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Most oxygen in blood is carried bound to hemoglobin, while only dissolved oxygen contributes to the partial pressure of oxygen. Cooperative binding gives the oxygen–hemoglobin dissociation curve its sigmoidal shape. Increased carbon dioxide or hydrogen ion concentration shifts the curve to the right, encouraging oxygen release in tissues. Carbon dioxide is transported mainly as bicarbonate, and deoxygenated hemoglobin carries more carbon dioxide. Ventilation-perfusion (V/Q) relationships also vary by lung region: gravity increases perfusion more than ventilation toward the base. A mismatch may take the form of dead space, where ventilation occurs without perfusion, or shunt, where perfusion occurs without ventilation. Diffusing capacity for carbon monoxide (DLCO) provides a measure relevant to gas diffusion efficiency.