You might see your veins under your skin, but when they visibly become more pronounced, you could have to call a doctor.

Take a look at the back of your hand, and you can probably trace a string of blue lines that lie just beneath your skin. As you likely know, these are your veins – and they serve a vital function in your cardiovascular system. Without them, deoxygenated blood would never make it back to the heart.

Sometimes, though, these little blue lines morph and take on completely different shapes and colors. Perhaps you have a dark red web of thin lines, or you might have noticed some of your veins twisting and turning. Either way, you might be able to see these blood vessels even more clearly through your skin now.

Of course, some of these conditions cause nothing more than a cosmetic concern. But, sometimes, when you can see veins through the skin, it can signify that trouble is brewing beneath the surface. And if those visible veins come with some other symptoms, then it’s definitely time to call a doctor – or even head straight to the ER.

For the most part, veins play a vital role in the functioning of your cardiovascular system. The heart does most of the work, sending oxygen and nutrient-rich blood to all areas of the body with each beat. And when the blood’s depleted of its supplies, the veins deliver it back to the heart.

Some may confuse veins with arteries, but each one serves a particular purpose in the cardiovascular system. While veins carry deoxygenated blood back to the heart, arteries do the opposite – they carry the oxygenated liquid out to the tissues. However, there’s a more specific way to differentiate each pathway.

Both the veins and arteries take place in pulmonary circulation, too, wherein the blood becomes infused with oxygen. In this process, veins return the oxygenated blood to the heart, while arteries pull the air-free blood from the heart to the lungs. So, as it turns out, we can’t differentiate the two circulatory components by the fact that they carry oxygenated or deoxygenated blood – they do both.

What can be said to distinguish between veins and arteries is the direction in which blood flows once inside each pathway. Veins carry blood toward the heart, while arteries move it away from the organ. So, the differentiation has nothing to do with oxygen content. However, in most areas of the body, the veins do carry deoxygenated blood and the arteries shuttle the fresh stuff to where it’s needed.

Although most veins carry dark red, oxygen-deprived blood to the heart, they typically appear through skin in a bluish color. This visual effect occurs because the body’s subcutaneous fat soaks in low-frequency light. Brighter wavelengths still show through this layer, including the blue hue in which our veins appear.

And, although veins perform such a vital function, they have relatively thin walls that expand with ease. While most veins serve the purpose of carrying blood, there remain a few other variations that exist for entirely different reasons. And sometimes, they can appear because of an underlying issue.

Veins contain valves, which open and close to let blood in, preventing it from backtracking on its way to the heart. When these valves malfunction due to weakness or damage, though, blood can’t go to its intended destination. Blood then pools, causing veins to bulge, which results in one of two common conditions.

Firstly, someone with valve-related issues may have varicose veins. Sometimes, superficial veins – or those nearest to the body’s surface – expand and change shape. They twist around and you can see their kinks through the skin. These are called varicose veins, and more than 40 million Americans have them, according to the Chicago Vein Institute.

Most cases of varicose veins come with only a few symptoms in tow. For instance, they can cause achy legs, or they can make limbs feel heavy. Some people report that their ankles swell, most commonly at night. And, sometimes, skin can become red and dry or discolored in a yellow or brownish hue.

Fortunately, those with varicose veins have plenty of options to assuage the aches and pains that come with them. For instance, reducing pressure by elevating the legs can temporarily help alleviate symptoms. Compression socks can also keep swelling at bay and can boost circulation to the area. And some take anti-inflammatories such as aspirin as part of their treatment, to mute the pain.

Varicose veins remain a common condition, although they tend to affect certain groups more often than others. Adults over 50 and women more often experience them most often, as do people with a history of the condition in their families. Those who work on their feet, such as nurses, actors or professors, develop varicose veins more frequently, too.

Similarly, spider veins can also appear in the legs and, sometimes, on faces, when a formerly healthy vein stops functioning properly. However, they have a completely different appearance than varicose veins. Indeed, rather than bulging out, spider veins appear as thin red, blue or purple lines.

Although they’re easy to see, spider veins often don’t carry any symptoms with them, nor do they cause any harm or pain. However, some people like to have them removed for cosmetic reasons and experts have a few ways to do so. They can use lasers to dry out spider veins, or they can use the same tool under the skin to heat them and cause them to break down.

Image: Instagram/Fox Vein & Laser Experts

Spider veins are hereditary for a whopping 90 percent of sufferers, according to a 2015 Oxford Academic study. And much like varicose veins, women, older adults and pregnant women are prone to spider veins, too. However, the latter form more commonly in those who work in the seated position, as opposed to varicose veins, which affect those who stand all day long.

These noticeably different types of veins aren’t the only ones that can show through the skin. Indeed, others can suggest a much more serious problem lingering below the surface. Sometimes, a vein can look darker than usual, or it may protrude above the skin, raising cause for concern.

Before panicking, though, you should go through the list of potential causes for such a striking appearance. For starters, regular exercise can cause veins to bulge out – just ask anyone who weight lifts seriously. There’s an explanation for this, although, surprisingly, it has nothing to do with a boost in blood flow.

Some people think that their post-weightlifting veins show because their workout got the blood pumping, too. But it turns out both blood volume and pressure actually fall during exercise, thus exerting less pressure on the veins. Indeed, bulging muscles have more to do with muscles than with the cardiovascular system.

