What are arteries? It sounds like a question from a middle school worksheet, and plenty of adults could not answer it cleanly. Most people know arteries and veins are both "blood vessels," and that one of them shows up blue under the skin. Ask how an artery differs from a vein, or where capillaries fit, and the answer gets vague fast.

The search data says a lot of people are asking right now. According to Google Trends, for US searches over the past seven days related to the seed term "arteries," the top related queries included "what is arteries" at 86, "veins" at 80, "artery" at 63, "arteries and veins" at 57, "what are arteries" at 53, "what do arteries do" at 33, "blood vessels" at 22 and "capillaries" at 17. The rising query "how many pulmonary arteries are there" was up 180%, and "are there arteries in your fingers" was up 40%.

A quick note on those numbers. Google Trends scores are relative within each seed term: 100 marks the most searched query in that set, and the others are scaled against it. They are not search counts. A rising percentage measures growth in interest, not volume.

So people want the names. This piece gives you the vocabulary: the three kinds of blood vessel, what each one carries, the pulmonary arteries as the textbook exception, and the small arteries that run along your fingers. Every fact comes with its source.

Blood vessels: the three types by name

Start with the umbrella term. Cleveland Clinic's blood vessel overview puts it plainly: "There are three main types of blood vessels," and lists them as arteries, veins and capillaries. The same page says "Your body contains about 60,000 miles of blood vessels." That figure covers all three types together.

The cleanest way to keep them apart is by direction. The National Cancer Institute's SEER Training Modules, in the section "Classification & Structure of Blood Vessels," give the two rules in seven words each:

• Arteries: "Arteries carry blood away from the heart."
• Veins: "Veins carry blood toward the heart."
• Capillaries: the SEER module calls them "the smallest and most numerous of the blood vessels," forming the connection between the two.

There are also two in-between names worth knowing. According to the SEER module, arteries branch into "microscopic arteries called arterioles," and after blood passes through the capillaries, "it enters the smallest veins, called venules." So the full sequence reads artery, arteriole, capillary, venule, vein.

What are arteries, and what do arteries do?

Cleveland Clinic defines arteries as "the blood vessels that bring oxygen-rich blood from your heart to all of your body's cells." It describes their job as "distributing oxygen, nutrients and hormones throughout your body." That is the textbook answer to "what do arteries do": they are the outbound routes.

You will also see the question typed as "what is arteries." It is the same search with the grammar slipped, and the answer is the same. One vessel is an artery; several are arteries.

Cleveland Clinic's page "Arteries" adds two pieces of vocabulary that show up in anatomy class:

• Two types. The page says "You have two types of arteries that have slightly different functions." Elastic arteries sit closer to the heart and contain more elastic tissue. Muscular arteries carry a larger share of smooth muscle and carry blood out toward the tissues. The smallest are the arterioles.
• Three wall layers. An artery wall is described in three layers: the tunica intima (inner), the tunica media (middle, mostly smooth muscle) and the tunica externa (outer, connecting to surrounding tissue and nerves).

The oxygen detail is where a related word comes in. The protein that carries oxygen in red blood cells is hemoglobin, and our explainer on what hemoglobin is and how it relates to iron covers that vocabulary.

Arteries and veins: the difference, and the exception

Here is the pair most people search together. Cleveland Clinic's page "Veins" says "Veins are blood vessels that carry oxygen-poor blood to your heart," and states the contrast directly: "The major difference between arteries and veins is the type of blood they carry." The same page notes that veins "sometimes also contain one-way valves that keep blood flowing in the right direction."

So the shorthand is arteries out, oxygen-rich; veins back, oxygen-poor. That shorthand is right almost everywhere, and the one place it flips is the lungs.

According to the SEER module, "Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs." Cleveland Clinic's "Arteries" page puts it the same way: "Your pulmonary artery is the only artery that carries deoxygenated blood." The mirror image is on the vein side. Cleveland Clinic's "Veins" page says "Pulmonary veins are an exception because they carry oxygen-rich blood from your lungs to your heart."

That exception is why the direction rule (away from or toward the heart) is the real definition, and the oxygen rule is only a pattern. A pulmonary artery is still an artery because it carries blood away from the heart, even though that blood is low in oxygen.

