A generic diagram of branched tubular system. By specifying where signaling molecules, transcription factors, and other molecular factors are localized or expressed, the lengths, the diameters, and the sites where branches form and the angles of bifurcation are specified. Dashed arrows show the lengths of the constituent segments; D1-D3, axial diameters (small dashed arrows) at various points of the length of a part; the arcs show the angles of bifurcation. Figures 345: Latex cast preparations of the airway (Figure 3), the venous (Figure 4), and the arterial- (Figure 5) systems of a mature lung of the pig (Sus scrofa). Product of complex branching process, the systems match to optimize gas exchange between air and blood. Tr, trachea. Scale bars: 1 cm. From Maina and van Gils .