Many transport proteins bind small molecules and transport them to other locations in the body. These proteins typically have a high binding affinity when their ligand is present in high concentrations and a low binding affinity at the target tissues. Hemoglobin is the quintessential transport protein, which transports oxygen from the lungs to the tissues. The binding of oxygen by hemoglobin is a cooperative process. Binding affinity is increased by the degree of oxygen saturation, with the first oxygens bound causing steric changes that facilitate binding for the next oxygens. Furthermore, low pH and high CO2 levels in the tissues decrease binding capacity of hemoglobin for oxygen, a phenomenon known as the Bohr effect.