A Field Trip Around the Heart

A Field Trip Around the Heart

by Lisa McNeil

Introduction

The circulatory system is a closed loop that transports blood gases, nutrients and waste around the body, helps regulate temperature, and protects against injury and disease. This book is going to focus on how blood circulates around the pulmonary (lungs) and systemic (body) pathways to exchange carbon dioxide and oxygen.

Blood that is carrying carbon dioxide is called deoxygenated (meaning without oxygen) and has a bluish tint, so the text will be blue.

Blood that is carrying oxygen (called oxygenated) has more of a red colour, so the text will be red. This colour-coding should help you remember what's happening in each stage.

Since the circulatory system is a closed loop, we could start anywhere and make the full cycle. We're going to start as blood coming back from the body.

Let's start our field trip!

Let's go!

We've been down to the big toe to pick up the carbon dioxide released from the cells and and now need to make our way back to the heart. The veins are the highways back to the heart, so we make our way up through the leg and the torso to the main highway, the inferior vena cava. "Inferior" means below, so this collects us blood from everywhere below the heart.

There is also the superior vena cava ("superior" means above) that collects everyone that went to places above the heart - the head, chest, and arms.

The vena cavae are the entry points into the right side of the heart. You're wondering why it's called the right side when clearly it's on the left side of the picture? You're looking at this heart from the front, so how it would look to you in the chest of someone sitting across the table.

As we all come in through the vena cavae, we first enter into the right atrium. It's a homecoming party because we're returning from all parts of the body. Those of us that were down to the big toe meet up again with those that travelled to the left ear lobe and oh, the stories to share!

The sinoatrial (SA) node, located in the wall of the right atrium, stimulates the contraction of the atrium. This pushes everyone on through the tricuspid atrioventricular (AV) valve and on to the right ventricle.

(A side note - it's called tricuspid because it's made up of three cusps or flaps.)

Coming through the tricuspid AV valve, we enter into the larger-sized right ventricle. I hope you brought everything with you because looking back at that valve you'll see that it's closed and won't let us back through to the atrium. It's the same with all the valves in the heart. The plan is to always have everyone moving in the one direction only, so a fully functional heart stops anyone from going back to where they just came from.

As you have a look around, you'll notice that the muscle walls here are thicker than in the right atrium. The atria only need to send us into the next room, but the ventricles needs to push us a further distance.

Now to move us on out of the right ventricle, the atrioventricular (AV) node is going to send a signal down the Purkinje fibres and around to the bottom of the heart to start the contraction there and push us on up out through the pulmonary semilunar valve.

What's really exciting is that coming out of the right ventricle through the pulmonary semilunar valve means we're into the pulmonary arteries and we're on our way to the lungs.

I love this part because the pulmonary artery splits in two, so you can choose if you'd like to go see the right or left lung. 

The lungs sit fairly close to the heart, so it's not that far to travel. No matter which side of the lungs you choose, the same thing is going to happen. We drop off the carbon dioxide coats we've been wearing as they're no longer needed.

We exchange them for new oxygen ones! This means we're now ready for the next trip out to the body to deliver the oxygen to the places it's required.

Before we can go out to the body, we need to travel through the other side of the heart first. We return to the heart, specifically to the left atrium, through the pulmonary veins.

Veins are what take blood back to the heart and this is the only time we're oxygenated blood in a vein because we're just coming back from the lungs.

We gather together again from both sides of the lungs in the left atrium. This is really just a rest stop as we wait to go into the left ventricle. The AV valve separating the left atrium and the left ventricle is a bicuspid valve this time because it's made up of just two sections.

The contraction of the left atrium actually happens at the same time as the contraction of the right atrium over on the other side.

The left ventricle is really the last chamber of the heart that we're going through. You're probably thinking, "Wow, the walls in here are really thick."

The reason is that this left ventricle gives the big push needed that sends us back out around the entire body. That's right, out to the right foot and the left ear lobe (and everywhere in between too!).

This happens at the same time as the right ventricle is contracting to send a new group of blood out to visit the lungs.

They throw open that aortic semilunar valve and send us out of the left ventricle to the aorta. This is the arch that goes up above the heart, anyone going to the places above (i.e. head and arms and left ear lobe) takes an exit, and then everyone else continues down around to the rest of the body. We're off to deliver the oxygen we picked up in the lungs to all the parts of the body.

This concludes the field trip around the heart as we're now back out to the body. See you back over in the right atrium in a little while?

Now that we've been through each chamber individually, have a look at the heart overall. The image shows that the left and right sides work at the same time.

The atria are contracting simultaneously to send blood into the right and left ventricles, which then contract at the same time to send it out to the lungs and the body.

Check out a CGI animation of a beating heart.

Or if you're really interested, see the real thing bop around in an open chest! 

References:

Title page image
Haggstrom, M. (2008, November 23). Heart (vessels only). Retrieved October 20, 2012 from http://commons.wikimedia.org/wiki/File:Heart_(vessels_only).gif

Original blank heart image
Cull, P. (Ed.). (1989). The Sourcebook of Medical Illustration. Park Ridge, NJ: Parthenon. Retrieved October 16, 2012 from http://acccn.net/Bio/book/Bio50/LecNotes/lecnot16a.html

Pulmonary exchange image
Blurpeace. (2009, May 7). Heart-and-lungs. Retrieved November 3, 2012 from http://en.wikipedia.org/wiki/File:Heart-and-lungs.jpg

Heart pumping image
Human Heart Healthy Pumping Pump. Retrieved October 20, 2012 from
http://pixabay.com/en/human-heart-healthy-pumping-pump-41546/

CGI heart video
Gulianelli. (2008, October 1). A Beating Human Heart (3D). Retrieved October 23, 2012 from http://www.youtube.com/watch?v=71olG7fyOCI

Real human heart video
Britannica. (2009, March 11). Heart Beating. Retrieved October 23, 2012 from http://www.youtube.com/watch?v=NYB-rJZQt4w