What is the cardiovascular system? It is the question behind a lot of biology homework this week, from a blank diagram of the heart to a quiz on what blood is made of. The short answer: a pump, a network of tubes, and the fluid that moves through them. Below is the tour, one part at a time, with the names anatomy texts use.

Plenty of people are asking. According to Google Trends, for US searches over the past seven days related to the seed term "cardiovascular system," the top related queries included "the cardiovascular system" at 100, "what is cardiovascular system" at 43, "what is the cardiovascular system" at 41, "heart cardiovascular system" at 34, "cardiovascular system function" at 16, "function of the cardiovascular system" at 13, "what is your cardiovascular system" at 10 and "cardiovascular system organs" at 9. Among rising queries, "what is the cardiovascular system made up of?" and "how much blood is in the human body" were both marked Breakout.

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. "Breakout" is Google's label for growth too large to show as a percentage, so it tells you interest jumped sharply, not how many people searched.

This piece sticks to plain anatomy and naming: the three parts the system is built from, the heart's chambers and valves by name, what blood contains, and how much of it an adult body holds. Every definition comes with its source, mainly the National Cancer Institute's SEER Training Modules, the National Heart, Lung, and Blood Institute (NHLBI) and MedlinePlus from the National Library of Medicine.

What is the cardiovascular system, in one definition

The SEER Training Modules give the cleanest definition. The cardiovascular system "is sometimes called the blood-vascular, or simply the circulatory, system." SEER says it consists of the heart, "a muscular pumping device," and "a closed system of vessels called arteries, veins, and capillaries." The blood inside, in SEER's words, "is pumped by the heart around a closed circle or circuit of vessels."

That answers the naming question too. "Cardiovascular" and "circulatory" are used for the same system: SEER lists them as alternate names for one thing. NHLBI uses the circulatory wording and puts the heart "at the center of your circulatory system," describing the system as "a network of blood vessels, such as arteries, veins, and capillaries."

So if someone asks what is your cardiovascular system, the textbook reply has three parts: the heart, the blood vessels and the blood.

What is the cardiovascular system made up of?

Here are the three components, with what each source calls them.

When people search cardiovascular system organs, the heart is the organ most texts single out. The vessels are the network it connects to, and blood is the tissue that moves through both.

The heart, by its parts

NHLBI describes the heart as "an organ about the size of your fist that pumps blood through your body" and places it "in the center of your chest, near your lungs." SEER adds that it is "shaped and sized roughly like a man's closed fist with two-thirds of the mass to the left of midline."

The four chambers

SEER says "The internal cavity of the heart is divided into four chambers": the right atrium, right ventricle, left atrium and left ventricle. The two upper chambers are the atria (singular: atrium), and the two lower chambers are the ventricles, per NHLBI.

• Atria: SEER calls them "thin-walled chambers that receive blood from the veins."
• Ventricles: SEER calls them "thick-walled chambers that forcefully pump blood out of the heart."

NHLBI names the dividing line: "An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum."

The four valves

According to SEER, "The heart has two types of valves that keep the blood flowing in the correct direction." The atrioventricular valves (also called cuspid valves) sit between the atria and ventricles. The semilunar valves sit at the bases of the large vessels leaving the ventricles. By name:

• Tricuspid valve: the right atrioventricular valve.
• Bicuspid, or mitral, valve: the left atrioventricular valve.
• Pulmonary semilunar valve: between the right ventricle and the pulmonary trunk.
• Aortic semilunar valve: between the left ventricle and the aorta.

The three layers of the wall

SEER says "Three layers of tissue form the heart wall": the epicardium on the outside, the myocardium in the middle and the endocardium on the inside. The whole heart "is enclosed in a pericardial sac."

The blood vessels, in one paragraph

SEER sorts the vessels into three types: "Arteries carry blood away from the heart," "Veins carry blood toward the heart," and capillaries are "the smallest and most numerous of the blood vessels," linking the two. We cover each type in its own post, linked below, so we will leave it at the names here.

