Vascular casts of human oxyntic mucosa, examined by scanning electron microscopy, revealed that (a) arterial breakup occurs only at the submucosal aspect of the mucosa, there being no arterial vessels further into the mucosa and (b) mucosal capillaries drain into the infrequent mucosal venules only in the most luminal region of the lamina propria immediately underlying the surface mucus cells; the mucosal venules do not receive capillary tributaries deeper within the mucosa. Thus, blood flow in the mucosal exchange vessels can only occur in a unidirectional fashion, from the serosal to the luminal aspect. Using transmission electron microscopy, an often intimate association was seen between the fenestrated mucosal capillaries and parietal cells. These findings are together incorporated into a hypothesis that proposes vascular transport of HCO3-, released interstitially by secreting parietal cells, from deep within the mucosa toward the surface epithelium, thereby assisting gastric mucosal defense against luminal acid.