretaker immune cells within the lungs and blood-derived immune cells that attack the microorganisms
However, in case of COVID19, the SARS-CoV2 virus being a new virus and very small in size, evades some of these defense mechanisms. It uses a spike-like protein to efficiently bind with a receptor called angiotensin-converting enzyme 2 (ACE-2) found on the surface of cells. Following binding, it injects the viral genome into the cells. The injected viral genome then hijacks the cell machinery to produce more copies of themselves, which then come out and spread to new cells. This first takes place in the upper respiratory tract, which includes the nose and back of throat. Symptoms become severe once the virus starts making its way to the lower respiratory tract, where the virus directly invades cells lining the air sacs (pneumocytes) and interferes with normal lung function causing Pneumonia This is experienced by the patient as breathlessness
Cough is the second most common symptom of COVID19, fever being the first. Some people infected with the virus have no symptoms in the beginning of the illness but majority of COVID19 patients eventually show high fever, dry cough, and shortness of breath. Those aged over 60 years, and those with underlying medical problems like cardiovascular disease, hypertension, diabetes, chronic respiratory disease, kidney disease and cancer are more likely to develop serious respiratory symptoms such as pneumonia. Children are also susceptible for SARS-CoV2 infection and present with fever and cough. However, a significant number of children do not seem to show severe symptoms and can easily transmit the virus to vulnerable people.
The pathophysiology of the COVID19 infection varies with patients and can be divided into at least four categories,
1) The uncomplicated (mild) illness which usually presents with symptoms of an upper respiratory tract viral infection, including mild fever, cough (dry), sore throat, nasal congestion, headache and muscle pain without shortness of breath
2) Moderate pneumonia, which results in respiratory symptoms such as cough and shortness of breath
3) Severe pneumonia is when the fever is associated with severe difficulty in breathing and decreased blood oxygen levels and the last stage
4)Acute Respiratory Distress Syndrome (ARDS) displaying varying degrees of respiratory failure.
Virus infection of the air sac cells in lungs and white blood cells leads to hyperactive responses and destruction of the tissues. The immune reaction also damages membrane separating the air sacs cells and blood vessels, eventually filling lungs with fluid and affecting the lung’s ability to oxygenate blood, leading to ARDS. Once this occurs, patients have to be put on mechanical ventilators to assist their breathing and specialized methods to ensure blood oxygen levels.