lung morphology trait blood-air barrier morphology trait Any measurable or observable characteristic related to the shape, structure, color, or pattern of the membrane between the capillary blood and alveolar air, comprised of the alveolar epithelium and the capillary epithelium and their adherent basement membranes and epithelial cell cytoplasm. Trait.ontology VT:0002324 alveolar-capillary barrier morphology trait alveolar-capillary membrane morphology trait alveolocapillary membrane morphology trait