Fig. 10. (A) Exchange tissue of the lung of the ostrich Struthio camelus showing air capillaries (*) and blood capillaries (filled circles). The blood capillaries contain erythrocytes (filled squares). Open circles, blood–gas (tissue) barrier; filled arrowheads, tissue not involved in gas exchange, i.e. areas where blood capillaries join or where air capillaries lie adjacent to each other. Scale bar, 5µm. (B) A blood capillary showing the blood–gas (tissue) barrier (open circle); blood plasma (p) and an erythrocyte (filled square). Note the sporadic attenuation of the blood–gas (tissue) barrier and the numerous micropinocytotic vesicles, filled arrowheads. Asterisks, air capillaries. Scale bar, 1µm.