Figure 2.

(a) The artery (A) is surrounded by a single layer of smooth muscle cells and loose connective tissue (CT). The vessel splits into smaller vessels (red arrows) that are no longer surrounded by connective tissue and form the capillary network in the inter-saccular septa. This artery provides blood for several adjacent sacculi. In this slice some parts of the inter-saccular ACN only contain a single layer of capillary vessels (sACN), while other parts contain a double-layered ACN (dACN). (b) Shows the same septa in a different slice, where it can be observed that there also is a double-layered capillary network (dACN) in these septa. The magenta part of the segmentation in the middle of this figure could neither be assigned to the blood vessels, the airspace nor the lymphatic vessel with certainty. (c) Slices where all inter-saccular septa contain a double-layered capillary network (dACN), while the septa that are located between the sacculi and non-parenchymal structures, e.g., lymphatic (L) or artery (A), only contain a single layer of capillaries (sACN).