Explainer: Here’s what the coronavirus does to the body

As the number of confirmed cases of Covid-19 surges past 2.5 million globally and deaths surpass 170,000, clinicians and pathologists are struggling to understand the damage wrought by the coronavirus as it tears through the body.

They are realising that although the lungs are ground zero, its reach can extend to many organs including the heart and blood vessels, kidneys, gut, and brain.

Here’s what coronavirus does to the body.

The onset of the infection

When an infected person expels virus-laden droplets and someone else inhales them, the novel coronavirus, called SARS-CoV-2, enters the nose and throat.

It finds a welcome home in the lining of the nose, according to a preprint from scientists at the Wellcome Sanger Institute and elsewhere.

They found that cells there are rich in a cell-surface receptor called angiotensin-converting enzyme 2 (ACE2).

Throughout the body, the presence of ACE2, which normally helps regulate blood pressure, marks tissues vulnerable to infection, because the virus requires that receptor to enter a cell.

Once inside, the virus hijacks the cell’s machinery, making myriad copies of itself and invading new cells.

The lungs: Ground zero

If the immune system doesn’t beat back SARS-CoV-2 during this initial phase, the virus then marches down the windpipe to attack the lungs, where it can turn deadly.

The thinner, distant branches of the lung’s respiratory tree end in tiny air sacs called alveoli, each lined by a single layer of cells that are also rich in ACE2 receptors.

Normally, oxygen crosses the alveoli into the capillaries, tiny blood vessels that lie beside the air sacs; the oxygen is then carried to the rest of the body.

For most patients, Covid-19 begins and ends in their lungs, because like the flu, coronaviruses are respiratory diseases.

Once infected, the immune system begins to fight the virus, a battle that disrupts this healthy oxygen transfer. 

White blood cells release inflammatory molecules called chemokines, which in turn summon more immune cells that target and kill virus-infected cells, leaving fluid and dead cells—pus—behind, which is then diagnosed as pneumonia, with its corresponding symptoms: coughing; fever; and rapid, shallow respiration. Some Covid-19 patients recover, sometimes with no more support than oxygen breathed in through nasal prongs.

In severe cases however, patients’ lungs are damaged resulting in respiratory failure. Even if death doesn’t occur, some patients survive with permanent lung damage.

According to the WHO, SARS punched holes in the lungs, giving them “a honeycomb-like appearance”—and these lesions are present in those afflicted by novel coronavirus, too.

When that occurs, patients often have to be put on ventilators to assist their breathing. Meanwhile, inflammation also makes the membranes between the air sacs and blood vessels more permeable, which can fill the lungs with fluid and affect their ability to oxygenate blood.

“In severe cases, you basically flood your lungs and you can’t breathe,” Frieman says. “That’s how people are dying,” said University of Maryland School of Medicine associate professor Matthew B. Frieman, at an interview.

The heart

Some patients in Italy have been diagnosed with infection of the heart muscle (viral myocarditis).

More than 1 in 5 patients developed heart damage as a result of Covid-19 in Wuhan, China, one small study published March 27 in the journal JAMA Cardiology suggested.

SARS-CoV-2 can infiltrate both the heart and the lungs, because they each contain cells covered with the surface proteins known as angiotensin-converting enzyme 2 (ACE2), which serves as the gateway for the virus to enter cells, Live Science previously reported.

The stomach: A shared gateway

But why does a respiratory virus bother the gut at all?

During the SARS and MERS outbreaks, nearly a quarter of patients had diarrhea—a much more significant feature of those zoonotic coronaviruses.

When any virus enters your body, it looks for human cells with its favorite doorways—proteins on the outside of the cells called receptors.

If the virus finds a compatible receptor on a cell, it can invade. Some viruses are picky about which door they choose, but others are a little more promiscuous.

“They can very easily penetrate into all types of cells,” says Anna Suk-Fong Lok, assistant dean for clinical research at the University of Michigan Medical School and former president of the American Association for the Study of Liver Diseases.

