1. We have studied the extensibility of circumferential strips of main pulmonary artery and large pulmonary veins obtained at post mortem from patients of all ages, dying from conditions other than heart and lung disease.

2. The vessel strips were submitted to increasing loads in a tension balance. The pulmonary arteries were found to be readily extensible. This extensibility became less with increasing age. The pulmonary veins were virtually inextensible at all ages.

3. It is postulated that the large extraparenchymal pulmonary veins have a capacitative role in supplying blood from the lungs to the left atrium. This may be accomplished by their collapsible nature, as they have little capability of distension.

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