A vascular system consists of a supplying arterial and a draining venous part which are connected by a terminal vascular network. The arterial segment can be characterized according to the structural features of the vessel wall. However, it is sometimes diffi- cult to distinguish the capillary from the postcapillary vessels on the basis of structural features alone. On the other hand, physiologic qualities such as permeability can hard- ly be associated with an equivalent histologic pattern of the vessel wall (lllig 1961; Rhodin 1967, 1968; Hauck 1971; Westergaard 1974). A defmition of a vascular seg- ment based on biologic significance should combine morphological and functional qualities of the vessel walls. During the ontogeny of the mammalian organism a variety of vascular patterns (e. g. , distribution of arteries and veins, arrangement of the capillaries) has been formed typical of each organ (Wolff et al. 1975; Baez 1977). The capillaries connect the feed- ing arterioles and the collecting venules in two different ways according to the branch- ing pattern of the terminal vessels (Hauck 1975, Wolff et al. , 1975).
The arterioles and venules are directly connected by capillary segments. Consequently a terminal vessel called arteriovenous (a-v) capillary results, or a closely meshed capillary network is de- veloped which connects arterioles and venules by a variable number of small capillary branches arranged parallel to the preexisting a-v capillary.