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11 Amazing Facts About Veins

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The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

Beneath your skin, and deeper within your body, run networks of veins. These thin, tube-like structures are an essential part of the circulatory system, which distributes blood and nutrients throughout the body. What Thomas E. Eidson, a phlebologist (vein disease specialist) at Atlas Vein Care in Arlington, Texas, finds most compelling about veins is "how absolutely intricate and fragile the circulatory system can seem and yet at the same time be so resilient and adaptive."

1. VEINS ARE ONE OF THREE KINDS OF BLOOD VESSELS.

Three types of blood vessels make up the human circulatory system: arteries, veins, and capillaries. All three of these vessels transport blood, oxygen, nutrients, and hormones to organs and cells. While arteries carry oxygenated blood away from the heart to the tissues of the body, veins carry oxygen-depleted blood from the tissues back to the heart, and in fact have special valves that help them to achieve this directional flow. Capillaries are tiny blood vessels that connect arteries to veins and allow nutrients in the blood to diffuse to the body's tissues.

2. A SINGLE VEIN IS COMPRISED OF THREE LAYERS.

Veins, small as they are, consist of three layers. According to Eidson, these layers are known as the tunica adventitia, tunica media, and tunica intima. The tunica adventitia is the tough outer layer of arteries and veins and is made mainly of connective tissues. The middle layer, tunica media, is all smooth muscle and elastic fibers. This layer is thinner in veins than in arteries. The innermost layer, tunica intima, comes in direct contact with blood as it flows through the vein. This structure is made up of smooth cells and has a hollow center known as the lumen.

3. OUR BODIES CONTAIN UP TO 100,000 MILES OF BLOOD VESSELS.

All the arteries, veins, and capillaries of a human child, stretched end to end, are estimated to wrap around the Earth about 2.5 times (the equivalent of about 60,000 miles). The amount of blood vessels in a human adult would circle our planet four times, equaling 100,000 miles, according to Eidson.

4. CAPILLARIES ARE SMALLER THAN THE WIDTH OF A HUMAN HAIR.

Capillaries are tiny—at their smallest, they're less than a third of the thickness of a human hair. But to really put it into perspective, consider that when red blood cells flow through capillaries, "[they] must travel through them one cell at a time in a single-file line," Eidson says.

5. PHYSICIANS HAD THE CIRCULATORY SYSTEM ALL WRONG UNTIL THE 17TH CENTURY.

"Physicians followed an incorrect model of the circulatory system proposed by Greek physician and philosopher Galen of Pergamon from about the 2nd century CE until the 1600s," Eidson says. According to a paper in the Journal of Thrombosis and Haemostasis, Galen thought there were two systems: one in which the liver, not the heart, produced blood that was distributed in the body centrifugally, and another where arteries carried air from the lungs and more blood to tissues. "Blood was not seen to circulate but rather to slowly ebb and flow," author W.C. Aird wrote. This attitude prevailed until 1628, when English physician William Harvey first correctly described the circulatory system and the function of the heart.

6. THE BODY CAN FORM NEW VESSELS WHEN ONE IS BLOCKED.

Eidson says the body can form new blood vessels if a pathway gets blocked, a process called angiogenesis or neovascularization. On the positive side, this is the process by which flesh wounds heal, drawing nutrients and oxygen from the nearest healthy capillaries to the site of those that are damaged; this isn't too hard given how numerous capillaries are in the body. On the negative side, this same process can lead to corneal neovascularization, in which new blood vessels invade the cornea from the limbus, a part of the eye where the cornea meets the sclera—the white part of the eye. The extra blood vessels can cause inflammation and scarring of the cornea, and even result in blindness.

7. ONE PHYSICIAN PERFORMED A PIONEERING EXPERIMENT ON VEINS IN THE ARM—HIS OWN.

German physician Werner Forssmann performed a cardiac catheterization on himself in 1929. In this procedure, a thin tube called a catheter is inserted into one of the large blood vessels in the arm that leads to the heart. The medical community at the time believed studying the heart was highly unorthodox, but Forssmann was determined to prove them wrong. If the procedure succeeded, Forssmann would be able to show that a catheter could assess the pressure in the organ and how well the heart is working.

He made an incision on the inside of his left elbow and threaded the thin tube into his heart—and had a technician take an X-ray to prove the penetration was a success. Then he calmly removed the catheter from his arm with no side effects. Now, "it's a procedure performed in the U.S. approximately 1 million times per year," Eidson says. Forssmann also went on to win the Nobel Prize for Medicine in 1956, shutting up his detractors.

