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.


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What Is the Rarest Blood Type in the World?

There are eight major blood types and some are more common than others. But what's the rarest of them all?

What Does It Mean If Your Blood Oxygen Level Is Low?

A low oxygen level in your blood is a good indicator of a COVID-19 infection, but what exactly does that mean and how do doctors test for it?

The Man Whose Blood Saved More Than 2 Million Babies

James Harrison's blood contains a rare and powerful antibody that's saved millions of Australian babies. Now, at age 81, after donating 1,173 times, he's hoping someone else will pick up where he left off.

What makes your heart beat?

What makes your heart beat is not a simple as you think. Take a look into what makes this crucial organ tick with the help of the information inside this article.

How and why does the heart pump blood to itself?

Every second of the day, blood bustles through your body, bound for your brain, toes and everything in between. How does your heart receive its fair share of the good stuff?

What determines the rhythm of your heart?

When it comes to the heart, timing is critical. An irregular heartbeat is inefficient and deprives the body of blood. So what sets your ticker's rhythm?

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?

Heart Pictures

The heart is crucial to life and the focus of love. Explore the parts of the heart and how your heart works, plus take a look at real pictures of human hearts with this gallery.

Serum Creatinine Overview

Creatinine is a chemical molecule that is present in the serum (liquid portion) of the blood. The amount of creatinine produced depends on a person's muscle mass. But how is it measured?

Why do you have two lungs but only one heart?

Why don't you have two hearts? Here's a hint: It's the same reason why you have one liver, but two eyes. Would a second heart help you live better?

The Heart: Introducing One Tough Muscle

The heart is one of the strongest muscles in the body. Learn about this extremely vital part of the body.

Are all blood types needed for for donation?

When you donate blood, your blood type is an important factor in how useful your donation may be. Just what are blood types?

How Blood Works

When you cut yourself accidentally, do you ever wonder what makes up this thing we call blood? It's pretty amazing stuff, considering how it wards off infections while supplying nutrients to every cell in the human body.

How Your Heart Works

The heart is a vital organ that basically serves as a pump. Learn about heart chambers and valves, blood flow, the heart's electrical system and blood supply. Read more about how this amazing organ works.