The Structure of the Intrapulmonary Vasculature of the Domestic Fowl

Summary

The walls of the small veins and arteries in the exchange tissue of the paleopulmo of the domestic fowl (G. domesticus) have been investigated by light and electron microscopy. The intra- and interparabronchial veins had one or two layers of thin smooth muscle cells devoid of nerve fibres. These vessels would have little capacity for regulating blood flow. The terminal arterioles possessed a single layer of non-innervated stout smooth muscle cells. The small interparabronchial arteries had two to four stout smooth muscle cells, innervated regularly but sparsely by noradrenergic, and perhaps also by cholinergic axons, in the adventitia. These arteries appeared capable of the local regulation of the microcirculation. One muscular valve was found in the opening of a small interparabronchial vein. The valve comprised two strong muscular components guarding a slit-like opening. The smooth muscle fibres in each component were arranged radially (dilator), with circular fibres (constrictor) at the base. This valve could have regulated the venous drainage from the vascular territory of about 5% to 10% of the length of two or three adjacent parabronchi of the longest type.