What blood is made of

MedlinePlus splits blood into a liquid part and a solid part. "The liquid part, called plasma, is made of water, salts, and protein." "The solid part of your blood contains red blood cells, white blood cells, and platelets." MedlinePlus also notes that "Over half of your blood is plasma."

The SEER introduction to blood gives the figures. Plasma "makes up about 55% of blood volume," and SEER describes it as the watery fluid in which the other parts are suspended. SEER calls the solid part the formed elements, "Cells and cell fragments suspended in the plasma," and lists three classes:

• Erythrocytes (red blood cells): the "Most numerous of the formed elements," per SEER.
• Leukocytes (white blood cells): larger than red cells and fewer in number, per SEER.
• Thrombocytes (platelets): SEER notes these are "not whole cells but rather small fragments" of very large cells called megakaryocytes.

How much blood is in the human body

Two federal sources give close figures. The SEER Training Modules say "In an adult human, blood volume measures about five liters and accounts for eight percent (8%) of the body weight." MedlinePlus puts it as a range: "The average adult has between 5 and 6 liters of blood or blood volume."

Both are averages for adults. The figure you see quoted will depend on which source a page is drawing from, which is why it is worth checking the source rather than the number alone.

Cardiovascular system function, as the anatomy texts describe it

When people search for the function of the cardiovascular system, the textbook answer is about movement and distribution. SEER describes the heart as "a muscular pump that provides the force necessary to circulate the blood to all the tissues in the body" and "the organ that keeps the blood moving through the vessels." The vessels, in SEER's words, are "the channels or conduits through which blood is distributed to body tissues."

Put together: the heart supplies the force, the vessels supply the route, and the blood is what travels.

Our take

We like starting with the parts list. Most confusion about the cardiovascular system is really confusion about names: which chamber is an atrium, which valve is the mitral, what "formed elements" means, and whether "cardiovascular" and "circulatory" are two systems or one. They are one.

The short version: a four-chambered heart with four valves, three types of vessels, and blood that is a little over half plasma, about five liters of it in an adult according to SEER.

For the routes blood takes, our piece on the pulmonary, systemic and coronary loops covers that ground instead of repeating it here. Our explainer on arteries, veins and capillaries by name takes the vessels one type at a time. And for the shopping side of a multi-jar listing, see what a three-pack of the same capsule actually buys you. Questions about your own heart or blood belong with a clinician.

Why we carry it at PRSPR

This article is anatomy, and the product tied to it is a different kind of thing: a three-pack of one jar. Our Fermented Beet Root & Maca 3 Pack Bundle is three of the same jar in one order. The current front label reads "Fermented Beet With Maca," "Dietary Supplement" and "60 Vegan Capsules." Those are label words, and the three-pack post above explains what buying the same jar in threes means in plain numbers.

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. The bundle is listed at $89.98.

To read the full label, see the Fermented Beet Root & Maca 3 Pack Bundle at PRSPR.

FAQ

What is the cardiovascular system?

The SEER Training Modules describe it as the heart, "a muscular pumping device," plus "a closed system of vessels called arteries, veins, and capillaries." SEER notes it is also called the blood-vascular or circulatory system.

What is the cardiovascular system made up of?

Three parts: the heart, the blood vessels (arteries, veins and capillaries) and the blood. Per SEER, the heart has four chambers: the right and left atria and the right and left ventricles.

How much blood is in the human body?

SEER puts adult blood volume at about five liters, about 8% of body weight. MedlinePlus gives a range of 5 to 6 liters for the average adult.

What is blood made of?

According to MedlinePlus, plasma (water, salts and protein) plus red blood cells, white blood cells and platelets. SEER says plasma is about 55% of blood volume.

Are the cardiovascular and circulatory systems the same thing?

Yes. SEER says the cardiovascular system "is sometimes called the blood-vascular, or simply the circulatory, system."