Tanno and Iguda are travelling around Professor Wendy’s body in her bloodstream. The blood is being pumped through the blood vessels by a large muscular organ called the heart.

All heart diagrams are drawn as if you are looking at someone else’s, so the left hand side of the diagram is actually their right side.

Blood returning from the body is brought back in a large vein called the vena cava, and enters a chamber called the right atrium.

The muscle in the walls of right atrium contract and force blood through a valve down into another chamber called the right ventricle.

The right ventricle walls then contract, forcing the blood up and out of the heart into the pulmonary artery.

The blood does not get pushed backwards because the right AV valve snaps shut, keeping the flow one way.

Image credit: Kelly Cassidy

When blood returns form the lungs in the pulmonary vein, it travels through the left atrium and left ventricle in the same way, and is then pushed out of the aorta and around the body. The septum separates the two sides of the heart so the blood never mixes.

Humans have a double circulatory system, which means the blood passes through the heart twice on each loop around the body. It travels from the heart to the lungs, back to the left hand side of the heart, then to the rest of the body before returning to the right hand side of the heart. This is like a ‘figure of eight’ system.

The left ventricle has incredibly thick muscle to pump blood all round the body.

This image shows a section through a human heart.
(Image credit: Joy Lincoln, PhD)

Can you see the heart of this unborn baby mouse?

The heart shows up in blue.

Image credit: Joy Lincoln, PhD.


Make a simple model of the heart

1) Fill a clean, clear jar just over half full with water and add a few drops of red food colouring.
2) Cut the neck off a balloon and stretch the rest over the opening of the jar. Save the balloon neck.
3) Use scissors (get an adult to help) to carefully poke two holes in the balloon. The size of each hole should allow a straw to be pushed through, so they fit tightly.
4) Push a straw through each hole.

5) Place the neck from the balloon over the end of one straw and seal the end of one straw with the balloon neck and attach in place with tape. This will act as a valve.
6) Press down on the balloon. The red liquid should be forced out of the unsealed straw. This is like the heart muscle contracting.
7) The ‘valve’ on the sealed straw stops the coloured liquid going back down.
8) Now remove the valve and the liquid should flow back down the straw.

You can go to the previous PostcardsFromTheBody secret pages by clicking the places below.

MouthStomachSmall Intestine
Photo of PostcardFromThe Mouth
Large IntestineBlood