8. STRONG VEINS ARE ESSENTIAL TO A STRONG BODY.

Veins return oxygen-depleted blood back to the heart against the force of gravity. "If veins are too weak—a condition called venous insufficiency—blood can pool in the legs and skin causing swelling, pain, discoloration, and wounds," says Albert Malvehy, a venous and lymphatic specialist and phlebology sonographer in Delray Beach, Florida. Chronic venous insufficiency is more common in people who are obese, pregnant, or who have a family history of the problem. It can also be caused by high blood pressure in the leg veins, as a result of sitting or standing for long stints; not enough exercise, smoking, or deep vein thrombosis (blood clots). Depending on the severity, treatments may range from medication to surgery.

9. VARICOSE VEINS ARE CAUSED BY DAMAGED VALVES.

When venous valves are damaged, blood can flow in the wrong direction and lead to stretched-out, bulging veins, Gregory P. Kezele, the medical director of Vein Clinics of Cleveland, tells Mental Floss. Varicose veins, which can range in color from purplish to neutral, appear twisted and gnarled, and may be raised on the skin's surface. (Don't confuse them with spider veins, which are clusters of bluish or reddish veins near the surface of the skin that resemble webs, hence the name.) Conditions like pregnancy, obesity, and genetic predisposition can cause them. Once varicose veins appear—usually on the legs—they require a medical procedure to get rid of them.

Veins are a critical part of normal circulation in the body, so varicose veins can be more than just a cosmetic issue. "They can be a sign of a deeper circulation problem," Malvehy says. "People with varicose veins, leg pain, restless legs syndrome, leg wounds, and leg swelling should be checked by a vein specialist."

One in five people have vein disease. As recently as 10 years ago, there were few treatments to offer varicose vein sufferers except for vein stripping surgery, in which problematic veins are removed. Malvehy says that over the past decade, "there has been a revolution in treatment, such that almost all vein issues can be treated in the office with no downtime."

One common treatment is sclerotherapy, in which a liquid solution is forced into the bulging vein to stop the flow of blood. The vein will eventually turn into scar tissue and fade away, though follow-up treatments might be needed.

Another treatment is thermal ablation, performed using ultrasound guidance. Kezele explains that a physician will insert a small catheter into the diseased veins, which then delivers heat; the heat will close off blood flow to the problem veins and improve circulation as blood diverts to healthy veins.

10. AN EARLY DEPICTION OF VEIN DISEASE APPEARS IN A SCULPTURE FROM 340 BCE.

According to Kezele, the first depiction of vein disease appears on a Greek tablet dating to the 4th century BCE. The carving, from the sanctuary of Amynos, shows a man clutching a giant, disembodied leg with a bulging vein. Kezele suggests on his website that "it shows the Greek official Lysimachides dedicating a fake leg suffering from a varicose vein to Amynos," an Athenian hero revered as a healer.

11. VEINS MIGHT "POP OUT" WHEN YOU EXERCISE.

There are lots of theories on why athletes often have big, bodaciously bulging veins visible on their arms or legs after they work out. The ropy look is completely normal and temporary. Writing in Scientific American, physiology professor Mark A. W. Andrews said that a likely cause of protruding veins is arterial blood pressure during exercise. Blood that would otherwise be resting in capillaries is forced out by the pressure into the surrounding muscle. That process—called filtration—makes the muscles swell, which pushes nearby veins closer to the skin's surface so they take on a bulging appearance. The process is more noticeable in athletes and body builders with very little subcutaneous fat.

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11 Facts About Fingernails
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Whether there's dirt beneath them or polish atop them, your fingernails serve more than just decorative purposes: They help keep your fingertips safe and have a multitude of special functions that even your doctor might not be aware of. “The nails occupy a unique space within dermatology and medicine in general, particularly because they are such a niche area about which few people have expertise,” Evan Rieder, assistant professor in the Ronald O. Perelman Department of Dermatology at NYU Langone Health, tells Mental Floss.

1. FINGERNAILS HAVE FOUR MAIN PARTS.

Along with skin and hair, nails are part of the body's integumentary system, whose main function is to protect your body from damage and infection. Fingernails have four basic structures: the matrix, the nail plate, the nail bed, and the skin around the nail (including the cuticle).

Fingernail cells grow continuously from a little pocket at the root of the nail bed called the matrix. The pale, crescent-shaped lunula—derived from Latin for "little moon"—on the nail itself is the visible portion of the matrix. If the lunula is injured, the  nail won't grow normally (a scarred lunula can result in a split nail), and changes in the lunula's appearance can also be signs of a systemic disease.

Fingernail cells are made of a protein called keratin (same as your hair). As the keratin cells push out of the matrix, they become hard, flat and compact, eventually forming the hard surface of the nail known as the nail plate. Beneath that is the nail bed, which almost never sees the light of day except when there's an injury or disease.

Surrounding the matrix is the cuticle, the semi-circle of skin that has a tendency to peel away from the nail. The skin just underneath the distal end of the fingernail is called the hyponychium, and if you've ever trimmed your nails too short, you know this skin can be slightly more sensitive than the rest of the fingertip.

