Like HowStuffWorks on Facebook!

Circulatory System

The heart is a hollow, muscular organ that maintains blood circulation throughout the body. Learn how the circulatory system - your heart, blood vessels and blood - bring nutrients to the body.

What causes the sound of a heartbeat?

Lub-dub, lub-dub, lub-dub, that's usually the sound of a happy heart. But what's responsible for that steady, comforting beat, and what if your heart doesn't sound like that?

What makes your heart beat?

What makes your heart beat? Learn more about what makes your heart beat from Discovery Health. See more »

How and why does the heart pump blood to itself?

Have you ever stopped to wonder how does the heart pump blood to itself and the rest of your body? Learn the answer to how does the heart pump blood. See more »

What determines the rhythm of your heart?

Your heart rhythm must be steady in order to be strong. Learn about your heart rhythm and find out what your heart does during every heartbeat. See more »

What causes the sound of a heartbeat?

We're all familiar with the sound of a heartbeat: lub-dub, lub-dub. Learn what causes the sound of a heartbeat and why yours might sound different. See more »

Heart Pictures

Take a look at these heart pictures to learn about the different parts of the heart and more. See our heart pictures to learn more. See more »

Serum Creatinine Overview

Serum Creatinine is a chemical molecule that is present in the liquid portion of the blood. Read more about serum creatinine and what it can do. See more »

Natriuretic Peptide

Natriuresis is the excretion of large amounts of salt in the urine, and natriuretic peptides cause natriuresis. Learn about naturiuresis and natriuretic peptides. See more »

Why do you have two lungs but only one heart?

The two lungs-one heart system evolved because that was all we needed to survive. Learn more about the evolution of the two lungs-one heart system. See more »

The Heart: Introducing One Tough Muscle

Knowing about the heart is good information for your overall health. Learn more about the heart from Discovery Health. See more »

Are all blood types needed for for donation?

I would like to donate blood and keep hearing about which blood types are needed most. How do human blood types work? See more »