As a person lifts weights, the targeted muscles begin to swell and tighten as they work. This growth pushes the veins toward the surface of the skin, thus giving them a bulging appearance. So, as physiology professor Mark. A. W. Andrews wrote for Scientific American in 2006, “This bulging is neither good nor bad, but simply a result of normal physiological mechanisms.”

On top of that, veins might seem to stick out even more against particular complexions. For example, the lighter a person’s skin or hair, the more visible they become. Plus, as we age, our veins pop more, too, as skin’s elasticity fades away and it thins. At the same time, older adults drop both fat and muscle, which makes veins more prominent.

Of course, aging is an all-natural process, which means that veins become a more pronounced feature as we get older. So, if an older adult notices a protruding or otherwise prominent vein pattern, it’s nothing to worry about. These superficial veins tend to reside close to the skin’s surface, which explains how they become so visible.

Another source of a bulging vein could simply be your genetic code. This means that if someone in your family has had similarly striking blood vessels, you might, too. Then, there’s the possibility that you’re suffering from a hormone imbalance. Indeed, during certain times in life, especially for women, veins can react to hormonal changes.

When a woman becomes pregnant, the volume of blood in her body raises between 20 and 40 percent. As such, the veins have a lot more work to do to make sure all of her blood makes it back to the heart for oxygenation. So, they get larger to accommodate the extra fluid, which makes veins seem larger. However, this only lasts for the duration of gestation.

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To that end, menopause also brings a shift in hormones, which can cause veins to bulge, too. Similarly, hot weather can trigger this same expansion of the blood vessels. That’s because as the heat rises, it makes the circulatory system work harder, thus etching veins more clearly on the skin’s surface.

Still, none of these vein-related conditions bring with them any reason to see a doctor. But some symptoms relating to visible veins should raise alarm bells. And most of them have to do with varicose veins – the ones mentioned above that typically do not cause anything more than cosmetic concerns.

Sometimes, varicose veins can signal that you have chronic venous insufficiency. This means that the veins in the legs prevent deoxygenated blood from pumping back up to the heart and lungs. So, the blood drips backward instead, leaving it to pool in the legs. Skin around the ankles may turn brown because of it, too.

A doctor can diagnose you with chronic venous insufficiency and the right treatment path can prevent a slew of painful side-effects. Those with the condition tend to experience everything from leg cramps and swelling to leg ulcers. Fortunately, though, this isn’t one of the fatal side effects that come with visible veins.

Another potentially dangerous situation can arise if you cut, bump or otherwise open up a varicose vein. Like any other vein, these visible kinks will begin to bleed, and oftentimes the flow will be profuse. To stall it, you can lift a leg into the air and apply pressure, but it can quickly become an emergency if the blood doesn’t stop. Indeed, that’s when it’s time to call an ambulance.

To that end, a visible vein might warrant a call to the doctor if skin starts feeling warm, turning red, or if you can feel a painful, dense bump beneath the surface. These symptoms can point to a superficial thrombophlebitis diagnosis, which means that either the vein wall has been damaged somehow, or that there’s a blood clot.

Although a blood clot sounds serious, superficial thrombophlebitis typically is not. Instead, the build-up breaks down on its own, and the redness and inflammation fade over several days. It is important to note that the condition shouldn’t cause swelling in the calf, nor should it stop a person from walking – it can be painful, though.

And yet, superficial thrombophlebitis isn’t the most serious condition that can arise within visible veins. Deep vein thrombosis occurs when a clot develops in a blood vessel that’s not superficial. Typically, doctors discover it has formed in the legs, particularly in the calf or thigh area. And these clots can become a serious health issue.

Scarily, deep vein thrombosis can occur and cause zero detectable symptoms. However, in some cases, sufferers do feel as though their legs – especially the calves – feel tender or swollen. They might also notice patches of redness on the skin, as well as warm-to-the-touch spots, heat that typically radiates from the clot.

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On top of that, deep vein thrombosis tends to affect either the left or right leg. Any pain that comes with the clot would worsen if you flex your foot so that the toes point to the knee. And if you have any of these symptoms, notify your doctor and seek medical attention as quickly as possible.

That’s because roughly ten percent of those with deep vein thrombosis will suffer from a pulmonary embolism, according to the U.K.’s National Health Service. And when a leg-based blood clot breaks apart, it can travel back up through the veins and to the lungs. This creates a blockage to the organ, which prevents blood from entering as designed.

Those suffering from a pulmonary embolism may experience a slew of symptoms. You might feel short of breath, a sensation that only gets worse if you exert yourself. Having chest pain, swollen legs, lightheadedness, discolored skin or a cough that brings up blood can also point to a pulmonary embolism.

So, if you have visible veins and any of the above symptoms, don’t hesitate to contact a doctor. A pulmonary embolism can cause death – a quarter of sufferers will pass away suddenly without symptoms. About 37 percent of people pass away within one year of the initial diagnosis, according to eMedicine.

Fortunately, you can take some precautionary measures to ward off varicose veins and keep their related clots at bay. For instance, losing weight can take extra pressure off of the veins in the legs. To that end, a diet low in salt will keep you from retaining too much water, which can otherwise cause swelling.

On top of that, you can fight varicose veins with regular exercise, which keeps blood circulating properly through the legs. You might also spend some time elevating your limbs above your heart to increase blood flow, too. No matter which measures you take, you’ll be on your way to improved health – all the way down to your blood vessels.