2. THEY GROW AT A RATE OF 0.1 MILLIMETERS A DAY ...

That's about 3 to 4 millimeters per month. But they don't always grow at the same speed: Fingernails grow more quickly during the day and in summer (this may be related to exposure to sunlight, which produces more nail-nourishing vitamin D). Nails on your bigger fingers also grow faster, and men's grow faster than women's. The pinky fingernail grows the slowest of all the fingernails. According to the American Academy of Dermatology, if you lose a fingernail due to injury, it can take up to six months to grow back (while a toenail could take as much as a year and a half).

3. ... BUT NOT AFTER YOU'RE DEAD.

You've probably heard that your fingernails keep growing after death. The truth is, they don't, according to the medical journal BMJ. What's actually happening is that the skin around the base of the fingernails retracts because the body is no longer pumping fluids into the tissues, and that creates a kind of optical illusion that makes the nails appear longer.

4. ITS ESTIMATED THAT 20 TO 30 PERCENT OF PEOPLE BITE THEIR NAILS.

Scientists say it's still unclear why, but they suspect nail-biters do it because they're bored, frustrated, concentrating, or because it just feels comforting (and anxiety doesn't seem to play a big role). Perfectionists who don't like to be idle are very likely to have the habit. Biters expose themselves to the dangerous crud that collects underneath the nail: The hyponychium attracts bacteria, including E. coli, and ingesting that through nail-biting can lead to gastrointestinal problems down the line. Biting can also damage teeth and jaws.

5. HUMAN FINGERNAILS ARE BASICALLY FLAT CLAWS.

Our primate ancestors had claws—which, like nails, are made of keratin. As human ancestors began using tools some 2.5 million years ago (or even earlier), evolutionary researchers believe that curved claws became a nuisance. To clutch and strike stone tools, our fingertips may have broadened, causing the claws to evolve into fingernails.

6. THE NAIL ACTUALLY MAKES YOUR FINGERTIP MORE SENSITIVE.

While the fingernail may be tough enough to protect tender flesh, it also has the paradoxical effect of increasing the sensitivity of the finger. It acts as a counterforce when the fingertip touches an object. "The finger is a particularly sensitive area because of very high density of nerve fibers," Rieder says.

7. FINGERNAILS CAN REVEAL LUNG, HEART, AND LIVER DISEASES.

"One of the most interesting facts about fingernails is that they are often a marker for disease within the body," Rieder says. Nail clubbing—an overcurvature of the nail plate and thickening of the skin around the nails—is a particularly significant sign of underlying illness, such as lung or heart disease, liver disease, or inflammatory bowel disease. Two-toned nails—whitish from the cuticle to the nail's midpoint and pink, brown, or reddish in the distal half—can be a sign of kidney and liver disease. Nails that are two-thirds whitish to one-third normal can also be a sign of liver disease. However, little white marks on your nails, known as milk spots (or punctate leukonychia) are just the remnants of any kind of trauma to the nail, from slamming it in a door to chewing on it too fervently.

8. YOU CAN GET A COMMON SKIN DISEASE ON YOUR NAILS.

Psoriasis is "typically thought of as a skin disease, but is actually a skin, joint, and nail disease, and when severe, a marker of cardiovascular risk," Rieder says. Psoriatic fingernails may have orange patches called oil spots, red lines known as splinter hemorrhages, lifting of the edges of the nails, and pits, "which look like a thumb tack was repeatedly and haphazardly pushed into the nails," he says.

Doctors often prescribe topical or injected corticosteroids to treat psoriatic nails, but using lasers is an emerging and potentially more cost-effective technique. Rieder relies on a pulsed dye laser, which uses an organic dye mixed with a solvent as the medium to treat nail psoriasis, "which can be both medically and aesthetically bothersome," he says. This laser is able to penetrate through the hard nail plate with minimal discomfort and "to treat targets of interest, in the case of psoriasis, blood vessels, and hyperactive skin," Rieder says.

9. ANCIENT CULTURES DISPLAYED SOCIAL STATUS WITH NAIL ART.

Painting and other forms of decorating nails have a history of offering social and aesthetic cues through variations in nail color, shape, and length, Rieder says. In fact, he adds, in some cultures ornate and well-decorated fingernails "serve as a proxy for social status."

Five thousand years ago in China, men and women of the Ming Dynasty aristocracy grew their nails long and covered them with golden nail guards or bright home-made polishes. The long nails allegedly announced to the world their social rank and their freedom from performing menial labor.

