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How does a scab help a cut or bruise?

How does a scab help a cut or bruise? asks reader Nia Ramsey.

When a scrape or a cut makes an opening in the skin, it’s a kind of body-wide emergency. An open cut will allow blood to spill out, like water from a broken fire hydrant. Blood carries oxygen and nutrients to all organs, from brain to liver to heart; without blood, the body dies. So when one of its carrying tubes — the blood vessels — is torn, blood acts fast to conserve every drop of itself. Even before we reach for the band-aids, our blood has begun to manufacture an even better covering — a scab.

You don't really want to see a picture of a scab, do you?

A skin injury–say, a paper cut–triggers a cascade of chemical signals. The body’s main objective: Make a clot to stop the bleeding, then recruit skin cells to repair the opening. Platelets, the key players in blood clotting, are always roaming through the blood, ready to swing into action. In fact, there are more than 1 billion platelets in each teaspoon of blood. So when the razor-like edge of an envelope slices your fingertip, platelets immediately glom on to the ragged blood vessel ends. Floating platelets are shaped like tiny discs. But at the site of an injury, platelets sprout burr-like protrusions. Wearing their homemade Velcro, platelets stick tightly together along the blood vessel tear. Meanwhile, like EMTs calling for help, platelets at the injury site release signaling chemicals, requesting help from their friends. The more platelets that stream in, the more signals are sent out. Result: a platelet pile-up.

From within the forming clot, platelets release chemicals that cause blood vessel walls to constrict, narrowing the vessel’s opening. With the opening narrowed and the platelets clumped, forming a physical barrier, the outrushing blood slows.

Next, the platelets and the vessel walls release an enzyme called thrombokinase, which converts the inactive blood enzyme prothrombin into active thrombin. Thrombin then helps change another protein, fibrinogen, into fibrin. Fibrin’s twisty fibers stitch a net over the wound where platelets are already clumped. This net traps trying-to-flee blood cells, like a spider’s web snares insects. The weave of fibrin, platelets, and trapped red blood cells is a clot, keeping blood from flowing out and germs from coming in. The jelly-like clot soon air-dries into a brownish crust — a scab.

Under a scab’s protective dome, white blood cells take care of bacteria that may have entered through the cut, and mop up dead cells. Meanwhile, healthy skin cells are recruited to the wound site. At the edges of the cut, the skin cells quickly multiply, creating new tissue to neatly cover the tear.

(In 2007, scientists in Germany discovered that a signal molecule called c-Met, which also regulates cell growth in growing embryos, seems to call the shots. Experiments show that if skin cells lack c-Met, they can’t multiply and migrate to where they’re needed in a wound.)

So think of a scab as a beehive of healing activity. Pull off a scab (ouch) and you also pull off the new tissue growing underneath it, simply delaying your cut’s disappearance.

How come onions make you cry?

How come onions make you cry? asks reader Jessica R.

Elaborate precautions and instructions. Special patented peeling devices. Goggles, with foam seals and anti-fog lenses. Chemical warfare protection, all to produce a slice of something you’ll happily eat on a sandwich.

An onion, uncut.

Onions are a vegetable that seems to resist being eaten. Cut into an onion, and it’s like a mini-mace attack. The result is stinging, watering eyes, which you struggle to keep open as you (valiantly) keep chopping. But hold an unpeeled, uncut onion to your face, and — nothing. Little to no scent. Zero tear gas.

How–and why–does an innocent-looking onion accomplish its tiny assault? Breaking an onion’s cells starts the ball rolling. Broken onion cells release molecules called amino acid sulfoxides, along with a special enzyme. The enzyme changes the sulfoxides into sulfenic acids. The acids in turn rearrange themselves into a chemical called syn-propanethial-S-oxide — which stealthily spreads as a gas into the air. Syn-propanethial-S-oxide is also called “lacrimatory factor,” because of its effects on the lacrimal, or tear glands.

About 30 seconds after being cut, an onion’s out-gassing lacrimatory factor reaches its peak. Wafting up into the eyes, lacrimatory factor reacts with the eyes’ thin film of water. One of the chemicals produced in the reaction is a mild form of sulfuric acid, the same acid our stomachs use to break down food. Irritated nerve endings immediately fire off messages to the brain; the eyes begin to sting. Meanwhile, the nerve signals cause tear glands to up production, releasing a flood of tears to dilute and wash away the offending chemical.

Scientists in Japan discovered the enzyme that synthesizes an onion’s lachrymatory factor, publishing their findings in 2002. (The newly discovered enzyme was named, not surprisingly, “lachrymatory-factor synthase.”) But although the “how” of stinging onion vapors has been mostly explained, the “why” remains something of a mystery.

Some say that the gas is a kind of plant-y defense against animals, the onion version of a rose’s thorns. But some point out that an onion’s chemistry is designed to protect it from other threats–invading bacteria, viruses, and fungus. The irritating gas, they say, may just be a by-product of a kind of chemical “bomb,” designed to kill invading microbes as quickly as possible.

So don’t take an onion’s reaction personally. But when cutting an onion, do try some techniques that really work: Chill the onion in the fridge before cutting; cold makes the lachrymatory factor tamer.

Cut the onion under running water, which short-circuits the gassy reaction by dissolving the chemical. Or set up a fan to blow the onion vapors away from you. And then there are those goggles….

Onions are so good at what they do that researchers studying tears often use onion vapors to make their volunteers cry. But the “reflex” tears produced, scientists say, are chemically different from those that roll down our faces when we’re upset. Emotional tears rid the body of some stress-produced hormones and contain natural pain-killers. This may help explain why we feel better after we cry–but get no emotional lift from chopping onions.

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