Step by Step
RA
Right atrium — deoxygenated blood enters
Deoxygenated blood returning from the body enters the right atrium via the superior and inferior vena cava.
RV
Right ventricle — pumps to the lungs
Blood passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary semilunar valve into the pulmonary arteries, heading to the lungs for gas exchange. This entire right-side pathway is the pulmonary circuit.
LA/LV
Left atrium and ventricle — oxygenated blood returns and is pumped out
Oxygenated blood returns from the lungs via the pulmonary veins into the left atrium, passes through the mitral (bicuspid) valve into the left ventricle, then exits through the aortic semilunar valve into the aorta to supply the whole body — this left-side pathway is the systemic circuit.
Key
Why the left ventricle has a thicker wall
The left ventricle must pump blood against much higher systemic resistance (throughout the entire body) than the right ventricle, which only pumps against the lower resistance of the nearby lungs — this is why the left ventricle wall is significantly thicker and more muscular.
Applied Walkthrough
1
Trace a single red blood cell starting in the right atrium. It flows through the tricuspid valve into the right ventricle, then out through the pulmonary valve toward the lungs — completing the pulmonary circuit.
2
In the lungs, the cell picks up oxygen and releases carbon dioxide, then returns via the pulmonary veins into the left atrium.
3
It passes through the mitral valve into the left ventricle, then is forcefully ejected through the aortic valve into the aorta — beginning its journey through the systemic circuit to deliver oxygen to body tissues.
4
Both the right and left sides contract simultaneously, meaning while the right side sends blood to the lungs, the left side is simultaneously sending oxygenated blood to the body — the two circuits operate as one coordinated pump.
Exam Application
Exams test whether you can trace the correct order of chambers and valves blood passes through in both the pulmonary and systemic circuits, and whether you understand why the left ventricle's wall is thicker than the right's.
⚠ Common Trap
The most common trap is reversing which side handles oxygenated versus deoxygenated blood — remember the right side always handles deoxygenated blood heading to the lungs, and the left side always handles oxygenated blood heading to the body.
✓ Quick Self-Check
1. What are the four chambers of the heart, in the order blood flows through them starting from the right atrium?
Right atrium → right ventricle → left atrium → left ventricle.
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2. Which side of the heart is part of the pulmonary circuit?
The right side.
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3. Which side of the heart is part of the systemic circuit?
The left side.
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4. Why is the left ventricle's wall thicker than the right ventricle's?
Because it must pump against much higher systemic resistance throughout the entire body, compared to the lower resistance of the nearby lungs.
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5. What valve does blood pass through going from the right atrium to the right ventricle?
The tricuspid valve.
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