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ABSTRACT
Using scanning electron microscopy (SEM) of vascular corrosion casts (VCCs), correlative light microscopy of paraplast embedded Goldner stained serial tissue sections, and 3D-morphometry, we studied the topographic microvascular anatomy of the septate paired lung of the adult African Clawed Toad, Xenopus laevis Daudin. Histomorphology showed that primary, secondary and tertiary septa arise from the lung walls, have a loose connective tissue core, and subdivide the lumen into primary, secondary and tertiary blind ending alveoli. SEM of VCCs revealed that right and left pulmonary arteries, which approached the lung along the lateral aspect of the bronchus, ran inside the lung parenchyma towards caudal. Circumferential arteries branched off in acute angles from dorsal and lateral sides of the pulmonary arteries and ran towards the medial pleural surface. Circumferential arteries gave off radial arteries which ran straight towards the visceral pleura, bifurcated into radial arterioles, and capillarised at regular intervals to form the alveolar capillary beds. The venous drainage basically mirrored the arterial supply (i.e. alveolar capillaries drained into radial venules) which emptied into the radial veins. The radial veins merged into the larger circumferential veins, which finally drained into the ipsilateral pulmonary vein. At the level of the bronchi, left and right pulmonary veins joined at the midline and formed a single pulmonary vein which emptied into the right atrium. Venous valves were absent. Peribronchial shunt vessels were not found. The interalveolar vessels (arterioles, venules and capillaries), found for the first time in the present study, are thought to function – depending on the respiratory cycle, rates of oxygen consumption or other physiological needs – as bypasses.