Day 13 of 30
Circulatory system: heart
Lessons are AI-drafted and being reviewed and improved over time.
Learning Objective
Explain how the four chambers and four valves of the heart keep blood moving in one direction through two separate loops.
Your heart is a muscle about the size of your fist, and it never takes a day off. At a resting rate of about 70 beats a minute, it beats roughly 100,000 times a day. Each beat pushes blood around a loop that delivers oxygen and nutrients to every cell and carries waste away. How does one small organ run two separate trips at once?
The heart has four chambers. The two upper chambers, the atria (one is an atrium), receive blood coming into the heart. The two lower chambers, the ventricles, pump blood out. A thick wall of muscle called the septum separates the right side from the left, so the two kinds of blood never mix. Heart diagrams are drawn as if you were facing another person, so the heart's left side appears on the right of the picture.
The two sides have different jobs. The right side receives oxygen-poor blood from the body and sends it to the lungs to pick up oxygen. This is the pulmonary circuit. The left side receives the oxygen-rich blood back from the lungs and pumps it out to the rest of the body through the aorta, the body's largest artery. This is the systemic circuit. The left ventricle has the thickest wall of all, because it must push blood through your whole body, while the right ventricle only has to reach the nearby lungs.
Blood must only travel one way, so valves act as one-way doors. Follow one trip: oxygen-poor blood enters the right atrium, passes through the tricuspid valve into the right ventricle, then through the pulmonary valve into the pulmonary artery on its way to the lungs. It returns to the left atrium, passes through the mitral valve into the left ventricle, and leaves through the aortic valve into the aorta. Strong cords anchored to the papillary muscles keep the valve flaps from flipping backward. The "lub-dub" sound of a heartbeat is the sound of valves snapping shut.
A common mistake is thinking blood in your veins is blue. It is not. Blood low in oxygen is dark red, and veins only look blue through your skin. Diagrams color it blue to show which blood has less oxygen. Use the interactive heart on this page to find every chamber and valve named above. Next lesson: the blood vessels that carry blood to and from the heart.
An upper chamber of the heart that receives blood coming in.
A lower chamber of the heart that pumps blood out; the left ventricle has the thickest wall.
The muscular wall that separates the right side of the heart from the left.
A flap of tissue that opens one way so blood cannot flow backward.
The largest artery; it carries oxygen-rich blood from the left ventricle to the body.
The involuntary muscle of the heart that contracts without you thinking about it.
Instructions
Materials Needed
- Clock or timer with seconds
- Paper and pencil
Which heart chambers receive blood, and which pump it out?
Why is the wall of the left ventricle thicker than the wall of the right ventricle?
List the four valves in the order blood passes them, starting from the right atrium.
Predict what would happen if a heart valve did not close all the way.
A friend says the blood in your veins is blue. How would you correct them?
Teaching Tip
Students often mix up left and right. Have them write L and R on the diagram from the patient's point of view before doing anything else. Ask "which ventricle would you expect to be stronger, and why?" before revealing the answer.
Go Deeper
Compare resting pulse before and after a minute of exercise. Research why athletes often have a lower resting heart rate, or what happens when a valve is damaged.
The AI tutor is not currently available.