How many pulmonary arteries are there

This is the rising question, and the answer depends on how you count. Cleveland Clinic's page "Pulmonary Arteries" explains that "After leaving your heart, your main pulmonary artery divides into your right pulmonary artery and left pulmonary artery." So you will see the answer given as one main pulmonary artery that splits into two, a right and a left. If you want the larger map these vessels sit on, our piece on the pulmonary, systemic and coronary loops names each circuit.

Capillaries: the smallest blood vessels

Capillaries are the link between the two systems. Cleveland Clinic's page "Capillaries" says "Capillaries connect arteries and veins," and defines them as "delicate blood vessels that deliver nutrients and oxygen to cells throughout your body."

The size numbers are what make them memorable. According to that same Cleveland Clinic page, "A capillary is about 5 micrometers (a micrometer is 0.001 millimeter) around," and it is "so tiny that red blood cells have to go through it lined up one behind the other." The page gives the wall as thin as well: "Capillaries have thin walls (1 micrometer thick) that let nutrients, fluids and gases go in and out."

That thin wall is the whole point of the word. Arteries and veins are the routes; capillaries are where the exchange with the surrounding tissue takes place.

Are there arteries in your fingers?

Yes, and they have names. Elsevier's anatomy reference describes the proper palmar digital arteries, which "arise from the common palmar digital arteries" in the palm. Each common palmar digital artery splits into two proper branches, "which course distally along medial and lateral aspects of the digits." In plain terms, they run along both sides of each finger toward the fingertip.

The same Elsevier entry says they "supply the metacarpophalangeal and interphalangeal joints, as well as the phalanges," which are the finger joints and finger bones. "Digital" here has nothing to do with screens: in anatomy, a digit is a finger or toe.

Our take

We think this is vocabulary worth having straight. You will run into these words on anatomy diagrams, in articles, and on labels, and each one has a precise meaning.

Here is the short version. There are three kinds of blood vessel. Arteries carry blood away from the heart. Veins carry blood toward it. Capillaries are the tiny vessels between them, where blood meets tissue. Arterioles and venules are the small branches on either side of the capillaries.

The detail to hold onto is the pulmonary exception. "Oxygen-rich" is a pattern, not the definition. Direction is the definition. Once you have that, the question of how many pulmonary arteries there are stops being a trick question.

That habit, reading a word for exactly what it means, is the same one we use for supplement labels. Our guide to the long chemical names on a D3 + K2 label does that for the bottle below. Questions about your own body belong with a clinician.

Why we carry it at PRSPR

This piece started from a product on our own shelf, the Vitamin D3 + K2 Plus MK7. It pairs vitamin D3 with vitamin K2 in the MK7 form, and it is listed at $49.99 in our Vitamins & Supplements collection.

Like everything we carry, it is made in a GMP-certified facility and third-party batch tested, so what's on the label is what's in the bottle.

If you want to read the label for yourself, see the Vitamin D3 + K2 Plus MK7 at PRSPR.

FAQ

What are arteries?

Cleveland Clinic defines arteries as "the blood vessels that bring oxygen-rich blood from your heart to all of your body's cells." The NCI SEER Training Modules put the rule simply: "Arteries carry blood away from the heart."

What do arteries do?

According to Cleveland Clinic, arteries are responsible for "distributing oxygen, nutrients and hormones throughout your body."

What is the difference between arteries and veins?

Direction. The SEER module says arteries carry blood away from the heart and "Veins carry blood toward the heart." Cleveland Clinic adds that veins carry oxygen-poor blood, with the pulmonary veins as the exception.

How many pulmonary arteries are there?

Cleveland Clinic says the main pulmonary artery "divides into your right pulmonary artery and left pulmonary artery." That is one main trunk branching into two.

Are there arteries in your fingers?

Yes. Elsevier's anatomy reference describes the proper palmar digital arteries, which run along the medial and lateral sides of the digits.

What are capillaries?

Cleveland Clinic says "Capillaries connect arteries and veins," and that a capillary is about 5 micrometers around, small enough that red blood cells pass through one behind the other.