Pulmonary Circuit: Definition, Pathway & Quiz

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Kelly Robson

Kelly has taught High School Science and Applied Communications. She holds an Education Specialist Degree in Ed. Leadership.

The pulmonary circuit is the path deoxygenated blood takes through the heart to the lungs. This lesson explains the reason for the journey as well as outlines the path through the pulmonary circuit.

We also recommend watching Pulmonary Surfactant Function and Ventilation and What Are Pulmonary Function Tests?

Cells need energy to perform their vital functions. Through a process known as cellular respiration, a cell creates adenosine triphosphate (ATP) to use as energy. Cellular respiration requires oxygen, and cells cannot store ATP, so every cell needs a continuous supply of oxygen to create ATP over and over again. Therefore, every cell needs a continuous supply of fresh oxygen.

Red blood cells carry oxygen through the body and deliver it to the cells. Red blood cells move throughout the body giving oxygen to the cells and taking the waste in the form of carbon dioxide out. The red blood cells get rid of the waste and obtain more oxygen as they travel through the pulmonary circuit.

Blood flow through the heart.
Blood flow through the heart

Journey through the Pulmonary Circuit

Let's look at the pulmonary circuit through the eyes of a blood cell. For this example, we'll call the blood cell 'Red'. We'll start this journey when Red returns home from a very hard day at work. Red's job is very important. He spends the whole day giving away oxygen and collecting carbon dioxide from other cells throughout the body. When he returns home he is very low in oxygen, or deoxygenated.

As a deoxygenated blood cell, Red travels through either the superior vena cava or the inferior vena cava. Don't worry, both vena cavas are fine to travel through; superior just means the one on top and inferior is the one on the bottom. These vena cavas function like slides, and Red and friends can choose which water slide they would like to travel down. The vena cavas are a one-way tunnel to the right atrium. The right atrium is the first holding tank near the top of the heart. Many other deoxygenated blood cells meet Red in the right atrium until it is full.

Once full, the right atrium contracts, sending Red and all of other deoxygenated blood through the atrioventricular valve (AV). The AV is another one-way valve that closes at the opening so the blood cannot flow back. The AV is also known as the tricuspid valve. This is due to the 3 flaps in its structure. All the blood brothers flow through the AV (another slide, like the vena cavas) and into the right ventricle, the second holding tank.

The right ventricle contracts, sending Red and the rest of the gang to the pulmonary artery. Red is finally getting close to becoming re-oxygenated. The pulmonary arteries take Red to the lungs. The artery branches off into millions of pulmonary capillaries. Finally, Red is able to release the carbon dioxide waste into the lungs to be exhaled out of the body. Red also collects the oxygen to be delivered on the next trip through the body.

Now, Red and the other oxygenated blood travel through the 2 pulmonary veins back to the left side of the heart. Here they will fill the left atrium and begin their journey through the systemic circuit. It is on their journey through the systemic circuit that they will deliver the much-needed oxygen to the other cells in the body and retrieve the waste in the form of CO2.

Pathway through the Pulmonary Circuit.
Pathway through the Pulmonary Circuit


Unoxygenated & Waste Rich Red Blood Cells > Vena Cava > Right Atrium > Atrioventricular Valve / Tricuspid Valve > Pulmonary Artery > Pulmonary Capillaries (Releases CO2 and gains O2 ) > Oxygenated Blood > Pulmonary Veins > Left Atrium > Systemic Circuit.


  • Vena Cava: Superior (top) Inferior (bottom) one way valve in which deoxygenated blood enters the heart.
  • Right Atrium: First holding tank that contracts pushing the blood into the AV.
  • Atrioventricular Valve: (aka: Tricuspid Valve) One-way valve with a flap that closes so that the blood cannot flow back up.
  • Pulmonary Artery: Blood vessel that takes deoxygenated blood from the heart to lungs.
  • Pulmonary Capillaries: Millions of little branches from the pulmonary artery where the deoxygenated blood obtains oxygen and releases carbon dioxide.
  • Pulmonary Veins: Blood vessels that carry the newly oxygenated blood back to the heart where it is then pumped through the rest of the body.
  • Systemic Circuit: The path the newly oxygenated blood takes through the rest of the body.

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