Blood reaches your lungs through a dedicated loop called pulmonary circulation, powered entirely by the right side of your heart. After traveling through your body and delivering oxygen to tissues, blood returns to the heart low on oxygen, gets pumped into the pulmonary arteries, and arrives at the lungs in under a second. There, it picks up fresh oxygen, drops off carbon dioxide, and heads back to the heart’s left side to be sent out to the body again.
The Path From Body to Lungs
Oxygen-depleted blood from your organs, muscles, and tissues flows back to the heart through two large veins and empties into the right atrium, the heart’s upper-right chamber. From there, it passes through the tricuspid valve into the right ventricle, the lower-right chamber. The right ventricle then contracts and pushes the blood through the pulmonary valve into the pulmonary artery, which is the only artery in your body that carries oxygen-poor blood. That artery splits into left and right branches, each heading to the corresponding lung.
Two valves keep this flow moving in one direction. The tricuspid valve has three small flaps (leaflets) that open to let blood into the right ventricle, then snap shut to prevent it from leaking backward into the atrium. The pulmonary valve works the same way at the exit of the right ventricle, stopping blood from sliding back in after the ventricle relaxes. If either valve doesn’t close properly, blood can pool or flow backward, making the heart work harder to push blood to the lungs.
Why the Right Side Pumps at Lower Pressure
Your lungs sit just inches from your heart, so the right ventricle doesn’t need to generate nearly as much force as the left ventricle, which pushes blood all the way to your feet. Normal pulmonary artery pressure tops out around 20 mmHg, with an average of about 12 mmHg. Compare that to the systemic side, where blood pressure typically runs around 120/80 mmHg. This lower pressure protects the delicate air sacs in your lungs from damage while still keeping blood flowing through them efficiently.
Blood in the pulmonary artery is only about 76% saturated with oxygen, even in a perfectly healthy person. That’s the whole point of the trip: it needs to reload.
What Happens Inside the Lungs
Once inside the lungs, the pulmonary arteries branch into smaller and smaller vessels until they become capillaries, blood vessels so narrow that red blood cells pass through in single file. These capillaries wrap tightly around tiny air sacs called alveoli, and this is where the actual gas swap takes place.
Oxygen from the air you breathe sits at a higher concentration inside the alveoli than in the blood, so it naturally moves across the membrane into the capillary. Carbon dioxide, a waste product your cells generated, is more concentrated in the blood, so it moves the opposite direction into the alveoli and gets exhaled. This exchange happens purely through diffusion: gases moving from where there’s more of them to where there’s less, no active pumping required.
To cross from the air sac into the bloodstream, oxygen passes through four ultra-thin layers: a coating of fluid lining the alveolus, the alveolar wall itself, a shared basement membrane, and the capillary wall. Despite these layers, the barrier is so thin that gas exchange reaches about 90% completion within roughly 0.4 seconds. At rest, a red blood cell spends approximately 0.7 seconds passing through the lung capillaries, giving it more than enough time. During intense exercise, that transit time can drop to 0.3 seconds, which still allows sufficient exchange in healthy lungs.
How the Lungs Direct Blood to the Right Places
Not every region of your lungs receives the same amount of air at any given moment. If a section of lung is poorly ventilated, perhaps from mucus, a collapsed airway, or a particular body position, the blood vessels in that area automatically constrict. This response, called hypoxic pulmonary vasoconstriction, diverts blood toward better-ventilated regions where it can actually pick up oxygen. It’s the lung’s built-in system for making sure blood doesn’t waste a trip to a part of the lung that has no fresh air to offer.
This response is unique to the lungs. Everywhere else in your body, low oxygen causes blood vessels to widen so more blood can reach oxygen-starved tissue. The lungs do the opposite because their job isn’t to receive oxygen from the blood but to load it. Sending blood to a poorly ventilated area would mean that blood returns to the heart still oxygen-poor, which helps no one.
The Return Trip to the Heart
After loading up on oxygen, blood drains from the capillary networks into progressively larger veins within the lungs. These converge into four pulmonary veins: a superior and an inferior vein from each lung. All four veins travel toward the heart and enter the left atrium through separate openings on its back wall. This is the only place in the body where veins carry oxygen-rich blood.
From the left atrium, the freshly oxygenated blood passes into the left ventricle, which pumps it out through the aorta to the rest of your body. The entire loop, from right atrium to lungs and back to left atrium, takes only a few seconds at rest. Your heart pumps your entire blood volume through this circuit roughly once per minute, meaning every drop of blood in your body cycles through your lungs about 60 times an hour to stay oxygenated.