What “pulmonary circulation” means (and what makes it different)
Pulmonary circulation is the short, low-pressure pathway that moves blood from the right side of the heart to the lungs for gas exchange, then returns it to the left side of the heart. Its job is to load oxygen (O2) onto hemoglobin and unload carbon dioxide (CO2) into the air spaces of the lungs.
Two features can feel “backwards” at first:
- Pulmonary arteries carry deoxygenated blood (because they carry blood away from the heart toward the lungs).
- Pulmonary veins carry oxygenated blood (because they carry blood toward the heart from the lungs).
Vessel names are based on direction of flow relative to the heart, not oxygen content.
Pressure and resistance: pulmonary vs systemic (why the right heart is a “low-pressure pump”)
The pulmonary circuit is designed for efficient gas exchange without flooding delicate alveoli. Compared with systemic circulation, pulmonary vessels have lower resistance and operate at lower pressures. Clinically typical values (approximate):
| Parameter | Pulmonary circulation | Systemic circulation |
|---|---|---|
| Arterial pressure | ~25/10 mmHg (mean ~15) | ~120/80 mmHg (mean ~90) |
| Primary function | Gas exchange | Deliver O2/nutrients to tissues |
| Vessel wall characteristics | Thinner, more compliant | Thicker, more muscular |
Practical implication: the right ventricle normally generates much less pressure than the left ventricle, because it is pushing blood through a low-resistance lung circuit rather than the high-resistance systemic circuit.
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Step-by-step pulmonary pathway (right heart → lungs → left heart)
Step 1: Systemic venous return enters the right atrium
Deoxygenated blood returning from the body arrives in the right atrium via the major systemic veins (e.g., superior and inferior vena cava). At this point, blood has:
- Lower O2 content (hemoglobin is less saturated)
- Higher CO2 content
- Low pressure (venous side of the circulation)
Practical checkpoint: If you were labeling a diagram, you would mark the right atrium as deoxygenated blood at low pressure.
Step 2: Right atrium → right ventricle (the filling-to-pumping handoff)
Blood moves from the right atrium into the right ventricle during ventricular filling. The right ventricle is the chamber that will generate the pressure needed to move blood through the pulmonary valve and into the pulmonary arteries.
Oxygenation status: still deoxygenated (no gas exchange has occurred yet).
Step 3: Right ventricle outflow through the pulmonary valve
When the right ventricle contracts, blood is ejected through the pulmonary valve into the pulmonary outflow tract. This is the gateway from the heart into the pulmonary arterial system.
Pressure note: right ventricular systolic pressure rises enough to open the pulmonary valve, but the peak pressure is far lower than what the left ventricle generates for systemic ejection.
Step 4: Pulmonary trunk → right and left pulmonary arteries
After crossing the pulmonary valve, blood enters the pulmonary trunk, which quickly divides into the right pulmonary artery and left pulmonary artery, each carrying blood to its respective lung.
This is where the “paired vessel approach” becomes especially useful:
- Pulmonary arteries (trunk and branches): carry blood away from the heart → therefore called arteries → blood is deoxygenated in this circuit.
- Pulmonary veins (later return): carry blood toward the heart → therefore called veins → blood is oxygenated in this circuit.
Practical example: If a question asks, “Which vessel carries deoxygenated blood but is still an artery?” the pulmonary arteries are the classic answer.
Step 5: Pulmonary arterioles → alveolar capillary networks (where gas exchange happens)
Within the lungs, pulmonary arteries branch repeatedly into smaller arteries and arterioles that feed dense capillary networks surrounding the alveoli. Gas exchange occurs across the alveolar–capillary membrane, a thin barrier optimized for diffusion.
What changes here:
- O2 diffuses from alveolar air into capillary blood.
- CO2 diffuses from capillary blood into alveolar air.
Anatomical location to name precisely: gas exchange occurs in the pulmonary capillaries that run in the alveolar walls (the alveolar septa), not in the large pulmonary arteries or veins.
Practical checkpoint: If you are asked to point to the “site of oxygenation,” you should identify the alveolar capillary bed, not the lung as a whole.
Step 6: Capillaries → venules → pulmonary veins (oxygenated blood returns)
After gas exchange, blood leaves the alveolar capillary networks into venules and then larger veins that form the pulmonary veins. At this point, blood is:
- Oxygenated (higher hemoglobin saturation)
- Lower in CO2
- Still relatively low pressure compared with systemic arterial blood
Most people have four pulmonary veins (two from each lung) returning to the heart.
Step 7: Pulmonary veins → left atrium (handoff to systemic circulation)
Pulmonary veins deliver oxygenated blood into the left atrium. This completes the pulmonary circuit and sets up the next step (systemic delivery) by providing oxygen-rich blood to the left heart.
Key naming reminder: pulmonary veins are called “veins” because they return blood to the heart, even though they carry oxygenated blood.
Paired vessel approach: keep direction and oxygenation separate in your mind
To avoid confusion, practice describing each vessel with two labels:
- Direction label: artery (away from heart) vs vein (toward heart)
- Oxygenation label: deoxygenated vs oxygenated
In pulmonary circulation, those labels pair like this:
| Segment | Artery/Vein (direction) | Oxygenation (typical) |
|---|---|---|
| Pulmonary trunk & pulmonary arteries | Arteries | Deoxygenated |
| Alveolar capillaries | Capillaries | Changing (gas exchange occurs) |
| Pulmonary veins | Veins | Oxygenated |
Guided flowchart exercise: annotate oxygen saturation and locate gas exchange
Copy the flowchart below into your notes. Then add (1) an oxygenation label and (2) a pressure label (low/moderate) for each box. Finally, circle the exact anatomical site of gas exchange.
Systemic veins → Right atrium → Right ventricle → Pulmonary valve → Pulmonary trunk → R/L pulmonary arteries → Pulmonary arterioles → Alveolar capillary networks → Pulmonary venules → Pulmonary veins → Left atriumExercise prompts
- Oxygenation annotation: Mark each segment as “deoxygenated,” “oxygenated,” or “transitioning.” Only one region should be “transitioning.”
- Gas exchange identification: Write “GAS EXCHANGE HERE” next to the anatomical structure where diffusion occurs (be specific: not just “lungs”).
- Pressure comparison: Put a “LOW-P” tag on segments that are characteristically low pressure in the pulmonary circuit; note where pressure is highest within this circuit (hint: immediately downstream of ventricular ejection).
- Direction check: Underline every structure that is an artery in this pathway and confirm it is defined by direction away from the heart, not oxygen content.
