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.

Why Is Pee Yellow?

Chloe Effron
Chloe Effron

WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Your body is kind of like a house. You bring things into your body by eating, drinking, and breathing. But just like the things we bring home to real houses, we don’t need every part of what we take in. So there are leftovers, or garbage. And if you let garbage sit around in your house or your body for too long, it gets gross and can make you sick. Your body takes out the garbage by peeing and pooping. These two things are part of your body’s excretory system (ECKS-krih-tore-eee SISS-tem), which is just a fancy way of saying “trash removal.” If your body is healthy, when you look in the toilet you should see brown poop and yellow pee.

Clear, light yellow pee is a sign that your excretory system and the rest of your body are working right. If your pee, or urine (YER-inn), is not see-through, that might mean you are sick. Dark yellow urine usually means that you aren’t drinking enough water. On the other hand, really pale or colorless pee can mean you might be drinking too much water! 

Your blood is filtered through two small organs called kidneys (KID-knees). Remember the garbage we talked about earlier? The chemicals called toxins (TOCK-sins) are like garbage in your blood. Your kidneys act like a net, catching the toxins and other leftovers and turning them into pee.

One part of your blood is called hemoglobin (HEE-moh-gloh-bin). This is what makes your blood red. Hemoglobin goes through a lot of changes as it passes through your body. When it reaches your kidneys, it turns yellow thanks to a chemical called urobilin (yer-ah-BY-lin). Urobilin is kind of like food coloring. The more water you add, the lighter it will be. That's why, if you see dark yellow pee in the toilet, it's time to ask your mom or dad for a cup of water. 

To learn more about pee, check out this article from Kids Health. 

12 Facts About Kidney Stones

Illustration by Mental Floss. Images: iStock
Illustration by Mental Floss. Images: iStock

Kidney stones are more common than ever. According to Harvard Medical School, every year more than 3 million people see a doctor for relief from these hard mineral and salt deposits, which form in your kidney when urine becomes too concentrated. Here's what we know about the condition formally called nephrolithiasis.

1. KIDNEY STONES TYPICALLY CAUSE REALLY PAINFUL SYMPTOMS.

At first you may notice your urine is cloudy, bloody, and foul smelling. Your back may begin to ache, and nausea may come over you. Then, as the stone moves from your kidney into your urinary tract or bladder, sometimes becoming trapped, there’s often an intense, stabbing pain that many people say they wouldn’t wish on their worst enemy.

2. MOST PEOPLE DEVELOP ONE TYPE OF STONE …

What kind of kidney stone you get depends on your diet, fluid intake, genetics, hereditary disorders, and even whether you take certain medications, but the vast majority of people get calcium oxalate stones. They're formed from a mix of calcium in urine and the compound oxalate, which is found naturally in food like nuts, chocolate, and some vegetables, including beets and spinach; oxalate is also produced by your liver. There's some evidence that people who take the seizure medicine topiramate can develop these stones in the form of calcium phosphate.

3. … BUT THERE ARE THREE OTHER KINDS TOO.

Struvite stones are fast-growing mineral deposits that typically develop in response to a urinary tract infection, and can grow large enough to block the kidney, ureter, or bladder before you notice any symptoms; they affect women more than men. Uric acid stones turn up in people who eat a lot of red meat, shellfish, and organ meats, which contain hefty doses of an organic compound called purine that can lead to more uric acid than the kidneys can excrete. Cystine stones are caused by a rare hereditary disorder called cistinuria in which your kidneys excrete excessive amounts of the amino acid cystine.

4. THEY'RE EXTREMELY COMMON—ESPECIALLY IN MEN.

There's a solid chance you could end up with a kidney stone. The National Kidney Foundation notes that one in 10 people will develop one during the course of their life. And if you’re male, take note: Your gender alone is considered a kidney stone risk factor. Men are twice as likely as women to develop them. Another factor is age: Although stones are most common from ages 20 through 50, they tend to peak around age 30.

5. IF YOU’VE HAD A KIDNEY STONE, YOU’LL PROBABLY DEVELOP ANOTHER ONE …

Sorry to say, but simply having a kidney stone puts you at risk for a recurrence. If you’ve had one, the U.S. National Library of Medicine notes that there’s a 30 to 50 percent chance more stones will form within five years.

6. … BUT YOU CAN TAKE STEPS TO PREVENT THEM.

Cutting back on sodium (i.e. deli meats, packaged soups, and processed foods) can help, because a stone can form from excessive salt consumption. You should also avoid too much animal protein—it produces urine containing more acid, which is known to increase your risk for kidney stones—and increase your intake of fruits, vegetables, whole grains, and low-fat dairy. And be sure to drink plenty of fluids, especially water—at least 12 glasses a day. (That's good advice for everyone, not just those prone to kidney stones.)

Don't drink much apple or cranberry juice as both contain oxalates and are linked to an increased risk of developing calcium oxalate stones. High doses of Vitamin C may boost the concentration of oxalate in urine; the Cleveland Clinic recommends a daily maximum of 500 milligrams.

