The pulmonary circulation is a short, low-pressure loop from the right ventricle through the lungs to the left atrium. Its job is to load oxygen into the blood and unload carbon dioxide.
The route
The pulmonary trunk leaves the right ventricle and splits into the right and left pulmonary arteries, the only arteries that carry low-oxygen blood. They branch alongside the airways down to capillaries around the alveoli, and four pulmonary veins return oxygen-rich blood to the left atrium.
Low pressure
Normal pulmonary artery pressure is about 25/10 mmHg, far lower than in the body's arteries. The thin-walled right ventricle can manage because the lung vessels offer little resistance.
Matching blood to air
When a region of lung is poorly ventilated, its small arteries narrow and send blood to better aerated areas (hypoxic vasoconstriction). This matching of ventilation and perfusion keeps blood oxygen up.
When the loop fails
A clot in the pulmonary arteries (pulmonary embolism) blocks flow to lung that is still ventilated, and long-term high pressure (pulmonary hypertension) strains the right ventricle. Left heart failure backs blood up into the lungs and pushes fluid into the alveoli.
At the bedside
Low oxygen saturation with clear lungs can point to a blood flow problem such as pulmonary embolism rather than an airway problem. Crackles at both lung bases suggest fluid backing up from the left heart.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).