Both SARS and MERS viruses can access the cells that line your intestines and large and small colon, and those infections appear to flourish in the gut, potentially causing the damage or the leakage of fluid that becomes diarrhoea.

Researchers believe Covid-19 uses the same receptor as SARS, and this doorway can be found in your lungs and small intestines.

Two studies—one in the New England Journal of Medicine and one preprint in medRxiv involving 1,099 cases—have also detected the virus in stool samples, which might indicate the virus could spread via feces.

But this is far from conclusive.

Blood storm

Coronaviruses can also cause problems in other systems of the body, due to the hyperactive immune response we mentioned earlier.

Among 184 Covid-19 patients in a Dutch ICU, 38 percent had blood that clotted abnormally, and almost one-third already had clots, according to a April 10 paper in Thrombosis Research.

Blood clots can break apart and land in the lungs, blocking vital arteries—a condition known as pulmonary embolism, which has reportedly killed Covid-19 patients.

Clots from arteries can also lodge in the brain, causing stroke. Many patients have “dramatically” high levels of D-dimer, a by-product of blood clots, said Behnood Bikdeli, a cardiovascular medicine fellow at Columbia University Medical Center.

“The more we look, the more likely it becomes that blood clots are a major player in the disease severity and mortality from COVID-19,” Bikdeli told Science Magazine.

Liver: Collateral damage

The liver is a large, meaty organ that sits on the right side of the belly. Normally you can't feel the reddish-brown organ that weighs about 1.4 kilos, because it's protected by the rib cage.

The liver is essential for digesting food and ridding your body of toxic substances. Its main job is to process your blood after it leaves the stomach, filtering out the toxins and creating nutrients your body can use.

It also makes the bile that helps your small intestine break down fats. Your liver also contains enzymes, which speed up chemical reactions in the body. However, any condition that damages the liver and prevents it from functioning well.

When a disease-causing virus like SARS-CoV-2 spreads from the respiratory system, your liver is often one downstream organ that suffers and doctors have seen indications of liver injury with SARS, MERS, and Covid-19—often mild, though more severe cases have led to severe liver damage and even liver failure.

In an interview with National Geographic, Anna Suk-Fong Lok, assistant dean for clinical research at the University of Michigan Medical School and former president of the American Association for the Study of Liver Diseases said: “Once a virus gets into your bloodstream, they can swim to any part of your body. The liver is a very vascular organ so [a coronavirus] can very easily get into your liver.”

In a normal body, Lok explains, liver cells are constantly dying off and releasing enzymes into your bloodstream.

The organ then quickly regenerates new cells and because of that regeneration process, the liver can withstand a lot of injury.

But at times one can have abnormally high levels of enzymes in their blood, as has been a common characteristic of patients suffering from SARS and MERS, making it a warning sign that the liver may have suffered an injury.

If it’s a mild injury the liver will quickly bounce back from but if the injury is more severe, it can lead to liver failure.

Kidney: It’s all connected

The worldwide fears of ventilator shortages for failing lungs have received plenty of attention. Not so a scramble for another type of equipment: dialysis machines.

“If these folks are not dying of lung failure, they’re dying of renal failure,” says neurologist Jennifer Frontera of New York University’s Langone Medical Center, which has treated thousands of Covid-19 patients.

The kidneys are caught up in this Covid-19 mess, too. Like the liver, your kidneys act as a filter of blood.

Each kidney is filled with about 800,000 microscopic distilling units called nephrons which have two main components: a filter to clean the blood and little tubes that return the cleaned blood back to your body or send the waste down to your bladder as urine.

When it comes to zoonotic diseases like coronaviruses, it’s the kidney tubules that seem to be most affected.

In fact, studies have shown the novel coronavirus can cause renal injury like SARS and MERS where six percent and a quarter of patients suffered these injuries.

After the SARS outbreak, the WHO reported that the virus was found in kidney tubules, which can become inflamed.