7. IT'S A MYTH THAT CALCIUM CREATES SOME KIDNEY STONES.

Despite the fact that the word calcium is part of the most common kind of kidney stone, you don’t need to treat calcium as the enemy. In fact, having too little calcium can actually increase the odds you’ll get these types of stones. According to the Cleveland Clinic, eating about two or three servings of calcium-rich foods daily reduces oxalate absorption, helping to keep calcium oxalate stones away. So get out the cheese.

8. IF YOU PASS A STONE, CONGRATULATIONS! NOW TAKE IT TO A DOCTOR.

Ninety percent of kidney stones are passed through urination. Getting one out this way may hurt a lot, but once the stone has finished causing you agony, it could provide clues that could help you avoid developing another one. If you’re able to retrieve the stone, bring it to your doctor, who can order an analysis. Identifying its components can reveal the kind of stone it is and potentially point to a treatment or prevention plan.

9. IF YOU CAN’T PASS A STONE, TREATMENTS ARE AVAILABLE …

In an attempt to exit the body, a stone travels from the kidney to the bladder through a narrow tube called the ureter. If the stone is larger than a quarter-inch, it's simply too big to pass through the ureter, and will get trapped there. (If it can make it through to your bladder, it's small enough to pass out out of your body through the urethra.) This causes intense pain, blocked urine flow, and possible bleeding from urinary tract walls. That's when it's time for treatment.

There are several methods for getting rid of a kidney stone, all of which aim to break the stone into smaller pieces so they can leave the body. In an extracorporeal shock wave lithotripsy (from the Greek for "crushed stone"), high-frequency sound waves are applied externally to break stones up, allowing them to pass when you pee. Laser lithotripsy takes a similar approach: Stones in the ureter are broken up with a laser and also leave the body naturally. More invasive is percutaneous ultrasonic lithotripsy, which involves passing narrow instruments (including a fiberoptic camera) through your back to your kidney; ultrasound breaks the stones up, and then fragments are removed by an instrument. Finally, a ureteroscopy is a treatment option in which a small scope is inserted in the ureter towards the bladder to determine the stone's location. Then it's broken up for natural passage or removed altogether. Luckily, you're unconscious under general anesthesia during the last procedure.

10. … AND THEY'RE FAR SUPERIOR TO THOSE USED IN THE PAST.

Kidney stones are nothing new—mentions of the painful formations go back more than 5000 years, to Mesopotamian medical texts—and medical interventions have occurred for just as long. Stones made it into the Hippocratic Oath, in which physicians swore they would "not use the knife, not even on sufferers from stone," leaving the procedure to "such men as are engaged in this work" [PDF]. Surgeons in ancient Greece and India were attempting stone removal as far back as the 7th century BCE.

The 16th to 18th centuries were a heyday for stone surgeons, who were largely self-taught. The most notorious of them was Frere Jacques Beaulieu. He pioneered the lateral perineal lithotomy—which involved making an incision in the perineum, inserting a terrifying cutting instrument into the bladder, cutting up the stone, and then extracting the pieces with the instrument or his fingers—in the late 17th century. Unfortunately for his patients, he had no technical training, and his method was often deadly; in 1698, after 25 of his 60 patients died, he was banned from doing the procedure—but he didn't stop. He's thought to have performed more than 5000 lithotomies. (And no, the song doesn't seem to be about him.)

11. IF ALL ELSE FAILS, TRY RIDING A ROLLER COASTER.

If you’re a thrill seeker who happens to have kidney stones (and some vacation time), you may be in luck. After a "notable number" of patients reported that riding the Big Thunder Mountain roller coaster at Walt Disney World in Orlando helped them to pass their kidney stones, Michigan State University urologist David Wartinger decided to investigate. He created a kidney replica—complete with kidney stones—put it in a backpack, and let it ride the roller coaster 60 times. It worked—but passing the stones depended on where the backpack was placed in the coaster. Rides in the last car were the most effective, with the stones passing 64 percent of the time, while the front few cars yielded only a 16 percent success rate.

Big Thunder Mountain was the only ride in the theme park that was effective. Neither Space Mountain nor Aerosmith's Rock 'n' Roller Coaster did the trick, likely because they were too fast, with a G-force that pinned the stones in place. Of course, while this is an interesting finding, if you suspect you have kidney stones, speak to your doctor before you high-tail it to Walt Disney World.

12. A KIDNEY STONE THE SIZE OF A MOUSE WAS REMOVED FROM A MAN IN 2004.

The stone measured 5.11 inches at its widest point—a world record. Five years later, a whopping 2.5-pound stone was surgically removed from a man in Hungary in 2009. Perhaps seeing a bunch of kidney stones in one place other than originating from your own body will put you at ease. If that’s the case, check out the International Museum of Surgical Science in Chicago, where a collection of stones is on display in glass jars.

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