Humans and many other organisms use a double circulatory system. This means that we have two separate circuits for blood in our body. Learn how these circuits differ and the function of each one.
What is a Double Circulation System?
The majority of mammals (including humans) utilize a double circulatory system. This means we have two loops in our body in which blood circulates. One is oxygenated, meaning oxygen rich, and the other is deoxygenated, which means it has little to no oxygen, but a lot of carbon dioxide.
Double circulatory systems are important because they ensure that we are giving our tissues and muscles blood full of oxygen, instead of a mixture of oxygenated and deoxygenated blood. While it may take a bit more energy than a single circulatory system, this system is much more efficient!
Some Basic Heart Anatomy
The organ that powers the circulatory system is the heart. The human heart pumps blood through four chambers: two atria and two ventricles. We also have four valves in our hearts that keep blood moving in one direction, preventing backflow back into the heart; two valves are found within the heart itself and two are found in the major arteries in the heart: the aorta and the pulmonary artery.
Humans have a 4-chambered heart (some organisms have only two or three chambers in their hearts; these animals are typically reptiles and fish), and we separate the heart into right and left halves. Each half of a human heart has one atrium, which sits 'on top' of one ventricle. The right and left halves of the heart are separated by a layer of muscle tissue called the septum. Keep in mind that when you look at a picture of a heart, it is shown how the heart would sit in your chest, so everything looks backwards.
There are two types of valves in the heart. The first set of valves are the atrioventricular valves that separate the atria from the ventricles. The tricuspid valve is the valve on the right side of the heart, and the bicuspid or mitral valve is the valve on the left side of the heart. The second set of valves are the semilunar valves that are found at the base of the aorta and the pulmonary artery. When you listen to your heart beat and you hear that 'lup-dub' sound, what you're hearing is the sound of these valves closing!
How Does Blood Move Through the Heart?
Let's start with the left side of the heart. Blood enters the left atrium, moves through the bicuspid, or mitral, valve, and into the left ventricle. Blood is then pushed through the aortic semilunar valve into the aorta. This blood is oxygenated and is distributed around the body, providing oxygen to our muscles and organs. Once this blood is deoxygenated, it returns to the heart through a few different veins.
Oxygenated blood moves through arteries, and deoxygenated blood moves through veins. Usually, these are shown on diagrams as red (oxygenated) and blue (deoxygenated). A simple way to remember this is that veins visit with deoxygenated blood, and arteries move away with oxygenated blood. The one big exception to this is the pulmonary artery. It carries deoxygenated blood!
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The deoxygenated blood returns to the heart in the right atrium. This blood moves through the tricuspid valve into the right ventricle, where it then moves through the pulmonary semilunar valve on its way to the lungs to pick up oxygen.
The basis of our double circulatory system is all about oxygen. The first loop is called the systemic circuit. In this loop, oxygenated blood moves from the heart to our body, where it off-loads oxygen to our muscles, tissues and organs. At the same time, this blood picks up carbon dioxide and other waste products. This blood is now considered to be deoxygenated, and it makes its way back to the heart.
The second loop is called the pulmonary circuit. Here, deoxygenated blood moves from the heart to the lungs, where it off-loads carbon dioxide and picks up oxygen. The now oxygenated blood then moves back to the heart, where it re-enters the systemic circuit.
So, the big picture here is that blood is always moving, picking up oxygen and off-loading it to tissues, muscles and organs that need it, and picking up carbon dioxide. However, in our bodies, oxygenated blood and deoxygenated blood never mix.
The human heart has four chambers, which help deliver blood to two different circuits. In the systemic circuit, oxygenated blood moves through the left side of the heart and delivers oxygen to the body. At the same time that this blood is off-loading oxygen, it is also picking up carbon dioxide. The deoxygenated blood then returns to the heart, where it moves through the right side of the heart and enters the pulmonary circuit. In the pulmonary circuit, deoxygenated blood moves to the lungs where it picks up oxygen and off-loads carbon dioxide. The oxygenated blood then returns to the left side of the heart where it re-enters the systemic circuit. While it requires more energy to have two different blood circuits, it is a more effective way of providing oxygen to our bodies!
This lesson on double circulation is designed to prepare you to realize these goals:
Note the importance of a double circulation system
Name the components of the four-chambered heart
Recount the process by which the double circulation system works to move oxygenated and deoxygenated blood through the body
Students often struggle to envision how the blood is actually flowing through double circulation. This activity seeks to address this as students create a walking tour of the circulation. This activity will require red and blue sidewalk chalk and a large space to draw outside, as students will draw a body, the lungs, and a diagram of the heart in colored chalk. Deoxygenated parts of the circulation can be drawn in blue and oxygenated parts in red. Then, students will take themselves or a friend on a "walking" tour of the circulation and point out important structures that were taught in the lesson.
In this activity, you're going to act like a blood cell traveling through the double circulation of a human being. To create this "walking tour" you'll need red and blue sidewalk chalk and a large space to draw. Follow the directions below to draw out the circulation, then lead a friend on a walking tour through your work.
Start by drawing a graphic heart in the center of your space.
Divide the heart into four sections and label each of the different chambers of the heart. Use red for chambers that have oxygenated blood and blue for chambers that have deoxygenated blood.
Next draw a set of lungs above the heart and a body below the heart.
Next draw tubes that represent arteries and veins that connect the circulatory system. Make sure to use the correct colors for oxygenated and deoxygenated blood.
Now that you have your circulation done, you can lead a friend on a walking tour of the circulation. Start in the body and flow back to the heart, then the lungs. Point out the important structures mentioned in the lesson as you go: right atrium, right ventricle, tricuspid valve, bicuspid valve, semilunar valves, left atrium and left ventricle.
How did you represent red blood cells in double circulation?
What could improve our model of double circulation? Were any components missing?
Students should be able to explain that they moved through the double circulation, just like red blood cells do. However, in this model, the red blood cells didn't actually "carry" oxygen or carbon dioxide, and this could be added in to make the model more accurate. Students might also report that their drawing could be more scientifically accurate.
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