10. A FORMER BEAUTICIAN HELD THE WORLD RECORD FOR THE LONGEST NAILS.

Lee Redmond of Utah started growing her nails in 1979 and kept at it until she held the world record for "longest fingernails on a pair of hands ever (female)" in 2008. Her right thumbnail was 2 feet, 11 inches and the collective length of all her nails was 28 feet, 4 inches. She also applied nail hardener daily and painted them a reflective gold. Unfortunately, she broke her nails in a 2009 car accident and has no plans to regrow them.

11. THE FIRST NAIL CLIPPERS WERE PATENTED IN 1875.

Today, biters don't have to use their teeth to trim their nails. While the earliest tools for cutting nails were most likely sharp rocks, sand, and knives, the purpose-built nail clipper—though it might be more accurately called a circular nail file—was designed by a Boston, Massachusetts inventor named Valentine Fogerty and patented in 1875. The nail clippers we know today were the design of inventors Eugene Heim and Oelestin Matz, who were granted their patent for a clamp-style fingernail clipper in 1881.

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What's Really Happening When We See 'Stars' After Rubbing Our Eyes?
Photo illustration by Mental Floss. Images: iStock.
Photo illustration by Mental Floss. Images: iStock.

It's likely happened to you before: You start rubbing your eyes and almost immediately begin seeing colors, specks, and swirls from behind your closed lids. So what's happening when you see these 2001-esque "stars"? Do they only occur upon rubbing? Does everyone experience them?

Before we can get to what causes the lights, we need to understand a bit about how the eyes work. Angie Wen, a cornea surgeon at New York Eye and Ear Infirmary of Mount Sinai, tells Mental Floss that the retina—the innermost layer of the eye—consists of millions of cells, or photoreceptors. These cells, she says, "are responsible for receiving information from the outside world and converting them to electrical impulses that are transmitted to the brain by the optic nerve. Then, the brain interprets them as images representing the world around us."

However, what we see doesn't just stop there. Sometimes "we see light that actually comes from inside our eyes or from electric stimulation of the brain rather than from the outside world," Wen says. "These bursts of seemingly random intense and colorful lights are called phosphenes, and appear due to electrical discharges from the cells inside our eyes that are a normal part of cellular function."

People have been writing and theorizing about phosphenes for thousands of years. Greek philosophers thought the bursts of light were the result of fire inside our heads: "The eye obviously has fire within it, for when the eye is struck fire flashes out," wrote Alcmaeon of Croton (6th–5th century BCE), a philosopher and early neuroscientist, of the swirls and specks someone sees after getting a blow to the head. A century later, Plato—who believed that a "visual current" [PDF] streamed out of the eye—wrote that "Such fire as has the property, not of burning, but of yielding a gentle light they [the Gods] contrived should become the proper body of each day."

Plato's take was still the dominant one through the Middle Ages. Eventually, Newton (1642–1727) theorized a concept that's more in line with what's believed today about these strange sparkly visions: The phenomenon is due to light that's produced and observed when pressure and motion is placed on the eyes.

Eleonora Lad, an associate professor of ophthalmology at Duke University Medical Center who has a background in neuroscience, explains exactly why eye rubbing generates these visions: "Most vision researchers believe that phosphenes result from the normal activity of the visual system after stimulation of one of its parts from some stimulus other than light," including putting external pressure on the eyes. (Interestingly, due to retinal damage, blind people can't see phosphenes caused by pressure, but they can see them when their visual cortex is electrically stimulated. In hopes of turning this phenomenon into improved vision for the blind, scientists have developed a cortical visual prosthesis, implanted in the visual cortex, that generates patterns of phosphenes. The device has been approved by the FDA for clinical trial.)

As Alcmaeon rightly pointed out, there are causes for the bursts of light beyond just rubbing your eyes: Getting hit in the eye can produce this phenomenon—as can a sneeze, a surprisingly powerful event that tends to clamp our eyes shut, Wen says.

Receiving an MRI or EEG may also trigger it. MRIs, for example, produce a changing magnetic field which can stimulate the visual cortex, making a person see these flashing lights. When it comes to an EEG, depending on the brain stimulation frequency band (Hz) used, some patients experience the phenomenon when closing their eyes, which is believed to come from retinal stimulation during the process.

And the activity doesn't only happen on Earth; astronauts in space have also been known to experience them. As reported in 2006 in the journal Vision Research, "over 80 percent of astronauts serving in today's NASA or ESA (European Space Agency) programs have perceived phosphenes at least in some missions and often over several orbits." They're mainly attributed to interactions between the eye and cosmic ray particles in space, outside the Earth's protective magnetic field.

No matter the cause, the bursts of light are perfectly normal—but that doesn't mean you should engage in excessive eye rubbing. Wen says ophthalmologists advise against rubbing your eyes or applying vigorous pressure; according to Lad, too much rubbing may be damaging to the cornea and lens or "result in a loss of fatty tissue around the eyes, causing the eyes to look deep-set."

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